Regime Shifts DataBase

Large persistent changes in ecosystem services

Aquatic ecosystems, such as ponds, canals, ditches or tropical lakes can experience shifts between submerged and floating plant-dominated regimes when the concentration of nutrients (i.e. nitrogen and phosphorus) in the water column changes. The two possible regimes and their associated ecosystem services are:

Submerged plant-dominated regime

This regime is dominated by submerged plants such as Elodea nuttallii (better known as waterweed, a rapidly growing, long and stringy plant) that grow underwater in aquatic ecosystems such as ponds, canals, ditches or tropical lakes. They can be found in water bodies with a low water depth, clear water, low turbidity and weak water-level fluctuations. Typically, their roots are connected to the sediment pool. The regime of submerged dominated plants is also characterized by a low-nutrient concentration in the water column.

Floating plant-dominated regime

This regime is dominated by floating plants such as Eichhornia crassipes, Salvinia molesta, Lemna gibba or Pistia stratiotes found in aquatic ecosystems such as ponds, canals, ditches or tropical lakes. In many places these are invasive exotic species. These plants have their leaf surface exposed to the atmosphere and their roots are not connected to the sediment floor. Typically, a floating plant dominated regime is characterized by dark, anoxic and high nutrient conditions in the water column. 

Submerged to floating plant dominance

The main direct driver that leads to the shift from submerged to floating plant dominance is nutrient enrichment of the water body, typically associated with fertilizer and manure runoff from agricultural activities in the catchment. Strong water-level fluctuations associated with large rainfall events or water abstraction for irrigation and other uses can also enhance nutrient input from the shoreline.

This shift can also be caused by a change in water depth (associated with rainfall events and water abstraction) or turbidity (resulting from pollution, sediment runoff, and algal growth). These both directly affect the availability of light, which in turn affects photosynthetic activity and therefore the growth of submerged plants. The deeper the water body the darker it gets with increasing water depth. The more turbid the water body the less light available for photosynthetic activity in the deeper water layers. Floating plants are therefore in a better position to compete for in situ light than submerged plants. Hence, the deeper or more turbid the water body is, the more likely a shift to a floating plant dominated system. Moreover, the floating plant regime shift can be caused by the introduction of invasive species. Invasive floating plant species might grow very rapidly and aggressively outside their natural environment with the consequence that they might take over an entire water body by reducing the incidence of light, which in turn lead to a decrease of submerged plants. A famous invasive floating plant is Eichhornia crassipes which causes substantial problems in many parts of the world.

When nutrient concentrations in the water are realtively low, nutrients settle onto the sediment floor and are taken up by submerged plants. Submerged plants can grow at low nutrient concentrations in the sediment, and their growth removes nutrients from the system, keeping the nutrient concentration in the system low.

However, once the nutrient concentrations in the water exceed a certain threshold due to excessive nutrient input from eg fertilizer runoff, it becomes possible for floating plants to establish and grow in the water body. The roots of floating plants are not connected to the sediment pool, and are therefore dependent on the availability of nutrients in the water column in order to grow. 

The growth of floating plants in the water column decreases the amount of light that reaches the deeper water layers, and therefore reduces photosynthetic activity of submerged plants, eventually leading to their death. The loss of the submerged plants in turn destabilizes the nutrients trapped in the sediment floor, which become resuspended in the water column, further increasing the nutrient concentration in the water column and enhancing the growth of the floating plants. Once the system has shifted to a floating plant regime, the dark and anoxic conditions under floating plants leave little opportunity for submerged plant life, and it may be very difficult to restore the system to a submerged plant regime. 

Floating plant dominated systems decrease fisheries and plant life due to the dark and anoxic conditions under the leaf surface. They can also have a negative impact on navigation in lakes and canals, water purification and recreation opportunities. Moreover, floating plants can clog water supply pipes for households, ditches or even a river mouth. They can also complicate fishing. In addition, there is a greater risk of pathogens that can negatively affect human health in floating plant dominated systems. 

The main management option to enhance the resilience of the submerged plant regime is to control the nutrient levels in the system, and prevent the establishment of significant numbers of floating plants through a mechanical or chemical removal.

A drastic harvest of floating plants in a shallow water body that has some submerged plants and a not too high nutrient level can shift to a floating plant-dominated regime back to submerged plant-dominated regime (see also the Dutch Ditches Case Study). The amount of harvest needed for a shift is predicted to increase with the water nutrient level. Harvesting the floating plants allows light to penetrate to the deeper water layers and enables submerged plants to re-establish. Once re-established, the submerged plants can help reduce nutrient levels by absorbing nutrients for their growth and trapping suspended nutrients in the sediment.

This regime shift entails extensive change in biodiversity and soil structure in the Arctic tundra due to increased climate warming. To date shrub invasion has been the main indicator of a potential regime shift.

Arctic tundra regime

This regime is characterized by low atmospheric temperatures that enable the formation of a layer of permanently frozen subsoil (permafrost) consisting mostly of gravel and finer material. The ecosystem has a low rate of primary production due to impeded mineral nutrient cycling. The short growing season in summer influences the vegetation and its root systems by limiting vertical expansion. Only plant species (lichens, liverworts, mosses) that are adapted to sweeping winds and soil disturbance can survive in these conditions. The atmospheric cold and existing permafrost restrains the plant growth ensuring that their roots are shallow and surface parts are low growing due to the frozen ground underneath the permafrost. Snow does not accumulate keeping the soil temperature low thus not allowing for permafrost to thaw and release carbon into atmosphere keeping the atmosphere cold.

Boreal forest regime

This regime is characterized by flora that mostly consists of cold-tolerant evergreen conifers, such as the evergreen spruce (Picea), fir (Abies), and pine (Pinus), and the deciduous larch or tamarack (Larix). Before these species are established in a region, there is an increased canopy of early-successional species such as aspen and paper birch (Frelich et al. 1995). Dwarf shrubs (e.g., Vaccinium uliginosum, Ledum decumbens, Betula nana and Cassiope tetragona) and low shrubs (e.g., Betula glandulosa, Salix glauca) act as pioneers that colonize the tundra before trees are able to establish (CAVM Team 2003). The shift to boreal forest can occur over a long time period varying from several decades to centuries depending on the migration time lag for each species and the occurrence of favorable conditions for seedling establishment.

The main external driver of this regime shift is CO₂ emissions that increase climate warming (IPCC 2007), leading to changes in the composition and abundance of arctic vegetation, animals and soil structure (Olofsson et al. 2009). Anthropogenic activities that elevate atmospheric greenhouse gas concentrations are generally considered to be the primary cause for this driver. Carbon release from anthropogenic sources is projected to continue and increase during the coming decades (IPCC 2007). This global driver is well established and could be looked as irreversible in the scale of next hundred years.

The tundra regime is maintained by the low atmospheric temperatures that ensure low soil temperature and the existence of permafrost. This determines the lack of microbial activity and scarce nutrient availability therefore ensuring species composition that are only able to exist in such circumstances.

A shift from the tundra regime to boreal forest is initiated by the atmospheric warming that is caused by continuing increase of CO₂ emissions. The atmospheric temperature increases the period of time when microbial activity and thus nutrient availability is sufficient enough for shrubs and boreal forest pioneer species (birch etc.) to withstand the winter conditions.

As a result the increasing number of shrubs ensure that snow drifting creates deep drifts that surround and extend downwind from shrub patches. These drifts increase the insulation of the soil below. This insulation elevates the soil temperature thus further allowing microbial activity to continue during the frigid arctic winter. The elevated soil temperatures under the snow lead to permafrost degradation that release carbon trapped in tundra soils, thereby further contributing to climate warming.

Shift from Tundra to Boreal Forest

Tundra provides biodiversity for different animal and plant species that characterize tundra ecosystem. The shift from tundra to boreal forest is projected to occur over large geographic areas throughout the tundra zone, with substantial impacts on the landscape and biodiversity (Bonan 1992). Changes in vegetation are also likely to affect composition of foraging mammals and birds (Hinzman et al. 2005). In addition to modifying wildlife habitat, increased woody shrub abundance will make traversing tundra more difficult for caribou, subsistence hunters and communities that rely on caribou for food, as well as hikers. Herbivores such as reindeers and microtine rodents can also be influenced by the shift towards woody vegetation as they prefer lichens, dwarf shrubs graminoids and deciduous shrubs over tall woody shrubs (Sturm et al. 2005). Increased expansion of woody vegetation may therefore result in tundra species extinctions or radical decrease in numbers. The loss of the climate regulating ecosystem service is a major concern as permafrost thawing is reducing the carbon sink service.

For local populations the tundra provides traditional recreation opportunities and contains their knowledge and educational as well as spiritual and religious values, which would be lost with the regime shift.

Shift from Boreal Forest to Tundra 

The change in biodiversity could provide new ecosystem services such as timber production and different wild animals and plant foods. This would benefit loggers as they would increase their income from tree cutting and local communities from goods that come from forest.

In order to prevent or reverse this regime shift it is essential that the input of carbon to the atmosphere is reduced in order to prevent further climate warming. The suggestion of a planetary boundary of 350 ppm for CO₂ levels is a first step in this direction (Rockstrom et al. 2009). Increased understanding of the influence of climate change on the complexity of arctic systems is essential for informing the adaptation of social, economic, and cultural systems to the changes taking place in the Arctic (Hinzman et al. 2005). Technology transfer could be a good initiative from developed countries as they can provide more advanced technological solutions to developing countries to help accelerate the learning curve on GHG emissions. A global response particularly from developed nations that are using the majority of the world's resources on a per capita basis should be in place to deal with such complex systems. Nevertheless, discussions about desirable management strategies and outcomes will be essential as there will be stakeholders who stand to lose from this regime shift (e.g. caribou herders) while others may see the change as desirable and beneficial (e.g. timber production, game hunting).

Forest regime
Forests are ecosystems typically dominated by trees, perennial plants taller than 5 meters. Tropical forest includes moist and dry forests (MA, 2005). A mature tropical forest contain at least four layers: emergent layer up to 45 - 80 meters tall, the canopy among 35 - 45 meters tall, the understory layer and the floor layer. Such structure gives a variety of habitats that host roughly half of the known plants and animals biodiversity (MA, 2005).


Savanna regime
Savannas, on the other hand, are drylands dominated by a mixture of grasslands and shrublands. The canopy in savannas never closes, and the floor layer is dominated by grasses, especially C4 species. Savannas, dry forest and shrublands conform 40% of the world’s land area and host up to 42% of human population (Reynolds et al. 2007, Falkenmark and Rockström 2008). About 25% of drylands, including savannas, are covered by croplands and sustain 50% of world’s livestock (MA, 2005).

Shift from forests to savannas

The most widely recognized driver is deforestation and consequently fragmentation of forest landscape, which reduces rainfall and increases surface temperature (Da Silva et al. 2008, Nobre et al. 2009). Reduction of forest cover accelerates albedo effect, loss of evapotranspiration and roughness length (Sternberg 2001), activates fire feedback, changes ocean circulation and warms up sea surface in the Amazon case, and ultimately changes the spatial vegetation structure.

Deforestation and forest degradation is driven by a complex, case specific interaction of social and economic drivers. The most important reported drivers include agriculture expansion, infrastructure development, the logging industry and fast population growth (Geist and Lambin 2002, MA, 2005). For example, in the Amazon, illegal logging is a critical threat that besides its damage to the forest, bring with them secondary effects like expansion of hunting areas, slash-and-burn farms, mining, the establishment of new road networks and therefore more logging facilities. The MA (2005) reports that 70 countries have problems with illegal logging leading to national income losses of $5 billions and total economic losses of about $10 billion. By 2001, Laurance and Williamson (2001) reported that 80% of brazilian logging activity were illegal; however, government counterintuitively sponsored colonization through cattle ranching projects.

Climate change and global warming are expected to enhance the regime shift; and the loss of forest areas are expected to exacerbate climate change (Laurance and Williamson 2001, Bonan 2008). While Laurance and Williamson (2001) report that forests like Amazon apparently change from carbon sinks to carbon sources during ENSO events; Nobre et al. (2009) suggest that deforestation of Amazon may actually increase ENSO variability; and Bonan (2008) confirms that deforestation would enhance global warming by decrease of evaporative cooling and release of carbon dioxide.

Shift from forest to savanna

The expansion of agriculture, increase in population as well as deforestation reduces rainfall and increases surface air temperature. This has a direct affect on plants, reducing evapotranspiration and photosynthesis and decreasing the supply of water vapour. Cattle pastures and regrowth forest areas become increasingly prone to frequent fires, which plays a fundamental role in the shift from forest to savanna since it is a feedback that actually maintains savanna state.

When tropical forests are replaced by less vegetated cover like savannas, or ultimately by sand in deserts, net radiation at the top of the atmosphere decreases inducing subsidence that inhibits precipitation (Oyama and Nobre 2004). While in the tropics land clearing affects the water balance and as consequence warms up the climate, in boreal forest such clearing affects mainly albedo and as results cools down climate (Foley et al. 2005). The albedo feedback is strengthened by changes in land cover, typically induced by deforestation for logging or agriculture activities. Warmer temperature and drier atmospheres, such as in savanna regime, result in an increase in lifting condensation level feedback that reduces the opportunity of cloud formation and therefore the likelihood of rainfalls (Pinto et al. 2009). Plants that do not have enough water responds by reducing transpiration and photosynthesis, interrupting the supply of water vapor that contribute to the recycled component of precipitation (Oyama and Nobre 2004, Saleska et al. 2007). Less evapotranspiration blocks the inland propagation of cold fronts responsible for precipitation, increasing the dry season length (Oyama and Nobre 2003, Pinto et al. 2009).

Savanna vegetation is better adapted to dry environments. Grasses usually have C4 photosynthesis type, a chemical pathway that reduces water consumption and helps to cope with nitrogen or CO2 limitations. Evapotranspiration depends on soil moisture and biomass. Thus, for instance, droughts frequency or grazing reduce biomass, weakening in turn the feedback effect (Dekker et al. 2007, Dekker et al. 2010). In addition, the spatial distribution of rainfall is affected by both the land-cover type and topography. Cattle pastures and regrowth forest areas become increasingly prone to frequent fires. In such zones fire can be produced after few days of dry conditions. On the regional scale, fire smoke may reduce rainfall by trapping moisture and inhibiting raindrops formation (Laurance and Williamson 2001). Fire plays a fundamental role in the shift from forest to savanna since it is a feedback that actually maintain savanna state (Laurance and Williamson 2001, Hutyra et al. 2005)

Forest provides a wide range of ecosystem services. Besides being hot spots of biodiversity, forest provides soil and water protection, it prevents soil erosion, floods and landslides. For example, soil erosion may be 10-20 times higher on areas cleared of forest (MA, 2005). Depending on soil conditions, at the local scale forest can also regulate the hydrological cycles by increasing precipitation and decreasing evaporation. They regulate below grown runoff and smoothing seasonal extreme events: heavy rainfalls or dry spells. Due to its regulating function in the water cycle, forested watersheds provides water supply to one third of the world’s largest cities (MA, 2005). 

Forest sustain about 200 million people belonging to indigenous groups, who depend on forest not only as source of resources (food, fiber, fuel) but also their culture and religious traditions (MA, 2005). Forest also maintain the agroforestry industry which, including temperate forests, produces 3.3 billion cubic meters of wood (MA, 2005). 

Managerial options for the forest to savanna regime shifts requires targeting the main drivers: deforestation and landscape fragmentation. Controlling illegal logging and implementing sustainable logging plans are part of the strategy. Sustainable logging needs to take into consideration reducing fragmentation and allowing deforested patches to regrow. Likewise, the expansion of agricultural frontier and grazing areas needs to be controlled and when unavoidable, it needs to be planned in order to prevent fragmentation. The fire frequency feedback accelerates the shift from forest to savanna regime. Laurance and Williamson (2001) suggest fundamental changes in prevailing land-use practices and development policies to avoid wildfires. Such changes include the management of logging and grazing areas in order to reduce fragmentation and therefore it would reduce the fire risk. Hence, fire and fragmentation management need to be coupled strategies.

Kelp forests are marine coastal ecosystems dominated by macroalgae typically found in temperate areas. This group of species forms submarine forest with three or four layers, which provides different habitats to several species. Kelp ecosystems are important to maintaining important industries such as, lobster and rockfish fisheries, tourism based on fishing, recreational diving and kayaking, as well food and pharmaceutical products derived from kelp (e.g. tooth paste).

Three different self-reinforcing regimes can be identified:

Kelp forests are highly productive ecosystems dominated by canopy-formed algae in cold-water rocky marine coastlines. Among the biota associated with kelp forest are marine mammals, fishes, crabs, sea urchins, mollusks, other algae and biota that live on the kelp themselves (Steneck et al. 2002). At least 4 trophic levels are found in kelp forests. Apex predators such as sea otters, cod, pollock, hake, and haddock are common. These predators regulate populations of species at lower trophic levels. In particular the regulation of sea urchins populations, who are an important consumer of kelp, is important to maintain kelp forests.

Urchin barrens are an alternative regime in which there is no kelp forest and substantial populations of urchins on the sea bed. This regime has low or no populations of apex predators and large populations of herbivores that keep macroalgae at very low population levels, preventing the regrowth of the kelp forest, an important habitat for apex predators (Steneck et al. 2004). Ecosystem services related to apex predator fisheries and recreational activities are substantially reduced. However populations of bottom dwelling species can increase. In some places urchins have themselves become a valuable fishery. While in others, populations of other harvested species such as lobster and crab have increased.

Turf-forming algae regime is similar to urchin barrens in that there is no kelp forest, however rather than dominance by urchins the seabed is dominated by turf forming algae. This regime also lacks apex predators (Gorman et al. 2009). Ecosystem services related to fisheries and recreational activities are substantially reduced. However, populations of invertebrate species such as lobster and crab can be higher. This regime shift is associated with a change in habitat in shallow marine coastal ecosystems. 

This regime shift is driven by two key direct drivers: reducing populations functional groups (Steneck et al. 2002; Steneck et al. 2004) and input of nutrients (Gorman and Connell 2009). Reductions in populations of key functional groups is due to the hunting of sea otter and the fishing of large apex predator fish species. Overfishing usually diminish the controlling function of top predators, and in severe cases, the trophic levels may be simplified. Nutrient input is often due to the output of wastewater from urban settlement and agriculture in nearby coastal areas. However it can also be due to naturally occurring offshore upwelling events. Nutrients favors the growth of turf algae over kelp. Strong rain events and floods can produce shock events for kelp ecosystems by providing a pulse input of nutrients and turbidity, which can stimulate the growth of turf algae.

Other drivers can also contribute to this regime shift. Pollution discharges and sedimentation may play a synergetic role as stressors. In Tasmania for example, global warming has favored the reproduction of urchins which acting in synergy with lobster fishing has reduced kelp resilience (Ling et al. 2009). El Niño events and global warming can generate water stratification. As a consequence, nitrogen concentration declines and kelp forest growth becomes limited by the availability of nitrogen (Steneck et al. 2002).

Shifts from kelp canopies to other regimes like urchin barrens and turfed landscapes have occurred in many regions worldwide (Steneck et al. 2002; Gorman and Connell 2009; Ling, et al. 2009). This regime shift is associated with a change in the biophysical structure in shallow marine coastal ecosystems. Kelp forest dynamics often include cycles of disturbance-recovery. However, increasing human-dominated coastlines modify environmental conditions that facilitate other regimes like turfs or urchin barrens, which in turn inhibits the reestablishment of kelp forests.

Overfishing and the input of nutrients are the main drivers of the regime shift. At least 4 trophic levels are found in kelp forests, and apex predators such as sea otters, cod, pollock, hake, and haddock are common. Overfishing usually diminish the controlling function of top predators, and in severe cases, the trophic levels may be simplified. In addition, urchins are prevented from grazing in the kelp forest through the effect of kelp foliage sweeping over the rocks due to its flexibility and the force of waves (Steneck et al. 2004). 

Overharvesting of sea otter, cod and haddock can release their prey populations in particular sea urchins from their predatory controls (Steneck et al. 2002; Steneck, et al. 2004). Large populations of urchins form grazing fronts that graze on kelp forest to its elimination (Lauzon-Guay et al. 2009). These populations of urchins can survive in adverse conditions by feeding on turf-forming algae. In some cases the removal of apex predators allows populations of other species to reach sufficient population sizes to exert some control on urchins, for example lobsters and crabs in the North Atlantic (Steneck et al. 2002). Therefore, in some cases the formation of urchin barrens open the opportunity for new fishing industry to establish, like urchin, lobster and crabs. In addition, declines in predatory fish have created a market for urchins, especially in Japan, establishing a new human induced control on the herbivore. In some cases, harvesting of urchins for this market has led to the re-establishment of kelp forest, despite urchin fishery being prohibited (Steneck et al. 2002; Scheffer 2009).

On the other hand, the turf-forming algae regime is maintained by its ability to persist under conditions of elevated nutrients, frequently attributed to coastal urban settlements, inhibiting the recruitment of kelp species (Gorman and Connell 2009). The loss of kelp dominated areas undermine kelp's ability to reestablish in disturbed areas (Gorman and Connell 2009). While deforested areas surrounded by kelp patches are more likely to return to the kelp regime, isolated kelp disturbed patches are more likely to stick in the turfed regime.

Shift from kelps forest to urchin barrens or turfs

The main ecosystem impact of the loss of kelp forests is the loss of habitat complexity. Kelp is a three-dimensional structure that offers shelter and food for many species; urchin barrens and turfs do not have such characteristics. This loss is associated with the reduction of the food web complexity and loss of functional groups (Steneck et al. 2004), with varying effects on fisheries. Some valuable fish species may diminish since kelp forests provide nursery areas. Invertebrate species such as lobster and crab can increases in population (Steneck et al. 2002). Along with hosting high marine biodiversity, kelp forests provide ecosystem services related to recreation for divers. In addition, kelp supports a multi-million dollar industry of canopy-cropping for alginates (Steneck et al. 2002). This product is commercially important in pharmaceutical and chemical industry. These services can be reduced or lost by this regime shift.

The ecosystem service impacts of algae turfs are likely to be similar to those related to coastal eutrophication. Such effects include abundance of rich-nutrient environment species as shellfish, bad odors and the associated consequences for recreational and aesthetic values.

Shift from urchin barrens or turfs back to kelp forest.

The recovery of kelp forest increases habitat complexity and is expected to increase biodiversity. On the other hand, the benefits gained by urchin and lobster fisheries may be diminished, since the recovery of functional groups and increasing biodiversity will control their populations. However, other species of commercial fisheries and tourism may recover, although it is not always the case.

Options for enhancing resilience

Restoring biodiversity in functional groups in kelp forests is particularly important to maintain apex predators (Estes et al. 2011). A high profile version of this strategy has been implemented to recover sea otter populations on the west coast of North America, by protecting sea otter populations (Steneck et al. 2002). Reducing nutrient runoff from agricultural areas is essential for enhancing resilience, as well as improving water treatment and managing runoff in urban areas (Gorman et al. 2009). The removal of sea urchins along with implementing measures to maintain the abundance of predators like cod, sea otters and sheep-head through fishery controls may provide stability to kelp forest ecosystems (Steneck et al. 2002; Steneck et al. 2004).

Options for reducing resilience to encourage restoration or transformation

Transplantation of kelp-forming species has been proposed as a restoration strategy in highly fragmented seascapes. This strategy is likely to increase the supply of kelp propagules to colonize new areas and maintain free substratum for canopy recruitment (Gorman and Connell 2009). However this strategy has not been tested.

Bivalve mollusks play an important role in aquatic ecosystems by filtering and sequestering nutrients. When bivalve abundance changes, it can have substantial ecosystem impacts, creating two different, self-reinforcing regimes.

Bivalve mollusk reef regime

When aquatic ecosystems have high bivalve mollusk density, they form reefs that include many bivalves and are shallow, nearing the water's surface. The filtering activity of the bivalves results in clear water with high levels of dissolved oxygen. Plankton populations are limited in these conditions. The bivalves themselves operate as reinforcing feedbacks in that their abundance ensures sufficient filtration to maintain this clear water regime. The clear water in turn reinforces bivalve abundance by maintaining hydrodynamics that are conducive to bivalve health (Scheffer 2009). Large, shallow bivalve mollusk reefs encourage biodiversity by providing habitat and filtering nutrients.

Isolated, low density bivalve mollusk regime

When aquatic ecosystems have low bivalve mollusk density, the reefs are small and deep. Water is turbid, with low levels of dissolved oxygen. Plankton and filamentous algae flourish under these conditions. Poor reef size and water filtration limit biodiversity. The low bivalve abundance state is reinforced as bivalve health and fecundity is weakened by turbid water conditions. The turbid water conditions are in turn reinforced by low bivalve abundance(Weijerman et al. 2005, Powell et al. 2008).

The regime of low bivalve abundance is expected to reduce fish populations, water cleansing and the ability of the ecosystem to maintain biodiversity. Such effects would have an impact on fisheries, human health, and recreation. 

Mechanized anthropogenic over-harvesting of bivalve mollusks is the main driver of the shift from high to low abundance (Thrush and Dayton 2002).  High inputs of nutrients from agricultural or urban sources can act as a slow driver that weakens bivalve health by creating plankton blooms that the bivalves are unable to filter.  Plankton blooms can further weaken the health of bivalves by producing organic matter, whose decay, reduces oxygen availability.  A disturbance that can trigger a regime shift is bivalve disease (Powell et al. 2008).  When populations are weakened by eutrophication and anoxia, they become physiologically susceptible to diseases that can rapidly devastate remaining populations (Leniham 1999). 

Bivalves are a class of mollusks containing over 30,000 species which include scallops, clams, oysters, and mussels. Bivalve mollusks, like corals, are often referred to as "ecosystem engineers" because they engineer the physical and hydrodynamic structures of the ecosystems they occupy in a way that produces conditions that support their own population growth. Bivalves live on the sediment surface, where they form reefs that provide increased surface area and habitat for a variety of species. They filter nutrients from the pelagic zone and transfer it to the benthic zone, maintaining the system in a clear water state. If bivalves are not able to maintain clear water, due to declines in bivalve populations or increases in low water quality, the ecosystem can shift to a turbid water state. The main drivers of this shift is direct anthropogenic over-harvesting of the bivalves, followed by a gradual increase of nutrients discharge from agricultural activities and coastal urban settlements.  Once mollusk abundance is reduced, plankton and filamentous algae populations can substantially increase. As result, oxygen and light available in the water column is reduced, while nutrients levels accumulate due to the lack of filtration.  These processes produce conditions which favour algae over bivalves.  If nutrient inputs are substantial, eutrophication and areas of hypoxia (low oxygen) are likely to appear in areas dominated by plankton instead of mollusks. 

Shift from high to low bivalve mollusk abundance regime

The loss of abundant bivalve mollusk reefs threatens diverse ecosystem services. The most direct impact is the loss of valuable shellfisheries. A secondary, but potentially substantial impact is the loss of the filtering service provided by bivalves.  In urbanized estuaries, it can necessitate the installation of costly synthetic water filtration technology (Gren et al. 2009).  In other areas it can lead to declines in water quality that shift populations and reduce recreation opportunities, and potentially the value of waterfront property.

The loss of bivalve habitat structure leads to lower species abundances, and to declines in species richness (Airoldi et al. 2008). Both structural and functional biodiversity is threatened by loss of bivalve abundance, with far reaching effects on marine ecosystems and human ability to exploit such systems (Worm et al. 2006).  The regime of low bivalve abundance is expected to reduce fisheries, water cleansing and the ability of the ecosystem to maintain biodiversity. Such effects would have an impact on fishing productivity and human health.

The loss of bivalve nutrient filtration can lead to other regime shifts, such as eutrophication and hypoxia, which can produce unproductive and undesirable ecosystems (see Hypoxia and Eutrophication regime shifts). 

Options for enhancing resilience

Estuaries are the world's most degraded marine ecosystems, receiving land-based pollution from crop cultivation and urbanization, and suffering from anthropogenic over-harvesting (Lotze et al. 2006).  The resulting problems are complex, impacting bivalve health and fecundity as well as many other species, with far reaching implications in both biological and social domains.

Experience has shown that management focused on one species or problem tends to be ineffective.  Newer research stresses the need to address natural resources as part of complex social-ecological systems, often with long histories of human exploitation (Jackson et al. 2001).   Rather than addressing problems on a species-by-species basis, multi -species management has shown to have synergistic effects.  For example, in Chesapeake bay, fisheries management looks at oyster, blue crab, striped bass and shad together (Boesch 2004).

Taking this approach further, Ecosystem-based fishery management efforts focus on recognizing interactions between multiple species and environmental stressors, such as low dissolved oxygen levels. Success is measured by the degree to which management efforts include ecosystem-based approaches, rather than by an assessment of fishing stocks (Lotze et al. 2006). 

Options for reducing resilience to encourage restoration or transformation

The main option exercised for preventing or reversing a regime shift regarding bivalve abundance is to import bivalve mollusks from another region for aquaculture (Van de Koppel 2008) .  Another practice commonly used is to hang bivalves from the surface or build artificial reefs to elevate bivalves in order to avoid the hypoxic conditions in deeper water (Carlsson et al. 2009). 

Savannas are systems that consist of a mixture of woody vegetation (trees or shrubs) and grasses. Savannas, dry forest and shrublands cover 40% of the world's land area, host up to 42% of the world's human population (Reynolds et al. 2007, Falkenmark and Rockström 2008), and together with drylands sustain 50% of the world's livestock (Millennium Ecosystem Assessment 2005).

At small scales (up to about 10 km²) savanna systems, especially those used for extensive cattle ranching, may stabilize in two different self-reinforcing regimes (Walker 1993, Scheffer et al. 2001, Scholes 2003):  

Open, grassy savanna regime

In this regime the landscape has a productive grass layer with few mature trees. The canopy in savannas never closes, and the floor layer is dominated by grass, especially C4 species. Most young trees are unable to establish because, while often numerous, seedlings are constantly knocked back to ground level by herbivory and fire. There is enough grass after grazing to support a fire with flame-length taller than the young saplings sufficiently often to keep them in a 'fire trap' (Dublin et al. 1990, Roques et al. 2001b, a). Open savanna systems are suitable for ranching, and are maintained by fire dynamics and grazing.

Closed, woody savanna regime

In this regime the landscape is dominated by woody shrubs or trees. Once established, woody vegetation is stable because adult trees are seldom killed by herbivory or fire. These alternate regimes can occur at a range of spatial scales. Sometimes larger areas (e.g. an entire cattle ranch) may shift from a grass-dominated to a persistent woody-dominated state (Dublin et al. 1990, Walker 1993). In other cases, the alternate regimes are expressed as a mosaic of small patches of trees or bush interspersed with patches of grass, where the respective patches are highly persistent over time (Rietkerk et al. 2004).   

Bush encroachment refers to a shift from a grassy system to a persistently woody system. It typically occurs in areas used for free-range cattle ranching, and is usually caused by a combination of grazing and fire management practices. Bush encroachment involves a change in the outcome of the competitive interaction between woody vegetation (shrubs and trees) and herbaceous vegetation (grasses and herbs), mediated by nutrients, grazing, fire, rainfall variability and use of either the woody or grassy components by humans (Anderies et al. 2002, Janssen et al. 2004, Wiegand et al. 2006). The encroachment typically occurs in episodes rather than continuously, and involves a particular set of encroaching species rather than the entire woody community.

Bush encroachment typically occurs in areas used for commercial cattle ranching (as opposed to subsistence, communal, or nomadic) and may follow episodes of sustained severe overgrazing, though not necessarily so. It may also occur under other land uses (Wiegand et al. 2006). It tends to be an episodic phenomenon, where the tree cohorts can often be linked to issues in the ranching enterprise – such as drought-induced debt or downturns in the cattle price cycle (Scholes 2003, Wiegand et al. 2006). 

There are several different hypotheses regarding the mechanism by which bush encroachment occurs. Different mechanisms (or combinations of mechanisms) may be important in different places. One proposed mechanism is based on changes in fire regime: in the sustained presence of high numbers of grazers (typically cattle) accumulation of grass fuel is reduced, leading to period without intense fire long enough for woody plants to grow beyond the fire-susceptible stage, which in turn suppresses grass production and fires, further enhancing the establishment of woody vegetation (Higgins et al. 2000, Staver et al. 2009, Staver et al. 2011). A related hypothesis notes the elimination of browsers (especially very large browsers such as elephant and giraffe, but also the more-numerous small browsers) from the system when cattle are introduced (Dublin et al. 1990). Similarly, alien species, such as Prosopis in South Africa or Acacia nilotica in Australia, both deliberately introduced, can play an important role in bush encroachment by affecting fire regimes (Poynton 1990). Another hypothesis focuses on changes in water availability based on the rooting depths of plants: grasses are thought to be more shallowly-rooted than trees, so if grass cover is reduced by overgrazing, this is more water available for trees, which promotes their growth and establishment, further suppressing grass growth (Noy-Meir 1982). Refinements of these hypotheses emphasize combinations of events, such as a multiyear drought or fireless period providing a "window" for the establishment of trees (Wiegand et al. 2006).

Yet other hypotheses focus on the role that increases in global CO2 levels may play in the observed proliferation of woody plants in many, widely-separated areas of the world during the 20th century. The underlying mechanism is still debated, but several possibilities have been proposed: i) that rising CO2 levels favour C3 (woody plant) photosynthesis relative to C4 (tropical grass) photosynthesis  ii) elevated CO2 may reduce transpiration of grasses, leading to greater water percolation and therefore favoring deeper rooted woody species, iii) faster growth of woody plants due to CO2 enrichment, and therefore faster escape of seedlings from susceptibility to fire, iv) investments in carbon-based defense compounds such as tannins, which are the main defense compounds in many encroaching trees but not in grasses (Midgley and Bond 2001, Wiegand et al. 2006). It is striking that encroachment is almost unheard of on communal land, and is not universal on commercial farms. A suggested explanation for this is that communal lands use the trees for firewood and run goats along with cattle, inhibiting the establishment of trees (Scholes 2003). Bush encroachment has been documented in East and Southern Africa (but not West Africa), South America (Uruguay/Argentina and Chile), North America (Texas, New Mexico) and Australia, but not in India, also a savanna environment. Furthermore, it did not happen simultaneously in those places, but 30-50 years after the widespread establishment of sedentary grazing management, what has been referred to as 'commercial' ranching above. This tends to disfavor the rising CO2 hypothesis, although rising CO2 may predispose the shift (Midgley and Bond 2001).  

Shift from grassy to woody savanna

Woody encroachment brings a relatively rapid change, over a decade or two, from a highly productive grass layer to a sparse and unproductive grass component. Since cattle are grass-eaters, this change substantially reduces cattle productivity (Anderies et al. 2002, Scholes 2003), with major impacts on cattle ranchers. On the other hand, encroachment increases the supply of tree-based ecosystem services, such as wood for fuel, charcoal-making and building material. This is somewhat dependent on the species involved. The increase in woody cover could potentially also have macro and micro-climatic effects through impacts on albedo and CO2 uptake, in addition to the decrease in methane emissions from cattle.

Difficulties in mustering the cattle in dense bush are a contributing factor. Therefore, wood encroachment leads to economic losses for cattle ranchers in what is frequently an economically marginal area for other agricultural uses such as croplands. 

Options for enhancing resilience

There is some agreement among researchers and extension workers that encroachment can be avoided by stocking lightly and burning frequently to prevent the establishment of trees and maintain grass crowns - the productive part of the grass that is less affected by fires (Roques et al. 2001b, Janssen et al. 2004). However, this is seldom reflected in management practice. 

Options for reducing resilience to encourage restoration or transformation

Bush encroachment is expensive to reverse, since rapid results rely on arboricides or repeated mechanical or manual clearing. A common method involves the manual removal of woody vegetation, with repeated follow-up control and the use of fire to enhance the establishment and competitive advantage of grasses (Scholes 1985, 2003). Attempts to reverse bush encroachment often have poor results, either due to the rapid resprouting of the trees or the conversion of the grass layer to less desirable species in the process. 

There are anecdotal reports of widespread mortality of near-dominant encroaching species after several decades, possibly related to disease, prolonged drought or simply old age, which provides windows for grass establishment and fuel load for intense fires.

The hypoxia regime shift involves a radical change in oxygen concentration in aquatic ecosystems such as rivers, lakes and marine ecosystems. The severity and persistence of hypoxic conditions varies. Episodic oxygen depletion represents 17% of known hypoxia cases, and occurs infrequently with several years sometimes elapsing between events. Episodic oxygen depletion is the first signal that a system has reached a critical point of eutrophication, which in combination with physical processes that stratify the water column, tips the system into hypoxic conditions (Diaz and Rosenberg 2008). Seasonal hypoxia tends to occur periodically during the summer, after algal spring blooms have sunk to the bottom and are being decomposed. It lasts from days to weeks, represents half of the known dead zones, and typically abates in the autumn (Diaz and Rosenberg 2008). Boom-and-bust cycles of animal populations are frequent. Persistent hypoxia occurs when hypoxia becomes persistent due to the build-up of organic matter in the sediments over time, particularly in systems prone to persistent stratification. Persistent hypoxia accounts for 8% of reported hypoxia cases. Anoxia occurs when DO levels fall below 0.2 ml per litre. The accumulation of organic matter is exacerbated, and poisonous hydrogen sulphide (H₂S) is released due to microbial metabolism.

Normoxia: This regime is characterized by normal levels of dissolved oxygen, typically 5 to 8 ml per liter. Most aquatic organisms are adapted to survive under these conditions. Particularly important are the benthic organisms. Benthos are a community of organisms which live in sedimentary environments at the bottom of water bodies. They constitute an important key functional group (scavengers and detritivores) in the food web responsible for the decay of dead matter. Macrobenthos facilitate bioturbation processes, in other words, they displace and mix particles facilitating chemical exchange between sediments and the water column. This allows the sediments to "capture" phosphorous and nitrogen from the water column. By removing these nutrients from the water column, bioturbation therefore reduces algal growth and helps ensure that the water remains oxygenated and suitable for the survival of benthos - creating a reinforcing feedback that maintains normoxia conditions.

Hypoxia: The hypoxic regime is reached when dissolved oxygen level falls below 2 ml per liter (Diaz and Rosenberg 2008). Hypoxia is associated with so-called "dead zones". These are areas where dissolved oxygen levels are so low that most life is not able to persist and only very few specialized microorganisms survive. In the hypoxic state, accumulation of organic matter produced by eutrophic processes favors the growth of microbes who, by decomposing the organic matter, consume the oxygen in the water column.

Anoxia: Anoxia occurs when hypoxia is exacerbated to DO levels under 0.2 ml per liter (Diaz and Rosenberg 2008). Under such conditions, benthic animals suffer from mass mortality and decay processes are carried out by microbial metabolism. As A result, hydrogen sulfide is released changing water acidity (pH) and then further favoring bacteria habitat. 

Hypoxia is driven by increasing nutrient input, both natural and anthropogenic. Natural sources come from the bottom of the ocean transported by upwellings, water currents that flow vertically driven by the interaction of winds and the temperature gradient between the ocean surface and bottom. Anthropogenic sources are related mainly with the use of fertilizers in agriculture. However, growing urban settlements in coastal areas also increase the water storm runoff and sewage.

The leakage of nutrients from agriculture is further exacerbated by rainfall variability and deforestation, since vegetation offers resistance to erosion and trap nutrients and moisture in the soil. Agriculture is driven in turn by increasing demand of food and fibers as well as population growth. Another important factor leading to hypoxic conditions is the stratification of the water column, which reduce the exchange of nutrients between the surface and the bottom. Water stratification is usually driven by anomalous increase in sea surface temperature. 

The fast variable in the hypoxia regime shift is DO, and can also include faunal composition. The slow variables are the accumulation of nutrients, mainly phosphorous and nitrogen in the water column. Large areas of the sea are oxygen minimum zones due to natural conditions. Hypoxic-prone areas generally have a semi-enclosed hydrogeomorphology and water-column stratification that restrict water exchange and reoxygenation of the deeper water layers (Diaz and Rosenberg 2008). Another natural condition that facilitates hypoxia and eutrophication is coastal upwelling, that lead to nutrient enrichment along continental margins.

The normoxia regime is usually  maintained by feedbacks that control the nutrients content in the water column and keep oxygen concentrations high. Zooplankton for example plays an important role controlling micro algae numbers, hence limiting their ability to use nutrients available and reduce oxygen through their metabolism. Macrophytes or macro algae also control micro algae by consuming the same resources. Under such conditions nutrients are kept on inorganic forms unavailable for micro algae to use.

Under the hypoxia regime the feedbacks above are weakened, allowing micro algae to escape predation and competition controls; and reducing the level of DO in water. As DO decreases, mass mortality of different organism increases the availability of organic matter in the water column, decreasing DO and promoting more micro algae growth. When DO reaches critical levels under 0.5mL per liter, hydrogen sulfide is released further decreasing pH in water and reducing biodiversity. Such extreme regime is called anoxia. 

Shift from normoxia to hypoxia and anoxia

Normoxia is associated with the provision of food for humans and wildife in aquatic systems as well as regulating services such as water purification and pest regulation. This regime shift into hypoxia, often characterised as a dead zone, has been reported in more than 400 systems affecting more than 245,000 square kilometers and including important fisheries such as the Baltic Sea, Kattegat, Black Sea, Gulf of Mexico, and East China Sea (Diaz and Rosenberg 2008). A major impact on ecosystems is a change in the flux of matter and energy through trophic levels. Consequently, fisheries and hence ecosystem services such as food production are affected. For example, Diaz & Rosenberg (2008) report that biomass in the Baltic Sea has been reduced by approximately 264,000 metric tons of carbon due to hypoxic conditions. Assuming that ~40% of benthic energy passes up the food chain, 106,000 metric tons of carbon of food energy for fisheries has been lost(Diaz and Rosenberg 2008). This implies a reduction in yields and consequent impact on employment in fisheries communities. Another example of such effects is the lobster fishery collapse in Norway (Diaz and Rosenberg 2008).

Besides its effect on fisheries and employment, hypoxia also affect the health and other cultural services of the people living near by dead zones. Decaying matter after mass mortality events create odors and risk of diseases. Recreation, aesthetic and touristic values are lost.

Shift from Hypoxia to Normoxia

When hypoxia is not severe, the system is likely to return to normoxia regime. It implies the recovery of fisheries when species hasn't been extinct locally. However, food webs not always fully recover, and usually top predators are lost. With the recovery of the system, fishing, recreational and touristic activities recover too, although not necessary to the same configuration than before the regime shift. 

Options for enhancing resilience

Since one of the main drivers of hypoxia is eutrophication, Diaz & Rosenberg (2008) recommend managing the input of fertilizers on agricultural land. They recognize the necessity of developing new methods to close the nutrient cycle on farms, in order to avoid the drainage of nutrients to water sources. Nutrient reductions can also be achieved relatively cost-efficiently by improving waste-water treatment system in regions where this is applicable (e.g. Baltic Sea).

Options for reducing resilience to encourage restoration or transformation

Only 4% of the reported cases of hypoxia have shown improvement, principally due to the reduction of organic and nutrient loading, stratification strength, and freshwater runoff (Diaz and Rosenberg 2008).

Managers could take advantage of windows of opportunity provided by these variables. For example, in the Baltic Sea the stratification of the water column is determined by input of saltwater from the North Sea (Conley et al. 2009). A policy of nutrient load reduction would therefore be more effective in years when saltwater input is low and it is a particularly rainy year. While nutrient loading is largely periodic and rain dependent, stratification and freshwater runoff rely more on physical processes and climate variability. Finally, it has been suggested that an appropriate goal is the reduction of nutrient loads to the level of the mid-1900s (Diaz and Rosenberg 2008). 

Aquatic ecosystems, such as ponds, canals, ditches or tropical lakes can experience shifts between submerged and floating plant-dominated regimes when the concentration of nutrients (i.e. nitrogen and phosphorus) in the water column changes. The two possible regimes and their associated ecosystem services are:

Submerged plant-dominated regime

This regime is dominated by submerged plants such as Elodea nuttallii (better known as waterweed, a rapidly growing, long and stringy plant) that grow underwater in aquatic ecosystems such as ponds, canals, ditches or tropical lakes. They can be found in water bodies with a low water depth, clear water, low turbidity and weak water-level fluctuations. Typically, their roots are connected to the sediment pool. The regime of submerged dominated plants is also characterized by a low-nutrient concentration in the water column.

Floating plant-dominated regime

This regime is dominated by floating plants such as Eichhornia crassipes, Salvinia molesta, Lemna gibba or Pistia stratiotes found in aquatic ecosystems such as ponds, canals, ditches or tropical lakes. In many places these are invasive exotic species. These plants have their leaf surface exposed to the atmosphere and their roots are not connected to the sediment floor. Typically, a floating plant dominated regime is characterized by dark, anoxic and high nutrient conditions in the water column. 

Submerged to floating plant dominance

The main direct driver that leads to the shift from submerged to floating plant dominance is nutrient enrichment of the water body, typically associated with fertilizer and manure runoff from agricultural activities in the catchment. Strong water-level fluctuations associated with large rainfall events or water abstraction for irrigation and other uses can also enhance nutrient input from the shoreline.

This shift can also be caused by a change in water depth (associated with rainfall events and water abstraction) or turbidity (resulting from pollution, sediment runoff, and algal growth). These both directly affect the availability of light, which in turn affects photosynthetic activity and therefore the growth of submerged plants. The deeper the water body the darker it gets with increasing water depth. The more turbid the water body the less light available for photosynthetic activity in the deeper water layers. Floating plants are therefore in a better position to compete for in situ light than submerged plants. Hence, the deeper or more turbid the water body is, the more likely a shift to a floating plant dominated system. Moreover, the floating plant regime shift can be caused by the introduction of invasive species. Invasive floating plant species might grow very rapidly and aggressively outside their natural environment with the consequence that they might take over an entire water body by reducing the incidence of light, which in turn lead to a decrease of submerged plants. A famous invasive floating plant is Eichhornia crassipes which causes substantial problems in many parts of the world.

When nutrient concentrations in the water are realtively low, nutrients settle onto the sediment floor and are taken up by submerged plants. Submerged plants can grow at low nutrient concentrations in the sediment, and their growth removes nutrients from the system, keeping the nutrient concentration in the system low.

However, once the nutrient concentrations in the water exceed a certain threshold due to excessive nutrient input from eg fertilizer runoff, it becomes possible for floating plants to establish and grow in the water body. The roots of floating plants are not connected to the sediment pool, and are therefore dependent on the availability of nutrients in the water column in order to grow. 

The growth of floating plants in the water column decreases the amount of light that reaches the deeper water layers, and therefore reduces photosynthetic activity of submerged plants, eventually leading to their death. The loss of the submerged plants in turn destabilizes the nutrients trapped in the sediment floor, which become resuspended in the water column, further increasing the nutrient concentration in the water column and enhancing the growth of the floating plants. Once the system has shifted to a floating plant regime, the dark and anoxic conditions under floating plants leave little opportunity for submerged plant life, and it may be very difficult to restore the system to a submerged plant regime. 

Floating plant dominated systems decrease fisheries and plant life due to the dark and anoxic conditions under the leaf surface. They can also have a negative impact on navigation in lakes and canals, water purification and recreation opportunities. Moreover, floating plants can clog water supply pipes for households, ditches or even a river mouth. They can also complicate fishing. In addition, there is a greater risk of pathogens that can negatively affect human health in floating plant dominated systems. 

The main management option to enhance the resilience of the submerged plant regime is to control the nutrient levels in the system, and prevent the establishment of significant numbers of floating plants through a mechanical or chemical removal.

A drastic harvest of floating plants in a shallow water body that has some submerged plants and a not too high nutrient level can shift to a floating plant-dominated regime back to submerged plant-dominated regime (see also the Dutch Ditches Case Study). The amount of harvest needed for a shift is predicted to increase with the water nutrient level. Harvesting the floating plants allows light to penetrate to the deeper water layers and enables submerged plants to re-establish. Once re-established, the submerged plants can help reduce nutrient levels by absorbing nutrients for their growth and trapping suspended nutrients in the sediment.

This regime shift entails extensive change in biodiversity and soil structure in the Arctic tundra due to increased climate warming. To date shrub invasion has been the main indicator of a potential regime shift.

Arctic tundra regime

This regime is characterized by low atmospheric temperatures that enable the formation of a layer of permanently frozen subsoil (permafrost) consisting mostly of gravel and finer material. The ecosystem has a low rate of primary production due to impeded mineral nutrient cycling. The short growing season in summer influences the vegetation and its root systems by limiting vertical expansion. Only plant species (lichens, liverworts, mosses) that are adapted to sweeping winds and soil disturbance can survive in these conditions. The atmospheric cold and existing permafrost restrains the plant growth ensuring that their roots are shallow and surface parts are low growing due to the frozen ground underneath the permafrost. Snow does not accumulate keeping the soil temperature low thus not allowing for permafrost to thaw and release carbon into atmosphere keeping the atmosphere cold.

Boreal forest regime

This regime is characterized by flora that mostly consists of cold-tolerant evergreen conifers, such as the evergreen spruce (Picea), fir (Abies), and pine (Pinus), and the deciduous larch or tamarack (Larix). Before these species are established in a region, there is an increased canopy of early-successional species such as aspen and paper birch (Frelich et al. 1995). Dwarf shrubs (e.g., Vaccinium uliginosum, Ledum decumbens, Betula nana and Cassiope tetragona) and low shrubs (e.g., Betula glandulosa, Salix glauca) act as pioneers that colonize the tundra before trees are able to establish (CAVM Team 2003). The shift to boreal forest can occur over a long time period varying from several decades to centuries depending on the migration time lag for each species and the occurrence of favorable conditions for seedling establishment.

The main external driver of this regime shift is CO₂ emissions that increase climate warming (IPCC 2007), leading to changes in the composition and abundance of arctic vegetation, animals and soil structure (Olofsson et al. 2009). Anthropogenic activities that elevate atmospheric greenhouse gas concentrations are generally considered to be the primary cause for this driver. Carbon release from anthropogenic sources is projected to continue and increase during the coming decades (IPCC 2007). This global driver is well established and could be looked as irreversible in the scale of next hundred years.

The tundra regime is maintained by the low atmospheric temperatures that ensure low soil temperature and the existence of permafrost. This determines the lack of microbial activity and scarce nutrient availability therefore ensuring species composition that are only able to exist in such circumstances.

A shift from the tundra regime to boreal forest is initiated by the atmospheric warming that is caused by continuing increase of CO₂ emissions. The atmospheric temperature increases the period of time when microbial activity and thus nutrient availability is sufficient enough for shrubs and boreal forest pioneer species (birch etc.) to withstand the winter conditions.

As a result the increasing number of shrubs ensure that snow drifting creates deep drifts that surround and extend downwind from shrub patches. These drifts increase the insulation of the soil below. This insulation elevates the soil temperature thus further allowing microbial activity to continue during the frigid arctic winter. The elevated soil temperatures under the snow lead to permafrost degradation that release carbon trapped in tundra soils, thereby further contributing to climate warming.

Shift from Tundra to Boreal Forest

Tundra provides biodiversity for different animal and plant species that characterize tundra ecosystem. The shift from tundra to boreal forest is projected to occur over large geographic areas throughout the tundra zone, with substantial impacts on the landscape and biodiversity (Bonan 1992). Changes in vegetation are also likely to affect composition of foraging mammals and birds (Hinzman et al. 2005). In addition to modifying wildlife habitat, increased woody shrub abundance will make traversing tundra more difficult for caribou, subsistence hunters and communities that rely on caribou for food, as well as hikers. Herbivores such as reindeers and microtine rodents can also be influenced by the shift towards woody vegetation as they prefer lichens, dwarf shrubs graminoids and deciduous shrubs over tall woody shrubs (Sturm et al. 2005). Increased expansion of woody vegetation may therefore result in tundra species extinctions or radical decrease in numbers. The loss of the climate regulating ecosystem service is a major concern as permafrost thawing is reducing the carbon sink service.

For local populations the tundra provides traditional recreation opportunities and contains their knowledge and educational as well as spiritual and religious values, which would be lost with the regime shift.

Shift from Boreal Forest to Tundra 

The change in biodiversity could provide new ecosystem services such as timber production and different wild animals and plant foods. This would benefit loggers as they would increase their income from tree cutting and local communities from goods that come from forest.

In order to prevent or reverse this regime shift it is essential that the input of carbon to the atmosphere is reduced in order to prevent further climate warming. The suggestion of a planetary boundary of 350 ppm for CO₂ levels is a first step in this direction (Rockstrom et al. 2009). Increased understanding of the influence of climate change on the complexity of arctic systems is essential for informing the adaptation of social, economic, and cultural systems to the changes taking place in the Arctic (Hinzman et al. 2005). Technology transfer could be a good initiative from developed countries as they can provide more advanced technological solutions to developing countries to help accelerate the learning curve on GHG emissions. A global response particularly from developed nations that are using the majority of the world's resources on a per capita basis should be in place to deal with such complex systems. Nevertheless, discussions about desirable management strategies and outcomes will be essential as there will be stakeholders who stand to lose from this regime shift (e.g. caribou herders) while others may see the change as desirable and beneficial (e.g. timber production, game hunting).

Forest regime
Forests are ecosystems typically dominated by trees, perennial plants taller than 5 meters. Tropical forest includes moist and dry forests (MA, 2005). A mature tropical forest contain at least four layers: emergent layer up to 45 - 80 meters tall, the canopy among 35 - 45 meters tall, the understory layer and the floor layer. Such structure gives a variety of habitats that host roughly half of the known plants and animals biodiversity (MA, 2005).


Savanna regime
Savannas, on the other hand, are drylands dominated by a mixture of grasslands and shrublands. The canopy in savannas never closes, and the floor layer is dominated by grasses, especially C4 species. Savannas, dry forest and shrublands conform 40% of the world’s land area and host up to 42% of human population (Reynolds et al. 2007, Falkenmark and Rockström 2008). About 25% of drylands, including savannas, are covered by croplands and sustain 50% of world’s livestock (MA, 2005).

Shift from forests to savannas

The most widely recognized driver is deforestation and consequently fragmentation of forest landscape, which reduces rainfall and increases surface temperature (Da Silva et al. 2008, Nobre et al. 2009). Reduction of forest cover accelerates albedo effect, loss of evapotranspiration and roughness length (Sternberg 2001), activates fire feedback, changes ocean circulation and warms up sea surface in the Amazon case, and ultimately changes the spatial vegetation structure.

Deforestation and forest degradation is driven by a complex, case specific interaction of social and economic drivers. The most important reported drivers include agriculture expansion, infrastructure development, the logging industry and fast population growth (Geist and Lambin 2002, MA, 2005). For example, in the Amazon, illegal logging is a critical threat that besides its damage to the forest, bring with them secondary effects like expansion of hunting areas, slash-and-burn farms, mining, the establishment of new road networks and therefore more logging facilities. The MA (2005) reports that 70 countries have problems with illegal logging leading to national income losses of $5 billions and total economic losses of about $10 billion. By 2001, Laurance and Williamson (2001) reported that 80% of brazilian logging activity were illegal; however, government counterintuitively sponsored colonization through cattle ranching projects.

Climate change and global warming are expected to enhance the regime shift; and the loss of forest areas are expected to exacerbate climate change (Laurance and Williamson 2001, Bonan 2008). While Laurance and Williamson (2001) report that forests like Amazon apparently change from carbon sinks to carbon sources during ENSO events; Nobre et al. (2009) suggest that deforestation of Amazon may actually increase ENSO variability; and Bonan (2008) confirms that deforestation would enhance global warming by decrease of evaporative cooling and release of carbon dioxide.

Shift from forest to savanna

The expansion of agriculture, increase in population as well as deforestation reduces rainfall and increases surface air temperature. This has a direct affect on plants, reducing evapotranspiration and photosynthesis and decreasing the supply of water vapour. Cattle pastures and regrowth forest areas become increasingly prone to frequent fires, which plays a fundamental role in the shift from forest to savanna since it is a feedback that actually maintains savanna state.

When tropical forests are replaced by less vegetated cover like savannas, or ultimately by sand in deserts, net radiation at the top of the atmosphere decreases inducing subsidence that inhibits precipitation (Oyama and Nobre 2004). While in the tropics land clearing affects the water balance and as consequence warms up the climate, in boreal forest such clearing affects mainly albedo and as results cools down climate (Foley et al. 2005). The albedo feedback is strengthened by changes in land cover, typically induced by deforestation for logging or agriculture activities. Warmer temperature and drier atmospheres, such as in savanna regime, result in an increase in lifting condensation level feedback that reduces the opportunity of cloud formation and therefore the likelihood of rainfalls (Pinto et al. 2009). Plants that do not have enough water responds by reducing transpiration and photosynthesis, interrupting the supply of water vapor that contribute to the recycled component of precipitation (Oyama and Nobre 2004, Saleska et al. 2007). Less evapotranspiration blocks the inland propagation of cold fronts responsible for precipitation, increasing the dry season length (Oyama and Nobre 2003, Pinto et al. 2009).

Savanna vegetation is better adapted to dry environments. Grasses usually have C4 photosynthesis type, a chemical pathway that reduces water consumption and helps to cope with nitrogen or CO2 limitations. Evapotranspiration depends on soil moisture and biomass. Thus, for instance, droughts frequency or grazing reduce biomass, weakening in turn the feedback effect (Dekker et al. 2007, Dekker et al. 2010). In addition, the spatial distribution of rainfall is affected by both the land-cover type and topography. Cattle pastures and regrowth forest areas become increasingly prone to frequent fires. In such zones fire can be produced after few days of dry conditions. On the regional scale, fire smoke may reduce rainfall by trapping moisture and inhibiting raindrops formation (Laurance and Williamson 2001). Fire plays a fundamental role in the shift from forest to savanna since it is a feedback that actually maintain savanna state (Laurance and Williamson 2001, Hutyra et al. 2005)

Forest provides a wide range of ecosystem services. Besides being hot spots of biodiversity, forest provides soil and water protection, it prevents soil erosion, floods and landslides. For example, soil erosion may be 10-20 times higher on areas cleared of forest (MA, 2005). Depending on soil conditions, at the local scale forest can also regulate the hydrological cycles by increasing precipitation and decreasing evaporation. They regulate below grown runoff and smoothing seasonal extreme events: heavy rainfalls or dry spells. Due to its regulating function in the water cycle, forested watersheds provides water supply to one third of the world’s largest cities (MA, 2005). 

Forest sustain about 200 million people belonging to indigenous groups, who depend on forest not only as source of resources (food, fiber, fuel) but also their culture and religious traditions (MA, 2005). Forest also maintain the agroforestry industry which, including temperate forests, produces 3.3 billion cubic meters of wood (MA, 2005). 

Managerial options for the forest to savanna regime shifts requires targeting the main drivers: deforestation and landscape fragmentation. Controlling illegal logging and implementing sustainable logging plans are part of the strategy. Sustainable logging needs to take into consideration reducing fragmentation and allowing deforested patches to regrow. Likewise, the expansion of agricultural frontier and grazing areas needs to be controlled and when unavoidable, it needs to be planned in order to prevent fragmentation. The fire frequency feedback accelerates the shift from forest to savanna regime. Laurance and Williamson (2001) suggest fundamental changes in prevailing land-use practices and development policies to avoid wildfires. Such changes include the management of logging and grazing areas in order to reduce fragmentation and therefore it would reduce the fire risk. Hence, fire and fragmentation management need to be coupled strategies.

Kelp forests are marine coastal ecosystems dominated by macroalgae typically found in temperate areas. This group of species forms submarine forest with three or four layers, which provides different habitats to several species. Kelp ecosystems are important to maintaining important industries such as, lobster and rockfish fisheries, tourism based on fishing, recreational diving and kayaking, as well food and pharmaceutical products derived from kelp (e.g. tooth paste).

Three different self-reinforcing regimes can be identified:

Kelp forests are highly productive ecosystems dominated by canopy-formed algae in cold-water rocky marine coastlines. Among the biota associated with kelp forest are marine mammals, fishes, crabs, sea urchins, mollusks, other algae and biota that live on the kelp themselves (Steneck et al. 2002). At least 4 trophic levels are found in kelp forests. Apex predators such as sea otters, cod, pollock, hake, and haddock are common. These predators regulate populations of species at lower trophic levels. In particular the regulation of sea urchins populations, who are an important consumer of kelp, is important to maintain kelp forests.

Urchin barrens are an alternative regime in which there is no kelp forest and substantial populations of urchins on the sea bed. This regime has low or no populations of apex predators and large populations of herbivores that keep macroalgae at very low population levels, preventing the regrowth of the kelp forest, an important habitat for apex predators (Steneck et al. 2004). Ecosystem services related to apex predator fisheries and recreational activities are substantially reduced. However populations of bottom dwelling species can increase. In some places urchins have themselves become a valuable fishery. While in others, populations of other harvested species such as lobster and crab have increased.

Turf-forming algae regime is similar to urchin barrens in that there is no kelp forest, however rather than dominance by urchins the seabed is dominated by turf forming algae. This regime also lacks apex predators (Gorman et al. 2009). Ecosystem services related to fisheries and recreational activities are substantially reduced. However, populations of invertebrate species such as lobster and crab can be higher. This regime shift is associated with a change in habitat in shallow marine coastal ecosystems. 

This regime shift is driven by two key direct drivers: reducing populations functional groups (Steneck et al. 2002; Steneck et al. 2004) and input of nutrients (Gorman and Connell 2009). Reductions in populations of key functional groups is due to the hunting of sea otter and the fishing of large apex predator fish species. Overfishing usually diminish the controlling function of top predators, and in severe cases, the trophic levels may be simplified. Nutrient input is often due to the output of wastewater from urban settlement and agriculture in nearby coastal areas. However it can also be due to naturally occurring offshore upwelling events. Nutrients favors the growth of turf algae over kelp. Strong rain events and floods can produce shock events for kelp ecosystems by providing a pulse input of nutrients and turbidity, which can stimulate the growth of turf algae.

Other drivers can also contribute to this regime shift. Pollution discharges and sedimentation may play a synergetic role as stressors. In Tasmania for example, global warming has favored the reproduction of urchins which acting in synergy with lobster fishing has reduced kelp resilience (Ling et al. 2009). El Niño events and global warming can generate water stratification. As a consequence, nitrogen concentration declines and kelp forest growth becomes limited by the availability of nitrogen (Steneck et al. 2002).

Shifts from kelp canopies to other regimes like urchin barrens and turfed landscapes have occurred in many regions worldwide (Steneck et al. 2002; Gorman and Connell 2009; Ling, et al. 2009). This regime shift is associated with a change in the biophysical structure in shallow marine coastal ecosystems. Kelp forest dynamics often include cycles of disturbance-recovery. However, increasing human-dominated coastlines modify environmental conditions that facilitate other regimes like turfs or urchin barrens, which in turn inhibits the reestablishment of kelp forests.

Overfishing and the input of nutrients are the main drivers of the regime shift. At least 4 trophic levels are found in kelp forests, and apex predators such as sea otters, cod, pollock, hake, and haddock are common. Overfishing usually diminish the controlling function of top predators, and in severe cases, the trophic levels may be simplified. In addition, urchins are prevented from grazing in the kelp forest through the effect of kelp foliage sweeping over the rocks due to its flexibility and the force of waves (Steneck et al. 2004). 

Overharvesting of sea otter, cod and haddock can release their prey populations in particular sea urchins from their predatory controls (Steneck et al. 2002; Steneck, et al. 2004). Large populations of urchins form grazing fronts that graze on kelp forest to its elimination (Lauzon-Guay et al. 2009). These populations of urchins can survive in adverse conditions by feeding on turf-forming algae. In some cases the removal of apex predators allows populations of other species to reach sufficient population sizes to exert some control on urchins, for example lobsters and crabs in the North Atlantic (Steneck et al. 2002). Therefore, in some cases the formation of urchin barrens open the opportunity for new fishing industry to establish, like urchin, lobster and crabs. In addition, declines in predatory fish have created a market for urchins, especially in Japan, establishing a new human induced control on the herbivore. In some cases, harvesting of urchins for this market has led to the re-establishment of kelp forest, despite urchin fishery being prohibited (Steneck et al. 2002; Scheffer 2009).

On the other hand, the turf-forming algae regime is maintained by its ability to persist under conditions of elevated nutrients, frequently attributed to coastal urban settlements, inhibiting the recruitment of kelp species (Gorman and Connell 2009). The loss of kelp dominated areas undermine kelp's ability to reestablish in disturbed areas (Gorman and Connell 2009). While deforested areas surrounded by kelp patches are more likely to return to the kelp regime, isolated kelp disturbed patches are more likely to stick in the turfed regime.

Shift from kelps forest to urchin barrens or turfs

The main ecosystem impact of the loss of kelp forests is the loss of habitat complexity. Kelp is a three-dimensional structure that offers shelter and food for many species; urchin barrens and turfs do not have such characteristics. This loss is associated with the reduction of the food web complexity and loss of functional groups (Steneck et al. 2004), with varying effects on fisheries. Some valuable fish species may diminish since kelp forests provide nursery areas. Invertebrate species such as lobster and crab can increases in population (Steneck et al. 2002). Along with hosting high marine biodiversity, kelp forests provide ecosystem services related to recreation for divers. In addition, kelp supports a multi-million dollar industry of canopy-cropping for alginates (Steneck et al. 2002). This product is commercially important in pharmaceutical and chemical industry. These services can be reduced or lost by this regime shift.

The ecosystem service impacts of algae turfs are likely to be similar to those related to coastal eutrophication. Such effects include abundance of rich-nutrient environment species as shellfish, bad odors and the associated consequences for recreational and aesthetic values.

Shift from urchin barrens or turfs back to kelp forest.

The recovery of kelp forest increases habitat complexity and is expected to increase biodiversity. On the other hand, the benefits gained by urchin and lobster fisheries may be diminished, since the recovery of functional groups and increasing biodiversity will control their populations. However, other species of commercial fisheries and tourism may recover, although it is not always the case.

Options for enhancing resilience

Restoring biodiversity in functional groups in kelp forests is particularly important to maintain apex predators (Estes et al. 2011). A high profile version of this strategy has been implemented to recover sea otter populations on the west coast of North America, by protecting sea otter populations (Steneck et al. 2002). Reducing nutrient runoff from agricultural areas is essential for enhancing resilience, as well as improving water treatment and managing runoff in urban areas (Gorman et al. 2009). The removal of sea urchins along with implementing measures to maintain the abundance of predators like cod, sea otters and sheep-head through fishery controls may provide stability to kelp forest ecosystems (Steneck et al. 2002; Steneck et al. 2004).

Options for reducing resilience to encourage restoration or transformation

Transplantation of kelp-forming species has been proposed as a restoration strategy in highly fragmented seascapes. This strategy is likely to increase the supply of kelp propagules to colonize new areas and maintain free substratum for canopy recruitment (Gorman and Connell 2009). However this strategy has not been tested.

Bivalve mollusks play an important role in aquatic ecosystems by filtering and sequestering nutrients. When bivalve abundance changes, it can have substantial ecosystem impacts, creating two different, self-reinforcing regimes.

Bivalve mollusk reef regime

When aquatic ecosystems have high bivalve mollusk density, they form reefs that include many bivalves and are shallow, nearing the water's surface. The filtering activity of the bivalves results in clear water with high levels of dissolved oxygen. Plankton populations are limited in these conditions. The bivalves themselves operate as reinforcing feedbacks in that their abundance ensures sufficient filtration to maintain this clear water regime. The clear water in turn reinforces bivalve abundance by maintaining hydrodynamics that are conducive to bivalve health (Scheffer 2009). Large, shallow bivalve mollusk reefs encourage biodiversity by providing habitat and filtering nutrients.

Isolated, low density bivalve mollusk regime

When aquatic ecosystems have low bivalve mollusk density, the reefs are small and deep. Water is turbid, with low levels of dissolved oxygen. Plankton and filamentous algae flourish under these conditions. Poor reef size and water filtration limit biodiversity. The low bivalve abundance state is reinforced as bivalve health and fecundity is weakened by turbid water conditions. The turbid water conditions are in turn reinforced by low bivalve abundance(Weijerman et al. 2005, Powell et al. 2008).

The regime of low bivalve abundance is expected to reduce fish populations, water cleansing and the ability of the ecosystem to maintain biodiversity. Such effects would have an impact on fisheries, human health, and recreation. 

Mechanized anthropogenic over-harvesting of bivalve mollusks is the main driver of the shift from high to low abundance (Thrush and Dayton 2002).  High inputs of nutrients from agricultural or urban sources can act as a slow driver that weakens bivalve health by creating plankton blooms that the bivalves are unable to filter.  Plankton blooms can further weaken the health of bivalves by producing organic matter, whose decay, reduces oxygen availability.  A disturbance that can trigger a regime shift is bivalve disease (Powell et al. 2008).  When populations are weakened by eutrophication and anoxia, they become physiologically susceptible to diseases that can rapidly devastate remaining populations (Leniham 1999). 

Bivalves are a class of mollusks containing over 30,000 species which include scallops, clams, oysters, and mussels. Bivalve mollusks, like corals, are often referred to as "ecosystem engineers" because they engineer the physical and hydrodynamic structures of the ecosystems they occupy in a way that produces conditions that support their own population growth. Bivalves live on the sediment surface, where they form reefs that provide increased surface area and habitat for a variety of species. They filter nutrients from the pelagic zone and transfer it to the benthic zone, maintaining the system in a clear water state. If bivalves are not able to maintain clear water, due to declines in bivalve populations or increases in low water quality, the ecosystem can shift to a turbid water state. The main drivers of this shift is direct anthropogenic over-harvesting of the bivalves, followed by a gradual increase of nutrients discharge from agricultural activities and coastal urban settlements.  Once mollusk abundance is reduced, plankton and filamentous algae populations can substantially increase. As result, oxygen and light available in the water column is reduced, while nutrients levels accumulate due to the lack of filtration.  These processes produce conditions which favour algae over bivalves.  If nutrient inputs are substantial, eutrophication and areas of hypoxia (low oxygen) are likely to appear in areas dominated by plankton instead of mollusks. 

Shift from high to low bivalve mollusk abundance regime

The loss of abundant bivalve mollusk reefs threatens diverse ecosystem services. The most direct impact is the loss of valuable shellfisheries. A secondary, but potentially substantial impact is the loss of the filtering service provided by bivalves.  In urbanized estuaries, it can necessitate the installation of costly synthetic water filtration technology (Gren et al. 2009).  In other areas it can lead to declines in water quality that shift populations and reduce recreation opportunities, and potentially the value of waterfront property.

The loss of bivalve habitat structure leads to lower species abundances, and to declines in species richness (Airoldi et al. 2008). Both structural and functional biodiversity is threatened by loss of bivalve abundance, with far reaching effects on marine ecosystems and human ability to exploit such systems (Worm et al. 2006).  The regime of low bivalve abundance is expected to reduce fisheries, water cleansing and the ability of the ecosystem to maintain biodiversity. Such effects would have an impact on fishing productivity and human health.

The loss of bivalve nutrient filtration can lead to other regime shifts, such as eutrophication and hypoxia, which can produce unproductive and undesirable ecosystems (see Hypoxia and Eutrophication regime shifts). 

Options for enhancing resilience

Estuaries are the world's most degraded marine ecosystems, receiving land-based pollution from crop cultivation and urbanization, and suffering from anthropogenic over-harvesting (Lotze et al. 2006).  The resulting problems are complex, impacting bivalve health and fecundity as well as many other species, with far reaching implications in both biological and social domains.

Experience has shown that management focused on one species or problem tends to be ineffective.  Newer research stresses the need to address natural resources as part of complex social-ecological systems, often with long histories of human exploitation (Jackson et al. 2001).   Rather than addressing problems on a species-by-species basis, multi -species management has shown to have synergistic effects.  For example, in Chesapeake bay, fisheries management looks at oyster, blue crab, striped bass and shad together (Boesch 2004).

Taking this approach further, Ecosystem-based fishery management efforts focus on recognizing interactions between multiple species and environmental stressors, such as low dissolved oxygen levels. Success is measured by the degree to which management efforts include ecosystem-based approaches, rather than by an assessment of fishing stocks (Lotze et al. 2006). 

Options for reducing resilience to encourage restoration or transformation

The main option exercised for preventing or reversing a regime shift regarding bivalve abundance is to import bivalve mollusks from another region for aquaculture (Van de Koppel 2008) .  Another practice commonly used is to hang bivalves from the surface or build artificial reefs to elevate bivalves in order to avoid the hypoxic conditions in deeper water (Carlsson et al. 2009). 

Savannas are systems that consist of a mixture of woody vegetation (trees or shrubs) and grasses. Savannas, dry forest and shrublands cover 40% of the world's land area, host up to 42% of the world's human population (Reynolds et al. 2007, Falkenmark and Rockström 2008), and together with drylands sustain 50% of the world's livestock (Millennium Ecosystem Assessment 2005).

At small scales (up to about 10 km²) savanna systems, especially those used for extensive cattle ranching, may stabilize in two different self-reinforcing regimes (Walker 1993, Scheffer et al. 2001, Scholes 2003):  

Open, grassy savanna regime

In this regime the landscape has a productive grass layer with few mature trees. The canopy in savannas never closes, and the floor layer is dominated by grass, especially C4 species. Most young trees are unable to establish because, while often numerous, seedlings are constantly knocked back to ground level by herbivory and fire. There is enough grass after grazing to support a fire with flame-length taller than the young saplings sufficiently often to keep them in a 'fire trap' (Dublin et al. 1990, Roques et al. 2001b, a). Open savanna systems are suitable for ranching, and are maintained by fire dynamics and grazing.

Closed, woody savanna regime

In this regime the landscape is dominated by woody shrubs or trees. Once established, woody vegetation is stable because adult trees are seldom killed by herbivory or fire. These alternate regimes can occur at a range of spatial scales. Sometimes larger areas (e.g. an entire cattle ranch) may shift from a grass-dominated to a persistent woody-dominated state (Dublin et al. 1990, Walker 1993). In other cases, the alternate regimes are expressed as a mosaic of small patches of trees or bush interspersed with patches of grass, where the respective patches are highly persistent over time (Rietkerk et al. 2004).   

Bush encroachment refers to a shift from a grassy system to a persistently woody system. It typically occurs in areas used for free-range cattle ranching, and is usually caused by a combination of grazing and fire management practices. Bush encroachment involves a change in the outcome of the competitive interaction between woody vegetation (shrubs and trees) and herbaceous vegetation (grasses and herbs), mediated by nutrients, grazing, fire, rainfall variability and use of either the woody or grassy components by humans (Anderies et al. 2002, Janssen et al. 2004, Wiegand et al. 2006). The encroachment typically occurs in episodes rather than continuously, and involves a particular set of encroaching species rather than the entire woody community.

Bush encroachment typically occurs in areas used for commercial cattle ranching (as opposed to subsistence, communal, or nomadic) and may follow episodes of sustained severe overgrazing, though not necessarily so. It may also occur under other land uses (Wiegand et al. 2006). It tends to be an episodic phenomenon, where the tree cohorts can often be linked to issues in the ranching enterprise – such as drought-induced debt or downturns in the cattle price cycle (Scholes 2003, Wiegand et al. 2006). 

There are several different hypotheses regarding the mechanism by which bush encroachment occurs. Different mechanisms (or combinations of mechanisms) may be important in different places. One proposed mechanism is based on changes in fire regime: in the sustained presence of high numbers of grazers (typically cattle) accumulation of grass fuel is reduced, leading to period without intense fire long enough for woody plants to grow beyond the fire-susceptible stage, which in turn suppresses grass production and fires, further enhancing the establishment of woody vegetation (Higgins et al. 2000, Staver et al. 2009, Staver et al. 2011). A related hypothesis notes the elimination of browsers (especially very large browsers such as elephant and giraffe, but also the more-numerous small browsers) from the system when cattle are introduced (Dublin et al. 1990). Similarly, alien species, such as Prosopis in South Africa or Acacia nilotica in Australia, both deliberately introduced, can play an important role in bush encroachment by affecting fire regimes (Poynton 1990). Another hypothesis focuses on changes in water availability based on the rooting depths of plants: grasses are thought to be more shallowly-rooted than trees, so if grass cover is reduced by overgrazing, this is more water available for trees, which promotes their growth and establishment, further suppressing grass growth (Noy-Meir 1982). Refinements of these hypotheses emphasize combinations of events, such as a multiyear drought or fireless period providing a "window" for the establishment of trees (Wiegand et al. 2006).

Yet other hypotheses focus on the role that increases in global CO2 levels may play in the observed proliferation of woody plants in many, widely-separated areas of the world during the 20th century. The underlying mechanism is still debated, but several possibilities have been proposed: i) that rising CO2 levels favour C3 (woody plant) photosynthesis relative to C4 (tropical grass) photosynthesis  ii) elevated CO2 may reduce transpiration of grasses, leading to greater water percolation and therefore favoring deeper rooted woody species, iii) faster growth of woody plants due to CO2 enrichment, and therefore faster escape of seedlings from susceptibility to fire, iv) investments in carbon-based defense compounds such as tannins, which are the main defense compounds in many encroaching trees but not in grasses (Midgley and Bond 2001, Wiegand et al. 2006). It is striking that encroachment is almost unheard of on communal land, and is not universal on commercial farms. A suggested explanation for this is that communal lands use the trees for firewood and run goats along with cattle, inhibiting the establishment of trees (Scholes 2003). Bush encroachment has been documented in East and Southern Africa (but not West Africa), South America (Uruguay/Argentina and Chile), North America (Texas, New Mexico) and Australia, but not in India, also a savanna environment. Furthermore, it did not happen simultaneously in those places, but 30-50 years after the widespread establishment of sedentary grazing management, what has been referred to as 'commercial' ranching above. This tends to disfavor the rising CO2 hypothesis, although rising CO2 may predispose the shift (Midgley and Bond 2001).  

Shift from grassy to woody savanna

Woody encroachment brings a relatively rapid change, over a decade or two, from a highly productive grass layer to a sparse and unproductive grass component. Since cattle are grass-eaters, this change substantially reduces cattle productivity (Anderies et al. 2002, Scholes 2003), with major impacts on cattle ranchers. On the other hand, encroachment increases the supply of tree-based ecosystem services, such as wood for fuel, charcoal-making and building material. This is somewhat dependent on the species involved. The increase in woody cover could potentially also have macro and micro-climatic effects through impacts on albedo and CO2 uptake, in addition to the decrease in methane emissions from cattle.

Difficulties in mustering the cattle in dense bush are a contributing factor. Therefore, wood encroachment leads to economic losses for cattle ranchers in what is frequently an economically marginal area for other agricultural uses such as croplands. 

Options for enhancing resilience

There is some agreement among researchers and extension workers that encroachment can be avoided by stocking lightly and burning frequently to prevent the establishment of trees and maintain grass crowns - the productive part of the grass that is less affected by fires (Roques et al. 2001b, Janssen et al. 2004). However, this is seldom reflected in management practice. 

Options for reducing resilience to encourage restoration or transformation

Bush encroachment is expensive to reverse, since rapid results rely on arboricides or repeated mechanical or manual clearing. A common method involves the manual removal of woody vegetation, with repeated follow-up control and the use of fire to enhance the establishment and competitive advantage of grasses (Scholes 1985, 2003). Attempts to reverse bush encroachment often have poor results, either due to the rapid resprouting of the trees or the conversion of the grass layer to less desirable species in the process. 

There are anecdotal reports of widespread mortality of near-dominant encroaching species after several decades, possibly related to disease, prolonged drought or simply old age, which provides windows for grass establishment and fuel load for intense fires.

The hypoxia regime shift involves a radical change in oxygen concentration in aquatic ecosystems such as rivers, lakes and marine ecosystems. The severity and persistence of hypoxic conditions varies. Episodic oxygen depletion represents 17% of known hypoxia cases, and occurs infrequently with several years sometimes elapsing between events. Episodic oxygen depletion is the first signal that a system has reached a critical point of eutrophication, which in combination with physical processes that stratify the water column, tips the system into hypoxic conditions (Diaz and Rosenberg 2008). Seasonal hypoxia tends to occur periodically during the summer, after algal spring blooms have sunk to the bottom and are being decomposed. It lasts from days to weeks, represents half of the known dead zones, and typically abates in the autumn (Diaz and Rosenberg 2008). Boom-and-bust cycles of animal populations are frequent. Persistent hypoxia occurs when hypoxia becomes persistent due to the build-up of organic matter in the sediments over time, particularly in systems prone to persistent stratification. Persistent hypoxia accounts for 8% of reported hypoxia cases. Anoxia occurs when DO levels fall below 0.2 ml per litre. The accumulation of organic matter is exacerbated, and poisonous hydrogen sulphide (H₂S) is released due to microbial metabolism.

Normoxia: This regime is characterized by normal levels of dissolved oxygen, typically 5 to 8 ml per liter. Most aquatic organisms are adapted to survive under these conditions. Particularly important are the benthic organisms. Benthos are a community of organisms which live in sedimentary environments at the bottom of water bodies. They constitute an important key functional group (scavengers and detritivores) in the food web responsible for the decay of dead matter. Macrobenthos facilitate bioturbation processes, in other words, they displace and mix particles facilitating chemical exchange between sediments and the water column. This allows the sediments to "capture" phosphorous and nitrogen from the water column. By removing these nutrients from the water column, bioturbation therefore reduces algal growth and helps ensure that the water remains oxygenated and suitable for the survival of benthos - creating a reinforcing feedback that maintains normoxia conditions.

Hypoxia: The hypoxic regime is reached when dissolved oxygen level falls below 2 ml per liter (Diaz and Rosenberg 2008). Hypoxia is associated with so-called "dead zones". These are areas where dissolved oxygen levels are so low that most life is not able to persist and only very few specialized microorganisms survive. In the hypoxic state, accumulation of organic matter produced by eutrophic processes favors the growth of microbes who, by decomposing the organic matter, consume the oxygen in the water column.

Anoxia: Anoxia occurs when hypoxia is exacerbated to DO levels under 0.2 ml per liter (Diaz and Rosenberg 2008). Under such conditions, benthic animals suffer from mass mortality and decay processes are carried out by microbial metabolism. As A result, hydrogen sulfide is released changing water acidity (pH) and then further favoring bacteria habitat. 

Hypoxia is driven by increasing nutrient input, both natural and anthropogenic. Natural sources come from the bottom of the ocean transported by upwellings, water currents that flow vertically driven by the interaction of winds and the temperature gradient between the ocean surface and bottom. Anthropogenic sources are related mainly with the use of fertilizers in agriculture. However, growing urban settlements in coastal areas also increase the water storm runoff and sewage.

The leakage of nutrients from agriculture is further exacerbated by rainfall variability and deforestation, since vegetation offers resistance to erosion and trap nutrients and moisture in the soil. Agriculture is driven in turn by increasing demand of food and fibers as well as population growth. Another important factor leading to hypoxic conditions is the stratification of the water column, which reduce the exchange of nutrients between the surface and the bottom. Water stratification is usually driven by anomalous increase in sea surface temperature. 

The fast variable in the hypoxia regime shift is DO, and can also include faunal composition. The slow variables are the accumulation of nutrients, mainly phosphorous and nitrogen in the water column. Large areas of the sea are oxygen minimum zones due to natural conditions. Hypoxic-prone areas generally have a semi-enclosed hydrogeomorphology and water-column stratification that restrict water exchange and reoxygenation of the deeper water layers (Diaz and Rosenberg 2008). Another natural condition that facilitates hypoxia and eutrophication is coastal upwelling, that lead to nutrient enrichment along continental margins.

The normoxia regime is usually  maintained by feedbacks that control the nutrients content in the water column and keep oxygen concentrations high. Zooplankton for example plays an important role controlling micro algae numbers, hence limiting their ability to use nutrients available and reduce oxygen through their metabolism. Macrophytes or macro algae also control micro algae by consuming the same resources. Under such conditions nutrients are kept on inorganic forms unavailable for micro algae to use.

Under the hypoxia regime the feedbacks above are weakened, allowing micro algae to escape predation and competition controls; and reducing the level of DO in water. As DO decreases, mass mortality of different organism increases the availability of organic matter in the water column, decreasing DO and promoting more micro algae growth. When DO reaches critical levels under 0.5mL per liter, hydrogen sulfide is released further decreasing pH in water and reducing biodiversity. Such extreme regime is called anoxia. 

Shift from normoxia to hypoxia and anoxia

Normoxia is associated with the provision of food for humans and wildife in aquatic systems as well as regulating services such as water purification and pest regulation. This regime shift into hypoxia, often characterised as a dead zone, has been reported in more than 400 systems affecting more than 245,000 square kilometers and including important fisheries such as the Baltic Sea, Kattegat, Black Sea, Gulf of Mexico, and East China Sea (Diaz and Rosenberg 2008). A major impact on ecosystems is a change in the flux of matter and energy through trophic levels. Consequently, fisheries and hence ecosystem services such as food production are affected. For example, Diaz & Rosenberg (2008) report that biomass in the Baltic Sea has been reduced by approximately 264,000 metric tons of carbon due to hypoxic conditions. Assuming that ~40% of benthic energy passes up the food chain, 106,000 metric tons of carbon of food energy for fisheries has been lost(Diaz and Rosenberg 2008). This implies a reduction in yields and consequent impact on employment in fisheries communities. Another example of such effects is the lobster fishery collapse in Norway (Diaz and Rosenberg 2008).

Besides its effect on fisheries and employment, hypoxia also affect the health and other cultural services of the people living near by dead zones. Decaying matter after mass mortality events create odors and risk of diseases. Recreation, aesthetic and touristic values are lost.

Shift from Hypoxia to Normoxia

When hypoxia is not severe, the system is likely to return to normoxia regime. It implies the recovery of fisheries when species hasn't been extinct locally. However, food webs not always fully recover, and usually top predators are lost. With the recovery of the system, fishing, recreational and touristic activities recover too, although not necessary to the same configuration than before the regime shift. 

Options for enhancing resilience

Since one of the main drivers of hypoxia is eutrophication, Diaz & Rosenberg (2008) recommend managing the input of fertilizers on agricultural land. They recognize the necessity of developing new methods to close the nutrient cycle on farms, in order to avoid the drainage of nutrients to water sources. Nutrient reductions can also be achieved relatively cost-efficiently by improving waste-water treatment system in regions where this is applicable (e.g. Baltic Sea).

Options for reducing resilience to encourage restoration or transformation

Only 4% of the reported cases of hypoxia have shown improvement, principally due to the reduction of organic and nutrient loading, stratification strength, and freshwater runoff (Diaz and Rosenberg 2008).

Managers could take advantage of windows of opportunity provided by these variables. For example, in the Baltic Sea the stratification of the water column is determined by input of saltwater from the North Sea (Conley et al. 2009). A policy of nutrient load reduction would therefore be more effective in years when saltwater input is low and it is a particularly rainy year. While nutrient loading is largely periodic and rain dependent, stratification and freshwater runoff rely more on physical processes and climate variability. Finally, it has been suggested that an appropriate goal is the reduction of nutrient loads to the level of the mid-1900s (Diaz and Rosenberg 2008). 

Thermohaline circulation is complex, but can be summarised as a global scale deep overturning of water masses, with sinking motions occurring in the northern Atlantic and around Antarctica and a transport of relatively warm surface waters towards these sinking regions (Steffen et al. 2004). The term thermohaline circulation thus refers to a particular driving mechanism; it is a physical, not an observational concept.

Strong Thermohaline circulation

THC regime can be characterized by the formation of deep water currents in the high latitudes of both hemispheres; the spreading of deep waters, as deep western boundary currents; upwelling (or upwards movement) of deep waters which is thought to take place mainly in the Antarctic Circumpolar Current region, possibly aided by the wind, and near-surface currents, required to close the flow (Rahmstorf et al. 2006).

Collapse of the Thermohaline circulation

This regime is characterized by weak formation of the dense and saline high latitude deep waters due to the increased influx of freshwater. The decreased salinity of the deep waters is weakening the density southwards driven water movement. The transport of less dense and saline waters from south towards north is decreased causing additional atmospheric temperature increase in tropical areas.

Increasing greenhouse gas concentration from anthropogenic sources is predicted to cause a rise in global mean temperatures (Cubasch et al. 2001). One of the most common anthropogenic greenhouse gases is carbon dioxide (CO2). It is considered to be the main driver for this regime shift and is typically operating in global scale due to the global input of CO2 in the atmosphere. Increasing CO2 levels in atmosphere increases the mean atmospheric temperature. This results in weakening ice-albedo mechanism for the initial regime as a result of increased atmospheric temperatures and changes in albedo.

The increasing freshwater influx from sea-ice loss is accelerated by freshwater-overturning and precipitation-river runoff. This leads to an in increased evaporation, river runoff, and changes to water salinity and density. The burning of fossile fuels such as coal and natural gas also drives this regime shift. This occurs in a regional scale but has global impact and is well established in the literature.

In the regime of strong THC, the most simple explanation of the main variable that maintains the regime would be the high-latitude cooling. In the deep ocean, the predominant driving force of movement of currents is differences in density, caused by salinity and temperature (the more saline the more denser and the colder the water, the more dense). The ocean's density distribution is itself affected by currents and mixing. Thermohaline and wind-driven currents therefore interact in non-linear ways and cannot be separated by oceanographic measurements. Changing the wind stress will alter the THC; altering thermohaline forcing will also change the wind driven currents.

The collapse of THC occurs due to increasing CO2 levels in atmosphere that slowly enhance the mean atmospheric temperature. As a result the high latitude cooling is weakened, increasing freshwater influx from the loss of ice, both from the Arctic, Greenland and possibly the north of Canada. This strengthens two main mechanisms for the new regime: freshwater-overturning mechanism and the precipitation-river runoff mechanism. Both increase the fresh water inflow into the ocean. Therefore new mechanism such as increased evaporation, river runoff, changes in water salinity and density are activated possibly reinforcing the new regime. Once the system has shifted, the warm conditions in Northern latitudes maintain a weak formation of the dense and saline high latitude deep waters due to the increased influx of freshwater.

Most ocean–atmosphere models predict a reduction of the THC for a warmer climate in the near future. Recent observational evidence provides some support to the model projections that the North Atlantic THC circulation could weaken or shut down later this century (Steffen et al. 2004). Lenton et al. (2008) report that the IPCC models predict the regime shift to happen after 2100, however, their models do not include freshwater runoff which is expected to accelerate the process. The models that project the collapse of THC under greater perturbations identify that the collapsed state can last for centuries and might be irreversible (Delworth et al. 2008). Nevertheless the latest IPCC report shows that it is very unlikely that the THC will undergo a large abrupt transition during the course of the 21st century (IPCC 2007). Some models show a recovery of the THC after the elimination of the freshwater perturbation whereas others do not. The mechanism for this different dynamical behavior of the THC is not completely understood (Stouffer et al. 2006).

The weakening of THC will impact global water regulation as water circulations in global scale are influenced by the changes in temperature, salinity and density. In response, climate regulation is altered and changes as the transport of heat is distributed in a different pattern. The water mass and heat circulation will no longer transport warm currents in a northly direction. This could increase the occurence of hurricanes, a southward shift of tropical rainfall belts with resulting agricultural impacts, as well as disruptions to marine ecosystems. Provisioning services such as fisheries, livestock and food crops are therefore potentially affected. Schmittner et al. (2007) show that changes in Atlantic circulation can have large effects on marine ecosystems and biogeochemical cycles, even in areas remote from the Atlantic, such as the Indian and North Pacific Oceans. Although loss of THC could affect Western Europe and result in colder climates. 

Food crisis and emigration are potential threats in the regions that are subjected to this regime shift (Barnett & Adger 2007). In addition, regional changes in sea level are predicted to rise of up to 80 cm in the North Atlantic (Knutti et al. 2002). This could affect the coastlines of the United States, Canada, and Europe causing coastal erosion and affect the security of housing and infrastructure (Delworth et al. 2008). The marine biodiversity of flora and fauna will also be affected. The change in salinity and temperature may facilitate the introduction and spread of invasive species that are able to thrive in the new conditions. This could affect the livelihoods and economic activity of local fishermen as they would have to adapt to new circumstances.  

This regime shift is mainly driven by the increase in greenhouse gases into the atmosphere at a global scale. Therefore a decrease in CO2 emissions and other greenhouse gases has to be achieved in order to prevent further climate warming leading to large influx of freshwater from the depletion of Arctic ice. In the best-case scenario, it may be possible to avert a regime change in a situation where the underlying driver(s) of change are amenable to fast manipulation through management, and where there are not substantial lags in ecosystem response. In the case of the THC neither of these factors hold: the underlying causes of global warming appear very difficult to address through rapid changes in management, and there are substantial inertias in ecosystem response to changes in the driving factors. Therefore an early warning indicators would be useful in providing advance warning of a substantial coming change in ecosystem conditions, but is unlikely to be useful in averting the regime change.

Aquatic ecosystems, such as ponds, canals, ditches or tropical lakes can experience shifts between submerged and floating plant-dominated regimes when the concentration of nutrients (i.e. nitrogen and phosphorus) in the water column changes. The two possible regimes and their associated ecosystem services are:

Submerged plant-dominated regime

This regime is dominated by submerged plants such as Elodea nuttallii (better known as waterweed, a rapidly growing, long and stringy plant) that grow underwater in aquatic ecosystems such as ponds, canals, ditches or tropical lakes. They can be found in water bodies with a low water depth, clear water, low turbidity and weak water-level fluctuations. Typically, their roots are connected to the sediment pool. The regime of submerged dominated plants is also characterized by a low-nutrient concentration in the water column.

Floating plant-dominated regime

This regime is dominated by floating plants such as Eichhornia crassipes, Salvinia molesta, Lemna gibba or Pistia stratiotes found in aquatic ecosystems such as ponds, canals, ditches or tropical lakes. In many places these are invasive exotic species. These plants have their leaf surface exposed to the atmosphere and their roots are not connected to the sediment floor. Typically, a floating plant dominated regime is characterized by dark, anoxic and high nutrient conditions in the water column. 

Submerged to floating plant dominance

The main direct driver that leads to the shift from submerged to floating plant dominance is nutrient enrichment of the water body, typically associated with fertilizer and manure runoff from agricultural activities in the catchment. Strong water-level fluctuations associated with large rainfall events or water abstraction for irrigation and other uses can also enhance nutrient input from the shoreline.

This shift can also be caused by a change in water depth (associated with rainfall events and water abstraction) or turbidity (resulting from pollution, sediment runoff, and algal growth). These both directly affect the availability of light, which in turn affects photosynthetic activity and therefore the growth of submerged plants. The deeper the water body the darker it gets with increasing water depth. The more turbid the water body the less light available for photosynthetic activity in the deeper water layers. Floating plants are therefore in a better position to compete for in situ light than submerged plants. Hence, the deeper or more turbid the water body is, the more likely a shift to a floating plant dominated system. Moreover, the floating plant regime shift can be caused by the introduction of invasive species. Invasive floating plant species might grow very rapidly and aggressively outside their natural environment with the consequence that they might take over an entire water body by reducing the incidence of light, which in turn lead to a decrease of submerged plants. A famous invasive floating plant is Eichhornia crassipes which causes substantial problems in many parts of the world.

When nutrient concentrations in the water are realtively low, nutrients settle onto the sediment floor and are taken up by submerged plants. Submerged plants can grow at low nutrient concentrations in the sediment, and their growth removes nutrients from the system, keeping the nutrient concentration in the system low.

However, once the nutrient concentrations in the water exceed a certain threshold due to excessive nutrient input from eg fertilizer runoff, it becomes possible for floating plants to establish and grow in the water body. The roots of floating plants are not connected to the sediment pool, and are therefore dependent on the availability of nutrients in the water column in order to grow. 

The growth of floating plants in the water column decreases the amount of light that reaches the deeper water layers, and therefore reduces photosynthetic activity of submerged plants, eventually leading to their death. The loss of the submerged plants in turn destabilizes the nutrients trapped in the sediment floor, which become resuspended in the water column, further increasing the nutrient concentration in the water column and enhancing the growth of the floating plants. Once the system has shifted to a floating plant regime, the dark and anoxic conditions under floating plants leave little opportunity for submerged plant life, and it may be very difficult to restore the system to a submerged plant regime. 

Floating plant dominated systems decrease fisheries and plant life due to the dark and anoxic conditions under the leaf surface. They can also have a negative impact on navigation in lakes and canals, water purification and recreation opportunities. Moreover, floating plants can clog water supply pipes for households, ditches or even a river mouth. They can also complicate fishing. In addition, there is a greater risk of pathogens that can negatively affect human health in floating plant dominated systems. 

The main management option to enhance the resilience of the submerged plant regime is to control the nutrient levels in the system, and prevent the establishment of significant numbers of floating plants through a mechanical or chemical removal.

A drastic harvest of floating plants in a shallow water body that has some submerged plants and a not too high nutrient level can shift to a floating plant-dominated regime back to submerged plant-dominated regime (see also the Dutch Ditches Case Study). The amount of harvest needed for a shift is predicted to increase with the water nutrient level. Harvesting the floating plants allows light to penetrate to the deeper water layers and enables submerged plants to re-establish. Once re-established, the submerged plants can help reduce nutrient levels by absorbing nutrients for their growth and trapping suspended nutrients in the sediment.

This regime shift entails extensive change in biodiversity and soil structure in the Arctic tundra due to increased climate warming. To date shrub invasion has been the main indicator of a potential regime shift.

Arctic tundra regime

This regime is characterized by low atmospheric temperatures that enable the formation of a layer of permanently frozen subsoil (permafrost) consisting mostly of gravel and finer material. The ecosystem has a low rate of primary production due to impeded mineral nutrient cycling. The short growing season in summer influences the vegetation and its root systems by limiting vertical expansion. Only plant species (lichens, liverworts, mosses) that are adapted to sweeping winds and soil disturbance can survive in these conditions. The atmospheric cold and existing permafrost restrains the plant growth ensuring that their roots are shallow and surface parts are low growing due to the frozen ground underneath the permafrost. Snow does not accumulate keeping the soil temperature low thus not allowing for permafrost to thaw and release carbon into atmosphere keeping the atmosphere cold.

Boreal forest regime

This regime is characterized by flora that mostly consists of cold-tolerant evergreen conifers, such as the evergreen spruce (Picea), fir (Abies), and pine (Pinus), and the deciduous larch or tamarack (Larix). Before these species are established in a region, there is an increased canopy of early-successional species such as aspen and paper birch (Frelich et al. 1995). Dwarf shrubs (e.g., Vaccinium uliginosum, Ledum decumbens, Betula nana and Cassiope tetragona) and low shrubs (e.g., Betula glandulosa, Salix glauca) act as pioneers that colonize the tundra before trees are able to establish (CAVM Team 2003). The shift to boreal forest can occur over a long time period varying from several decades to centuries depending on the migration time lag for each species and the occurrence of favorable conditions for seedling establishment.

The main external driver of this regime shift is CO₂ emissions that increase climate warming (IPCC 2007), leading to changes in the composition and abundance of arctic vegetation, animals and soil structure (Olofsson et al. 2009). Anthropogenic activities that elevate atmospheric greenhouse gas concentrations are generally considered to be the primary cause for this driver. Carbon release from anthropogenic sources is projected to continue and increase during the coming decades (IPCC 2007). This global driver is well established and could be looked as irreversible in the scale of next hundred years.

The tundra regime is maintained by the low atmospheric temperatures that ensure low soil temperature and the existence of permafrost. This determines the lack of microbial activity and scarce nutrient availability therefore ensuring species composition that are only able to exist in such circumstances.

A shift from the tundra regime to boreal forest is initiated by the atmospheric warming that is caused by continuing increase of CO₂ emissions. The atmospheric temperature increases the period of time when microbial activity and thus nutrient availability is sufficient enough for shrubs and boreal forest pioneer species (birch etc.) to withstand the winter conditions.

As a result the increasing number of shrubs ensure that snow drifting creates deep drifts that surround and extend downwind from shrub patches. These drifts increase the insulation of the soil below. This insulation elevates the soil temperature thus further allowing microbial activity to continue during the frigid arctic winter. The elevated soil temperatures under the snow lead to permafrost degradation that release carbon trapped in tundra soils, thereby further contributing to climate warming.

Shift from Tundra to Boreal Forest

Tundra provides biodiversity for different animal and plant species that characterize tundra ecosystem. The shift from tundra to boreal forest is projected to occur over large geographic areas throughout the tundra zone, with substantial impacts on the landscape and biodiversity (Bonan 1992). Changes in vegetation are also likely to affect composition of foraging mammals and birds (Hinzman et al. 2005). In addition to modifying wildlife habitat, increased woody shrub abundance will make traversing tundra more difficult for caribou, subsistence hunters and communities that rely on caribou for food, as well as hikers. Herbivores such as reindeers and microtine rodents can also be influenced by the shift towards woody vegetation as they prefer lichens, dwarf shrubs graminoids and deciduous shrubs over tall woody shrubs (Sturm et al. 2005). Increased expansion of woody vegetation may therefore result in tundra species extinctions or radical decrease in numbers. The loss of the climate regulating ecosystem service is a major concern as permafrost thawing is reducing the carbon sink service.

For local populations the tundra provides traditional recreation opportunities and contains their knowledge and educational as well as spiritual and religious values, which would be lost with the regime shift.

Shift from Boreal Forest to Tundra 

The change in biodiversity could provide new ecosystem services such as timber production and different wild animals and plant foods. This would benefit loggers as they would increase their income from tree cutting and local communities from goods that come from forest.

In order to prevent or reverse this regime shift it is essential that the input of carbon to the atmosphere is reduced in order to prevent further climate warming. The suggestion of a planetary boundary of 350 ppm for CO₂ levels is a first step in this direction (Rockstrom et al. 2009). Increased understanding of the influence of climate change on the complexity of arctic systems is essential for informing the adaptation of social, economic, and cultural systems to the changes taking place in the Arctic (Hinzman et al. 2005). Technology transfer could be a good initiative from developed countries as they can provide more advanced technological solutions to developing countries to help accelerate the learning curve on GHG emissions. A global response particularly from developed nations that are using the majority of the world's resources on a per capita basis should be in place to deal with such complex systems. Nevertheless, discussions about desirable management strategies and outcomes will be essential as there will be stakeholders who stand to lose from this regime shift (e.g. caribou herders) while others may see the change as desirable and beneficial (e.g. timber production, game hunting).

Forest regime
Forests are ecosystems typically dominated by trees, perennial plants taller than 5 meters. Tropical forest includes moist and dry forests (MA, 2005). A mature tropical forest contain at least four layers: emergent layer up to 45 - 80 meters tall, the canopy among 35 - 45 meters tall, the understory layer and the floor layer. Such structure gives a variety of habitats that host roughly half of the known plants and animals biodiversity (MA, 2005).


Savanna regime
Savannas, on the other hand, are drylands dominated by a mixture of grasslands and shrublands. The canopy in savannas never closes, and the floor layer is dominated by grasses, especially C4 species. Savannas, dry forest and shrublands conform 40% of the world’s land area and host up to 42% of human population (Reynolds et al. 2007, Falkenmark and Rockström 2008). About 25% of drylands, including savannas, are covered by croplands and sustain 50% of world’s livestock (MA, 2005).

Shift from forests to savannas

The most widely recognized driver is deforestation and consequently fragmentation of forest landscape, which reduces rainfall and increases surface temperature (Da Silva et al. 2008, Nobre et al. 2009). Reduction of forest cover accelerates albedo effect, loss of evapotranspiration and roughness length (Sternberg 2001), activates fire feedback, changes ocean circulation and warms up sea surface in the Amazon case, and ultimately changes the spatial vegetation structure.

Deforestation and forest degradation is driven by a complex, case specific interaction of social and economic drivers. The most important reported drivers include agriculture expansion, infrastructure development, the logging industry and fast population growth (Geist and Lambin 2002, MA, 2005). For example, in the Amazon, illegal logging is a critical threat that besides its damage to the forest, bring with them secondary effects like expansion of hunting areas, slash-and-burn farms, mining, the establishment of new road networks and therefore more logging facilities. The MA (2005) reports that 70 countries have problems with illegal logging leading to national income losses of $5 billions and total economic losses of about $10 billion. By 2001, Laurance and Williamson (2001) reported that 80% of brazilian logging activity were illegal; however, government counterintuitively sponsored colonization through cattle ranching projects.

Climate change and global warming are expected to enhance the regime shift; and the loss of forest areas are expected to exacerbate climate change (Laurance and Williamson 2001, Bonan 2008). While Laurance and Williamson (2001) report that forests like Amazon apparently change from carbon sinks to carbon sources during ENSO events; Nobre et al. (2009) suggest that deforestation of Amazon may actually increase ENSO variability; and Bonan (2008) confirms that deforestation would enhance global warming by decrease of evaporative cooling and release of carbon dioxide.

Shift from forest to savanna

The expansion of agriculture, increase in population as well as deforestation reduces rainfall and increases surface air temperature. This has a direct affect on plants, reducing evapotranspiration and photosynthesis and decreasing the supply of water vapour. Cattle pastures and regrowth forest areas become increasingly prone to frequent fires, which plays a fundamental role in the shift from forest to savanna since it is a feedback that actually maintains savanna state.

When tropical forests are replaced by less vegetated cover like savannas, or ultimately by sand in deserts, net radiation at the top of the atmosphere decreases inducing subsidence that inhibits precipitation (Oyama and Nobre 2004). While in the tropics land clearing affects the water balance and as consequence warms up the climate, in boreal forest such clearing affects mainly albedo and as results cools down climate (Foley et al. 2005). The albedo feedback is strengthened by changes in land cover, typically induced by deforestation for logging or agriculture activities. Warmer temperature and drier atmospheres, such as in savanna regime, result in an increase in lifting condensation level feedback that reduces the opportunity of cloud formation and therefore the likelihood of rainfalls (Pinto et al. 2009). Plants that do not have enough water responds by reducing transpiration and photosynthesis, interrupting the supply of water vapor that contribute to the recycled component of precipitation (Oyama and Nobre 2004, Saleska et al. 2007). Less evapotranspiration blocks the inland propagation of cold fronts responsible for precipitation, increasing the dry season length (Oyama and Nobre 2003, Pinto et al. 2009).

Savanna vegetation is better adapted to dry environments. Grasses usually have C4 photosynthesis type, a chemical pathway that reduces water consumption and helps to cope with nitrogen or CO2 limitations. Evapotranspiration depends on soil moisture and biomass. Thus, for instance, droughts frequency or grazing reduce biomass, weakening in turn the feedback effect (Dekker et al. 2007, Dekker et al. 2010). In addition, the spatial distribution of rainfall is affected by both the land-cover type and topography. Cattle pastures and regrowth forest areas become increasingly prone to frequent fires. In such zones fire can be produced after few days of dry conditions. On the regional scale, fire smoke may reduce rainfall by trapping moisture and inhibiting raindrops formation (Laurance and Williamson 2001). Fire plays a fundamental role in the shift from forest to savanna since it is a feedback that actually maintain savanna state (Laurance and Williamson 2001, Hutyra et al. 2005)

Forest provides a wide range of ecosystem services. Besides being hot spots of biodiversity, forest provides soil and water protection, it prevents soil erosion, floods and landslides. For example, soil erosion may be 10-20 times higher on areas cleared of forest (MA, 2005). Depending on soil conditions, at the local scale forest can also regulate the hydrological cycles by increasing precipitation and decreasing evaporation. They regulate below grown runoff and smoothing seasonal extreme events: heavy rainfalls or dry spells. Due to its regulating function in the water cycle, forested watersheds provides water supply to one third of the world’s largest cities (MA, 2005). 

Forest sustain about 200 million people belonging to indigenous groups, who depend on forest not only as source of resources (food, fiber, fuel) but also their culture and religious traditions (MA, 2005). Forest also maintain the agroforestry industry which, including temperate forests, produces 3.3 billion cubic meters of wood (MA, 2005). 

Managerial options for the forest to savanna regime shifts requires targeting the main drivers: deforestation and landscape fragmentation. Controlling illegal logging and implementing sustainable logging plans are part of the strategy. Sustainable logging needs to take into consideration reducing fragmentation and allowing deforested patches to regrow. Likewise, the expansion of agricultural frontier and grazing areas needs to be controlled and when unavoidable, it needs to be planned in order to prevent fragmentation. The fire frequency feedback accelerates the shift from forest to savanna regime. Laurance and Williamson (2001) suggest fundamental changes in prevailing land-use practices and development policies to avoid wildfires. Such changes include the management of logging and grazing areas in order to reduce fragmentation and therefore it would reduce the fire risk. Hence, fire and fragmentation management need to be coupled strategies.

Kelp forests are marine coastal ecosystems dominated by macroalgae typically found in temperate areas. This group of species forms submarine forest with three or four layers, which provides different habitats to several species. Kelp ecosystems are important to maintaining important industries such as, lobster and rockfish fisheries, tourism based on fishing, recreational diving and kayaking, as well food and pharmaceutical products derived from kelp (e.g. tooth paste).

Three different self-reinforcing regimes can be identified:

Kelp forests are highly productive ecosystems dominated by canopy-formed algae in cold-water rocky marine coastlines. Among the biota associated with kelp forest are marine mammals, fishes, crabs, sea urchins, mollusks, other algae and biota that live on the kelp themselves (Steneck et al. 2002). At least 4 trophic levels are found in kelp forests. Apex predators such as sea otters, cod, pollock, hake, and haddock are common. These predators regulate populations of species at lower trophic levels. In particular the regulation of sea urchins populations, who are an important consumer of kelp, is important to maintain kelp forests.

Urchin barrens are an alternative regime in which there is no kelp forest and substantial populations of urchins on the sea bed. This regime has low or no populations of apex predators and large populations of herbivores that keep macroalgae at very low population levels, preventing the regrowth of the kelp forest, an important habitat for apex predators (Steneck et al. 2004). Ecosystem services related to apex predator fisheries and recreational activities are substantially reduced. However populations of bottom dwelling species can increase. In some places urchins have themselves become a valuable fishery. While in others, populations of other harvested species such as lobster and crab have increased.

Turf-forming algae regime is similar to urchin barrens in that there is no kelp forest, however rather than dominance by urchins the seabed is dominated by turf forming algae. This regime also lacks apex predators (Gorman et al. 2009). Ecosystem services related to fisheries and recreational activities are substantially reduced. However, populations of invertebrate species such as lobster and crab can be higher. This regime shift is associated with a change in habitat in shallow marine coastal ecosystems. 

This regime shift is driven by two key direct drivers: reducing populations functional groups (Steneck et al. 2002; Steneck et al. 2004) and input of nutrients (Gorman and Connell 2009). Reductions in populations of key functional groups is due to the hunting of sea otter and the fishing of large apex predator fish species. Overfishing usually diminish the controlling function of top predators, and in severe cases, the trophic levels may be simplified. Nutrient input is often due to the output of wastewater from urban settlement and agriculture in nearby coastal areas. However it can also be due to naturally occurring offshore upwelling events. Nutrients favors the growth of turf algae over kelp. Strong rain events and floods can produce shock events for kelp ecosystems by providing a pulse input of nutrients and turbidity, which can stimulate the growth of turf algae.

Other drivers can also contribute to this regime shift. Pollution discharges and sedimentation may play a synergetic role as stressors. In Tasmania for example, global warming has favored the reproduction of urchins which acting in synergy with lobster fishing has reduced kelp resilience (Ling et al. 2009). El Niño events and global warming can generate water stratification. As a consequence, nitrogen concentration declines and kelp forest growth becomes limited by the availability of nitrogen (Steneck et al. 2002).

Shifts from kelp canopies to other regimes like urchin barrens and turfed landscapes have occurred in many regions worldwide (Steneck et al. 2002; Gorman and Connell 2009; Ling, et al. 2009). This regime shift is associated with a change in the biophysical structure in shallow marine coastal ecosystems. Kelp forest dynamics often include cycles of disturbance-recovery. However, increasing human-dominated coastlines modify environmental conditions that facilitate other regimes like turfs or urchin barrens, which in turn inhibits the reestablishment of kelp forests.

Overfishing and the input of nutrients are the main drivers of the regime shift. At least 4 trophic levels are found in kelp forests, and apex predators such as sea otters, cod, pollock, hake, and haddock are common. Overfishing usually diminish the controlling function of top predators, and in severe cases, the trophic levels may be simplified. In addition, urchins are prevented from grazing in the kelp forest through the effect of kelp foliage sweeping over the rocks due to its flexibility and the force of waves (Steneck et al. 2004). 

Overharvesting of sea otter, cod and haddock can release their prey populations in particular sea urchins from their predatory controls (Steneck et al. 2002; Steneck, et al. 2004). Large populations of urchins form grazing fronts that graze on kelp forest to its elimination (Lauzon-Guay et al. 2009). These populations of urchins can survive in adverse conditions by feeding on turf-forming algae. In some cases the removal of apex predators allows populations of other species to reach sufficient population sizes to exert some control on urchins, for example lobsters and crabs in the North Atlantic (Steneck et al. 2002). Therefore, in some cases the formation of urchin barrens open the opportunity for new fishing industry to establish, like urchin, lobster and crabs. In addition, declines in predatory fish have created a market for urchins, especially in Japan, establishing a new human induced control on the herbivore. In some cases, harvesting of urchins for this market has led to the re-establishment of kelp forest, despite urchin fishery being prohibited (Steneck et al. 2002; Scheffer 2009).

On the other hand, the turf-forming algae regime is maintained by its ability to persist under conditions of elevated nutrients, frequently attributed to coastal urban settlements, inhibiting the recruitment of kelp species (Gorman and Connell 2009). The loss of kelp dominated areas undermine kelp's ability to reestablish in disturbed areas (Gorman and Connell 2009). While deforested areas surrounded by kelp patches are more likely to return to the kelp regime, isolated kelp disturbed patches are more likely to stick in the turfed regime.

Shift from kelps forest to urchin barrens or turfs

The main ecosystem impact of the loss of kelp forests is the loss of habitat complexity. Kelp is a three-dimensional structure that offers shelter and food for many species; urchin barrens and turfs do not have such characteristics. This loss is associated with the reduction of the food web complexity and loss of functional groups (Steneck et al. 2004), with varying effects on fisheries. Some valuable fish species may diminish since kelp forests provide nursery areas. Invertebrate species such as lobster and crab can increases in population (Steneck et al. 2002). Along with hosting high marine biodiversity, kelp forests provide ecosystem services related to recreation for divers. In addition, kelp supports a multi-million dollar industry of canopy-cropping for alginates (Steneck et al. 2002). This product is commercially important in pharmaceutical and chemical industry. These services can be reduced or lost by this regime shift.

The ecosystem service impacts of algae turfs are likely to be similar to those related to coastal eutrophication. Such effects include abundance of rich-nutrient environment species as shellfish, bad odors and the associated consequences for recreational and aesthetic values.

Shift from urchin barrens or turfs back to kelp forest.

The recovery of kelp forest increases habitat complexity and is expected to increase biodiversity. On the other hand, the benefits gained by urchin and lobster fisheries may be diminished, since the recovery of functional groups and increasing biodiversity will control their populations. However, other species of commercial fisheries and tourism may recover, although it is not always the case.

Options for enhancing resilience

Restoring biodiversity in functional groups in kelp forests is particularly important to maintain apex predators (Estes et al. 2011). A high profile version of this strategy has been implemented to recover sea otter populations on the west coast of North America, by protecting sea otter populations (Steneck et al. 2002). Reducing nutrient runoff from agricultural areas is essential for enhancing resilience, as well as improving water treatment and managing runoff in urban areas (Gorman et al. 2009). The removal of sea urchins along with implementing measures to maintain the abundance of predators like cod, sea otters and sheep-head through fishery controls may provide stability to kelp forest ecosystems (Steneck et al. 2002; Steneck et al. 2004).

Options for reducing resilience to encourage restoration or transformation

Transplantation of kelp-forming species has been proposed as a restoration strategy in highly fragmented seascapes. This strategy is likely to increase the supply of kelp propagules to colonize new areas and maintain free substratum for canopy recruitment (Gorman and Connell 2009). However this strategy has not been tested.

Bivalve mollusks play an important role in aquatic ecosystems by filtering and sequestering nutrients. When bivalve abundance changes, it can have substantial ecosystem impacts, creating two different, self-reinforcing regimes.

Bivalve mollusk reef regime

When aquatic ecosystems have high bivalve mollusk density, they form reefs that include many bivalves and are shallow, nearing the water's surface. The filtering activity of the bivalves results in clear water with high levels of dissolved oxygen. Plankton populations are limited in these conditions. The bivalves themselves operate as reinforcing feedbacks in that their abundance ensures sufficient filtration to maintain this clear water regime. The clear water in turn reinforces bivalve abundance by maintaining hydrodynamics that are conducive to bivalve health (Scheffer 2009). Large, shallow bivalve mollusk reefs encourage biodiversity by providing habitat and filtering nutrients.

Isolated, low density bivalve mollusk regime

When aquatic ecosystems have low bivalve mollusk density, the reefs are small and deep. Water is turbid, with low levels of dissolved oxygen. Plankton and filamentous algae flourish under these conditions. Poor reef size and water filtration limit biodiversity. The low bivalve abundance state is reinforced as bivalve health and fecundity is weakened by turbid water conditions. The turbid water conditions are in turn reinforced by low bivalve abundance(Weijerman et al. 2005, Powell et al. 2008).

The regime of low bivalve abundance is expected to reduce fish populations, water cleansing and the ability of the ecosystem to maintain biodiversity. Such effects would have an impact on fisheries, human health, and recreation. 

Mechanized anthropogenic over-harvesting of bivalve mollusks is the main driver of the shift from high to low abundance (Thrush and Dayton 2002).  High inputs of nutrients from agricultural or urban sources can act as a slow driver that weakens bivalve health by creating plankton blooms that the bivalves are unable to filter.  Plankton blooms can further weaken the health of bivalves by producing organic matter, whose decay, reduces oxygen availability.  A disturbance that can trigger a regime shift is bivalve disease (Powell et al. 2008).  When populations are weakened by eutrophication and anoxia, they become physiologically susceptible to diseases that can rapidly devastate remaining populations (Leniham 1999). 

Bivalves are a class of mollusks containing over 30,000 species which include scallops, clams, oysters, and mussels. Bivalve mollusks, like corals, are often referred to as "ecosystem engineers" because they engineer the physical and hydrodynamic structures of the ecosystems they occupy in a way that produces conditions that support their own population growth. Bivalves live on the sediment surface, where they form reefs that provide increased surface area and habitat for a variety of species. They filter nutrients from the pelagic zone and transfer it to the benthic zone, maintaining the system in a clear water state. If bivalves are not able to maintain clear water, due to declines in bivalve populations or increases in low water quality, the ecosystem can shift to a turbid water state. The main drivers of this shift is direct anthropogenic over-harvesting of the bivalves, followed by a gradual increase of nutrients discharge from agricultural activities and coastal urban settlements.  Once mollusk abundance is reduced, plankton and filamentous algae populations can substantially increase. As result, oxygen and light available in the water column is reduced, while nutrients levels accumulate due to the lack of filtration.  These processes produce conditions which favour algae over bivalves.  If nutrient inputs are substantial, eutrophication and areas of hypoxia (low oxygen) are likely to appear in areas dominated by plankton instead of mollusks. 

Shift from high to low bivalve mollusk abundance regime

The loss of abundant bivalve mollusk reefs threatens diverse ecosystem services. The most direct impact is the loss of valuable shellfisheries. A secondary, but potentially substantial impact is the loss of the filtering service provided by bivalves.  In urbanized estuaries, it can necessitate the installation of costly synthetic water filtration technology (Gren et al. 2009).  In other areas it can lead to declines in water quality that shift populations and reduce recreation opportunities, and potentially the value of waterfront property.

The loss of bivalve habitat structure leads to lower species abundances, and to declines in species richness (Airoldi et al. 2008). Both structural and functional biodiversity is threatened by loss of bivalve abundance, with far reaching effects on marine ecosystems and human ability to exploit such systems (Worm et al. 2006).  The regime of low bivalve abundance is expected to reduce fisheries, water cleansing and the ability of the ecosystem to maintain biodiversity. Such effects would have an impact on fishing productivity and human health.

The loss of bivalve nutrient filtration can lead to other regime shifts, such as eutrophication and hypoxia, which can produce unproductive and undesirable ecosystems (see Hypoxia and Eutrophication regime shifts). 

Options for enhancing resilience

Estuaries are the world's most degraded marine ecosystems, receiving land-based pollution from crop cultivation and urbanization, and suffering from anthropogenic over-harvesting (Lotze et al. 2006).  The resulting problems are complex, impacting bivalve health and fecundity as well as many other species, with far reaching implications in both biological and social domains.

Experience has shown that management focused on one species or problem tends to be ineffective.  Newer research stresses the need to address natural resources as part of complex social-ecological systems, often with long histories of human exploitation (Jackson et al. 2001).   Rather than addressing problems on a species-by-species basis, multi -species management has shown to have synergistic effects.  For example, in Chesapeake bay, fisheries management looks at oyster, blue crab, striped bass and shad together (Boesch 2004).

Taking this approach further, Ecosystem-based fishery management efforts focus on recognizing interactions between multiple species and environmental stressors, such as low dissolved oxygen levels. Success is measured by the degree to which management efforts include ecosystem-based approaches, rather than by an assessment of fishing stocks (Lotze et al. 2006). 

Options for reducing resilience to encourage restoration or transformation

The main option exercised for preventing or reversing a regime shift regarding bivalve abundance is to import bivalve mollusks from another region for aquaculture (Van de Koppel 2008) .  Another practice commonly used is to hang bivalves from the surface or build artificial reefs to elevate bivalves in order to avoid the hypoxic conditions in deeper water (Carlsson et al. 2009). 

Savannas are systems that consist of a mixture of woody vegetation (trees or shrubs) and grasses. Savannas, dry forest and shrublands cover 40% of the world's land area, host up to 42% of the world's human population (Reynolds et al. 2007, Falkenmark and Rockström 2008), and together with drylands sustain 50% of the world's livestock (Millennium Ecosystem Assessment 2005).

At small scales (up to about 10 km²) savanna systems, especially those used for extensive cattle ranching, may stabilize in two different self-reinforcing regimes (Walker 1993, Scheffer et al. 2001, Scholes 2003):  

Open, grassy savanna regime

In this regime the landscape has a productive grass layer with few mature trees. The canopy in savannas never closes, and the floor layer is dominated by grass, especially C4 species. Most young trees are unable to establish because, while often numerous, seedlings are constantly knocked back to ground level by herbivory and fire. There is enough grass after grazing to support a fire with flame-length taller than the young saplings sufficiently often to keep them in a 'fire trap' (Dublin et al. 1990, Roques et al. 2001b, a). Open savanna systems are suitable for ranching, and are maintained by fire dynamics and grazing.

Closed, woody savanna regime

In this regime the landscape is dominated by woody shrubs or trees. Once established, woody vegetation is stable because adult trees are seldom killed by herbivory or fire. These alternate regimes can occur at a range of spatial scales. Sometimes larger areas (e.g. an entire cattle ranch) may shift from a grass-dominated to a persistent woody-dominated state (Dublin et al. 1990, Walker 1993). In other cases, the alternate regimes are expressed as a mosaic of small patches of trees or bush interspersed with patches of grass, where the respective patches are highly persistent over time (Rietkerk et al. 2004).   

Bush encroachment refers to a shift from a grassy system to a persistently woody system. It typically occurs in areas used for free-range cattle ranching, and is usually caused by a combination of grazing and fire management practices. Bush encroachment involves a change in the outcome of the competitive interaction between woody vegetation (shrubs and trees) and herbaceous vegetation (grasses and herbs), mediated by nutrients, grazing, fire, rainfall variability and use of either the woody or grassy components by humans (Anderies et al. 2002, Janssen et al. 2004, Wiegand et al. 2006). The encroachment typically occurs in episodes rather than continuously, and involves a particular set of encroaching species rather than the entire woody community.

Bush encroachment typically occurs in areas used for commercial cattle ranching (as opposed to subsistence, communal, or nomadic) and may follow episodes of sustained severe overgrazing, though not necessarily so. It may also occur under other land uses (Wiegand et al. 2006). It tends to be an episodic phenomenon, where the tree cohorts can often be linked to issues in the ranching enterprise – such as drought-induced debt or downturns in the cattle price cycle (Scholes 2003, Wiegand et al. 2006). 

There are several different hypotheses regarding the mechanism by which bush encroachment occurs. Different mechanisms (or combinations of mechanisms) may be important in different places. One proposed mechanism is based on changes in fire regime: in the sustained presence of high numbers of grazers (typically cattle) accumulation of grass fuel is reduced, leading to period without intense fire long enough for woody plants to grow beyond the fire-susceptible stage, which in turn suppresses grass production and fires, further enhancing the establishment of woody vegetation (Higgins et al. 2000, Staver et al. 2009, Staver et al. 2011). A related hypothesis notes the elimination of browsers (especially very large browsers such as elephant and giraffe, but also the more-numerous small browsers) from the system when cattle are introduced (Dublin et al. 1990). Similarly, alien species, such as Prosopis in South Africa or Acacia nilotica in Australia, both deliberately introduced, can play an important role in bush encroachment by affecting fire regimes (Poynton 1990). Another hypothesis focuses on changes in water availability based on the rooting depths of plants: grasses are thought to be more shallowly-rooted than trees, so if grass cover is reduced by overgrazing, this is more water available for trees, which promotes their growth and establishment, further suppressing grass growth (Noy-Meir 1982). Refinements of these hypotheses emphasize combinations of events, such as a multiyear drought or fireless period providing a "window" for the establishment of trees (Wiegand et al. 2006).

Yet other hypotheses focus on the role that increases in global CO2 levels may play in the observed proliferation of woody plants in many, widely-separated areas of the world during the 20th century. The underlying mechanism is still debated, but several possibilities have been proposed: i) that rising CO2 levels favour C3 (woody plant) photosynthesis relative to C4 (tropical grass) photosynthesis  ii) elevated CO2 may reduce transpiration of grasses, leading to greater water percolation and therefore favoring deeper rooted woody species, iii) faster growth of woody plants due to CO2 enrichment, and therefore faster escape of seedlings from susceptibility to fire, iv) investments in carbon-based defense compounds such as tannins, which are the main defense compounds in many encroaching trees but not in grasses (Midgley and Bond 2001, Wiegand et al. 2006). It is striking that encroachment is almost unheard of on communal land, and is not universal on commercial farms. A suggested explanation for this is that communal lands use the trees for firewood and run goats along with cattle, inhibiting the establishment of trees (Scholes 2003). Bush encroachment has been documented in East and Southern Africa (but not West Africa), South America (Uruguay/Argentina and Chile), North America (Texas, New Mexico) and Australia, but not in India, also a savanna environment. Furthermore, it did not happen simultaneously in those places, but 30-50 years after the widespread establishment of sedentary grazing management, what has been referred to as 'commercial' ranching above. This tends to disfavor the rising CO2 hypothesis, although rising CO2 may predispose the shift (Midgley and Bond 2001).  

Shift from grassy to woody savanna

Woody encroachment brings a relatively rapid change, over a decade or two, from a highly productive grass layer to a sparse and unproductive grass component. Since cattle are grass-eaters, this change substantially reduces cattle productivity (Anderies et al. 2002, Scholes 2003), with major impacts on cattle ranchers. On the other hand, encroachment increases the supply of tree-based ecosystem services, such as wood for fuel, charcoal-making and building material. This is somewhat dependent on the species involved. The increase in woody cover could potentially also have macro and micro-climatic effects through impacts on albedo and CO2 uptake, in addition to the decrease in methane emissions from cattle.

Difficulties in mustering the cattle in dense bush are a contributing factor. Therefore, wood encroachment leads to economic losses for cattle ranchers in what is frequently an economically marginal area for other agricultural uses such as croplands. 

Options for enhancing resilience

There is some agreement among researchers and extension workers that encroachment can be avoided by stocking lightly and burning frequently to prevent the establishment of trees and maintain grass crowns - the productive part of the grass that is less affected by fires (Roques et al. 2001b, Janssen et al. 2004). However, this is seldom reflected in management practice. 

Options for reducing resilience to encourage restoration or transformation

Bush encroachment is expensive to reverse, since rapid results rely on arboricides or repeated mechanical or manual clearing. A common method involves the manual removal of woody vegetation, with repeated follow-up control and the use of fire to enhance the establishment and competitive advantage of grasses (Scholes 1985, 2003). Attempts to reverse bush encroachment often have poor results, either due to the rapid resprouting of the trees or the conversion of the grass layer to less desirable species in the process. 

There are anecdotal reports of widespread mortality of near-dominant encroaching species after several decades, possibly related to disease, prolonged drought or simply old age, which provides windows for grass establishment and fuel load for intense fires.

The hypoxia regime shift involves a radical change in oxygen concentration in aquatic ecosystems such as rivers, lakes and marine ecosystems. The severity and persistence of hypoxic conditions varies. Episodic oxygen depletion represents 17% of known hypoxia cases, and occurs infrequently with several years sometimes elapsing between events. Episodic oxygen depletion is the first signal that a system has reached a critical point of eutrophication, which in combination with physical processes that stratify the water column, tips the system into hypoxic conditions (Diaz and Rosenberg 2008). Seasonal hypoxia tends to occur periodically during the summer, after algal spring blooms have sunk to the bottom and are being decomposed. It lasts from days to weeks, represents half of the known dead zones, and typically abates in the autumn (Diaz and Rosenberg 2008). Boom-and-bust cycles of animal populations are frequent. Persistent hypoxia occurs when hypoxia becomes persistent due to the build-up of organic matter in the sediments over time, particularly in systems prone to persistent stratification. Persistent hypoxia accounts for 8% of reported hypoxia cases. Anoxia occurs when DO levels fall below 0.2 ml per litre. The accumulation of organic matter is exacerbated, and poisonous hydrogen sulphide (H₂S) is released due to microbial metabolism.

Normoxia: This regime is characterized by normal levels of dissolved oxygen, typically 5 to 8 ml per liter. Most aquatic organisms are adapted to survive under these conditions. Particularly important are the benthic organisms. Benthos are a community of organisms which live in sedimentary environments at the bottom of water bodies. They constitute an important key functional group (scavengers and detritivores) in the food web responsible for the decay of dead matter. Macrobenthos facilitate bioturbation processes, in other words, they displace and mix particles facilitating chemical exchange between sediments and the water column. This allows the sediments to "capture" phosphorous and nitrogen from the water column. By removing these nutrients from the water column, bioturbation therefore reduces algal growth and helps ensure that the water remains oxygenated and suitable for the survival of benthos - creating a reinforcing feedback that maintains normoxia conditions.

Hypoxia: The hypoxic regime is reached when dissolved oxygen level falls below 2 ml per liter (Diaz and Rosenberg 2008). Hypoxia is associated with so-called "dead zones". These are areas where dissolved oxygen levels are so low that most life is not able to persist and only very few specialized microorganisms survive. In the hypoxic state, accumulation of organic matter produced by eutrophic processes favors the growth of microbes who, by decomposing the organic matter, consume the oxygen in the water column.

Anoxia: Anoxia occurs when hypoxia is exacerbated to DO levels under 0.2 ml per liter (Diaz and Rosenberg 2008). Under such conditions, benthic animals suffer from mass mortality and decay processes are carried out by microbial metabolism. As A result, hydrogen sulfide is released changing water acidity (pH) and then further favoring bacteria habitat. 

Hypoxia is driven by increasing nutrient input, both natural and anthropogenic. Natural sources come from the bottom of the ocean transported by upwellings, water currents that flow vertically driven by the interaction of winds and the temperature gradient between the ocean surface and bottom. Anthropogenic sources are related mainly with the use of fertilizers in agriculture. However, growing urban settlements in coastal areas also increase the water storm runoff and sewage.

The leakage of nutrients from agriculture is further exacerbated by rainfall variability and deforestation, since vegetation offers resistance to erosion and trap nutrients and moisture in the soil. Agriculture is driven in turn by increasing demand of food and fibers as well as population growth. Another important factor leading to hypoxic conditions is the stratification of the water column, which reduce the exchange of nutrients between the surface and the bottom. Water stratification is usually driven by anomalous increase in sea surface temperature. 

The fast variable in the hypoxia regime shift is DO, and can also include faunal composition. The slow variables are the accumulation of nutrients, mainly phosphorous and nitrogen in the water column. Large areas of the sea are oxygen minimum zones due to natural conditions. Hypoxic-prone areas generally have a semi-enclosed hydrogeomorphology and water-column stratification that restrict water exchange and reoxygenation of the deeper water layers (Diaz and Rosenberg 2008). Another natural condition that facilitates hypoxia and eutrophication is coastal upwelling, that lead to nutrient enrichment along continental margins.

The normoxia regime is usually  maintained by feedbacks that control the nutrients content in the water column and keep oxygen concentrations high. Zooplankton for example plays an important role controlling micro algae numbers, hence limiting their ability to use nutrients available and reduce oxygen through their metabolism. Macrophytes or macro algae also control micro algae by consuming the same resources. Under such conditions nutrients are kept on inorganic forms unavailable for micro algae to use.

Under the hypoxia regime the feedbacks above are weakened, allowing micro algae to escape predation and competition controls; and reducing the level of DO in water. As DO decreases, mass mortality of different organism increases the availability of organic matter in the water column, decreasing DO and promoting more micro algae growth. When DO reaches critical levels under 0.5mL per liter, hydrogen sulfide is released further decreasing pH in water and reducing biodiversity. Such extreme regime is called anoxia. 

Shift from normoxia to hypoxia and anoxia

Normoxia is associated with the provision of food for humans and wildife in aquatic systems as well as regulating services such as water purification and pest regulation. This regime shift into hypoxia, often characterised as a dead zone, has been reported in more than 400 systems affecting more than 245,000 square kilometers and including important fisheries such as the Baltic Sea, Kattegat, Black Sea, Gulf of Mexico, and East China Sea (Diaz and Rosenberg 2008). A major impact on ecosystems is a change in the flux of matter and energy through trophic levels. Consequently, fisheries and hence ecosystem services such as food production are affected. For example, Diaz & Rosenberg (2008) report that biomass in the Baltic Sea has been reduced by approximately 264,000 metric tons of carbon due to hypoxic conditions. Assuming that ~40% of benthic energy passes up the food chain, 106,000 metric tons of carbon of food energy for fisheries has been lost(Diaz and Rosenberg 2008). This implies a reduction in yields and consequent impact on employment in fisheries communities. Another example of such effects is the lobster fishery collapse in Norway (Diaz and Rosenberg 2008).

Besides its effect on fisheries and employment, hypoxia also affect the health and other cultural services of the people living near by dead zones. Decaying matter after mass mortality events create odors and risk of diseases. Recreation, aesthetic and touristic values are lost.

Shift from Hypoxia to Normoxia

When hypoxia is not severe, the system is likely to return to normoxia regime. It implies the recovery of fisheries when species hasn't been extinct locally. However, food webs not always fully recover, and usually top predators are lost. With the recovery of the system, fishing, recreational and touristic activities recover too, although not necessary to the same configuration than before the regime shift. 

Options for enhancing resilience

Since one of the main drivers of hypoxia is eutrophication, Diaz & Rosenberg (2008) recommend managing the input of fertilizers on agricultural land. They recognize the necessity of developing new methods to close the nutrient cycle on farms, in order to avoid the drainage of nutrients to water sources. Nutrient reductions can also be achieved relatively cost-efficiently by improving waste-water treatment system in regions where this is applicable (e.g. Baltic Sea).

Options for reducing resilience to encourage restoration or transformation

Only 4% of the reported cases of hypoxia have shown improvement, principally due to the reduction of organic and nutrient loading, stratification strength, and freshwater runoff (Diaz and Rosenberg 2008).

Managers could take advantage of windows of opportunity provided by these variables. For example, in the Baltic Sea the stratification of the water column is determined by input of saltwater from the North Sea (Conley et al. 2009). A policy of nutrient load reduction would therefore be more effective in years when saltwater input is low and it is a particularly rainy year. While nutrient loading is largely periodic and rain dependent, stratification and freshwater runoff rely more on physical processes and climate variability. Finally, it has been suggested that an appropriate goal is the reduction of nutrient loads to the level of the mid-1900s (Diaz and Rosenberg 2008). 

Thermohaline circulation is complex, but can be summarised as a global scale deep overturning of water masses, with sinking motions occurring in the northern Atlantic and around Antarctica and a transport of relatively warm surface waters towards these sinking regions (Steffen et al. 2004). The term thermohaline circulation thus refers to a particular driving mechanism; it is a physical, not an observational concept.

Strong Thermohaline circulation

THC regime can be characterized by the formation of deep water currents in the high latitudes of both hemispheres; the spreading of deep waters, as deep western boundary currents; upwelling (or upwards movement) of deep waters which is thought to take place mainly in the Antarctic Circumpolar Current region, possibly aided by the wind, and near-surface currents, required to close the flow (Rahmstorf et al. 2006).

Collapse of the Thermohaline circulation

This regime is characterized by weak formation of the dense and saline high latitude deep waters due to the increased influx of freshwater. The decreased salinity of the deep waters is weakening the density southwards driven water movement. The transport of less dense and saline waters from south towards north is decreased causing additional atmospheric temperature increase in tropical areas.

Increasing greenhouse gas concentration from anthropogenic sources is predicted to cause a rise in global mean temperatures (Cubasch et al. 2001). One of the most common anthropogenic greenhouse gases is carbon dioxide (CO2). It is considered to be the main driver for this regime shift and is typically operating in global scale due to the global input of CO2 in the atmosphere. Increasing CO2 levels in atmosphere increases the mean atmospheric temperature. This results in weakening ice-albedo mechanism for the initial regime as a result of increased atmospheric temperatures and changes in albedo.

The increasing freshwater influx from sea-ice loss is accelerated by freshwater-overturning and precipitation-river runoff. This leads to an in increased evaporation, river runoff, and changes to water salinity and density. The burning of fossile fuels such as coal and natural gas also drives this regime shift. This occurs in a regional scale but has global impact and is well established in the literature.

In the regime of strong THC, the most simple explanation of the main variable that maintains the regime would be the high-latitude cooling. In the deep ocean, the predominant driving force of movement of currents is differences in density, caused by salinity and temperature (the more saline the more denser and the colder the water, the more dense). The ocean's density distribution is itself affected by currents and mixing. Thermohaline and wind-driven currents therefore interact in non-linear ways and cannot be separated by oceanographic measurements. Changing the wind stress will alter the THC; altering thermohaline forcing will also change the wind driven currents.

The collapse of THC occurs due to increasing CO2 levels in atmosphere that slowly enhance the mean atmospheric temperature. As a result the high latitude cooling is weakened, increasing freshwater influx from the loss of ice, both from the Arctic, Greenland and possibly the north of Canada. This strengthens two main mechanisms for the new regime: freshwater-overturning mechanism and the precipitation-river runoff mechanism. Both increase the fresh water inflow into the ocean. Therefore new mechanism such as increased evaporation, river runoff, changes in water salinity and density are activated possibly reinforcing the new regime. Once the system has shifted, the warm conditions in Northern latitudes maintain a weak formation of the dense and saline high latitude deep waters due to the increased influx of freshwater.

Most ocean–atmosphere models predict a reduction of the THC for a warmer climate in the near future. Recent observational evidence provides some support to the model projections that the North Atlantic THC circulation could weaken or shut down later this century (Steffen et al. 2004). Lenton et al. (2008) report that the IPCC models predict the regime shift to happen after 2100, however, their models do not include freshwater runoff which is expected to accelerate the process. The models that project the collapse of THC under greater perturbations identify that the collapsed state can last for centuries and might be irreversible (Delworth et al. 2008). Nevertheless the latest IPCC report shows that it is very unlikely that the THC will undergo a large abrupt transition during the course of the 21st century (IPCC 2007). Some models show a recovery of the THC after the elimination of the freshwater perturbation whereas others do not. The mechanism for this different dynamical behavior of the THC is not completely understood (Stouffer et al. 2006).

The weakening of THC will impact global water regulation as water circulations in global scale are influenced by the changes in temperature, salinity and density. In response, climate regulation is altered and changes as the transport of heat is distributed in a different pattern. The water mass and heat circulation will no longer transport warm currents in a northly direction. This could increase the occurence of hurricanes, a southward shift of tropical rainfall belts with resulting agricultural impacts, as well as disruptions to marine ecosystems. Provisioning services such as fisheries, livestock and food crops are therefore potentially affected. Schmittner et al. (2007) show that changes in Atlantic circulation can have large effects on marine ecosystems and biogeochemical cycles, even in areas remote from the Atlantic, such as the Indian and North Pacific Oceans. Although loss of THC could affect Western Europe and result in colder climates. 

Food crisis and emigration are potential threats in the regions that are subjected to this regime shift (Barnett & Adger 2007). In addition, regional changes in sea level are predicted to rise of up to 80 cm in the North Atlantic (Knutti et al. 2002). This could affect the coastlines of the United States, Canada, and Europe causing coastal erosion and affect the security of housing and infrastructure (Delworth et al. 2008). The marine biodiversity of flora and fauna will also be affected. The change in salinity and temperature may facilitate the introduction and spread of invasive species that are able to thrive in the new conditions. This could affect the livelihoods and economic activity of local fishermen as they would have to adapt to new circumstances.  

This regime shift is mainly driven by the increase in greenhouse gases into the atmosphere at a global scale. Therefore a decrease in CO2 emissions and other greenhouse gases has to be achieved in order to prevent further climate warming leading to large influx of freshwater from the depletion of Arctic ice. In the best-case scenario, it may be possible to avert a regime change in a situation where the underlying driver(s) of change are amenable to fast manipulation through management, and where there are not substantial lags in ecosystem response. In the case of the THC neither of these factors hold: the underlying causes of global warming appear very difficult to address through rapid changes in management, and there are substantial inertias in ecosystem response to changes in the driving factors. Therefore an early warning indicators would be useful in providing advance warning of a substantial coming change in ecosystem conditions, but is unlikely to be useful in averting the regime change.

The Greenland Ice Sheet (GIS) is approximately 1.7 million km² in areas covering approximately 80%  of Greenland. It is grounded on bedrock that mostly rests near or above sea level thus would contribute to the globally averaged sea-level rise of 7.3 m if melted completely (Parizek et al. 2004, Lemke et al. 2007).  Anticipated future climate warming has the potential to permanently reduce large areas of GIS or even abate it completely. The evidence suggests nearly total ice-sheet loss may result from warming of more than a few degrees above mean 20th century values, but this threshold is poorly defined (perhaps as little as 2oC or more than 7^oC) (Alley et al. 2010).

Greenland with permanent ice sheet

This regime can be described as permanent ice body cover over major parts (~80%) of Greenland only exposing the bedrock in western and southern parts. The Greenland Ice Sheet has been more closely tied to temperature than to anything else. It shrinks with warming and grows with cooling, thus the volume and cover vary throughout the seasons, but in case of cold climate the relationship between the ice growth/decline would be approximately evenly balanced or with a slightly increased ice growth. In winter when the atmospheric temperatures decrease below freezing point and precipitation levels decline, the accumulation of the ice steadily increases (Bamber et al. 2007).

Greenland without permanent ice sheet

Due to the loss of the ice-sheet volume as a result of the warming, Greenland territory in future could become free from permanent ice sheet cover. This would happen as a consequence of the negative relation to ice growth/ice decline during the winter and summer where the lost ice volume in summer could not be reproduced in the following winter. Rising sea level as a result of warming tends to float marginal regions of ice sheets and force further retreat, so the generally positive relation between sea level and temperature means that typically both reduce the volume of the ice sheet (Alley et al. 2010).

Increasing greenhouse gas concentration from anthropogenic sources is predicted to cause a rise in global mean temperatures (Cubasch et al. 2001). One of the most common anthropogenic greenhouse gases is carbon dioxide (CO2). The influence of this driver is well established as confirmed by many studies. The indirect driver that is increasing anthropogenic CO2 levels in atmosphere is the burning of fossil fuels such as coal and natural gas. It is occurring regionally but has global impact and is well established in literature.

The initial regime would typically occur in cold climate conditions where the relationship between the ice growth/decline would be approximately evenly balanced or with a slightly increased ice growth. The two main mechanisms that maintain this regime are ice-albedo mechanism and meltwater-ice sliding mechanism.

Increasing CO2 levels in atmosphere - the key driver of the regime shift, initiates the increase of atmospheric temperatures and changes in albedo. As a result - increased absorption of solar energy promotes higher air, ice, water and land temperatures which leads towards degrading sea ice. Also the inland surface temperature increase can cause surface melting in the ablation zone that presently accounts for roughly half of the mass loss from the GIS (Parizek et al. 2004). Thus this driver indirectly is increasing drainage of meltwater feeding into crevasses close to the glacier margin resulting in higher calving rates (Murray et al. 2010). Furthermore, thinning and retreating of the glacier tongue due to these increased rates cause reduced effective pressures beneath the glacier, promoting faster flow that results in decrease of ice volume.  

The increase in surface air temperatures changes the ice-albedo feedback thermodynamics. This means that the heat exchange within the sea ice, as well as between the top and bottom of the ice is changed. This leads to a decrease in sea ice volume. The resulting increased amount of open land and water surface in summer decreases the albedo, as the dark ocean surface absorbs more solar radiation (Lindsay et al. 2005).  The annually integrated absorption of solar radiation is observed to increase when the surface albedo is relatively low (Rigor et al. 2002, Holland et al. 2006). This increasingly accumulated amount of heat on the surface reinforces the initial warming. Due to the loss of the ice-sheet volume as a result of the warming, Greenland territory in future could become free from permanent ice sheet cover. This would happen as a consequence of the negative relation to ice growth/ice decline during the winter and summer where the lost ice volume in summer could not be reproduced in the following winter. The Greenland without permanent ice sheet regime is characterized by other dominant feedback mechanisms. For example ice volume-wave action, the water temperature-density and meltwater-ice sliding velocity mechanisms.

The shift to the regime of Greenland without ice sheet will mainly result in loss of some desirable ecosystem services. The ecosystem service of desirable climate regulation could be lost as the change in movement of currents (change in thermohaline circulation) and air masses would alter the transport of heat. This could lead to increased hurricane activity, a southward shift of tropical rainfall belts with resulting agricultural impacts, and disruptions to marine ecosystems.

The loss of certain animal and plant food species as provisioning services is predicted in the future. These changes may have important consequences for food webs and could well be extremely significant for the Greenland economy, which is highly dependent on fisheries (AMAP 2007). Such cultural services like recreation and aesthetical values would also be affected. Each of those services attracts more people to see the Ice sheet thus also bringing in more tourists. Water regulation as regulating ecosystem services could be altered through the large input of freshwater in the water cycle.  The vast amount of "stored" water entering the water cycle within warmer climate would result in severe winter precipitation.

A new ecosystem service is possible as the thawing ice sheet will potentially form glacial freshwater lakes in Greenland. This will generate new recreation opportunities in summer – using lakes for different purposes from different social groups. Flora could expand deeper into Greenland and new species could be introduced as the climate warms giving the local population the chance to gain additional plant foods.

The potential options for preventing or reversing this potential regime shift mainly relates to the decrease of greenhouse gas input into the atmosphere at a global scale. This has to be achieved in order to prevent further climate warming leading to the loss of Greenland Ice Sheets. Options include reduction of deforestation, use of fossil fuels and charcoal as energy source, and cleaner economies. Geoengineering strategies has also been proposed, large scale experiments to decrease global temperature and CO2 concentration in the atmosphere. However, their applicability is debated and the usefulness is contested. As this system boundaries are set mainly around geophysical variables it is necessary to look at the social mechanisms involved to limit the influence on the main direct driver of greenhouse gas emissions. Nevertheless even if CO2 levels in atmosphere leads to atmospheric temperature increase, it is very hard to achieve from the local to regional management perspective. It does require global coordination and cooperation in order to achieve CO2 reduction targets.

Aquatic ecosystems, such as ponds, canals, ditches or tropical lakes can experience shifts between submerged and floating plant-dominated regimes when the concentration of nutrients (i.e. nitrogen and phosphorus) in the water column changes. The two possible regimes and their associated ecosystem services are:

Submerged plant-dominated regime

This regime is dominated by submerged plants such as Elodea nuttallii (better known as waterweed, a rapidly growing, long and stringy plant) that grow underwater in aquatic ecosystems such as ponds, canals, ditches or tropical lakes. They can be found in water bodies with a low water depth, clear water, low turbidity and weak water-level fluctuations. Typically, their roots are connected to the sediment pool. The regime of submerged dominated plants is also characterized by a low-nutrient concentration in the water column.

Floating plant-dominated regime

This regime is dominated by floating plants such as Eichhornia crassipes, Salvinia molesta, Lemna gibba or Pistia stratiotes found in aquatic ecosystems such as ponds, canals, ditches or tropical lakes. In many places these are invasive exotic species. These plants have their leaf surface exposed to the atmosphere and their roots are not connected to the sediment floor. Typically, a floating plant dominated regime is characterized by dark, anoxic and high nutrient conditions in the water column. 

Submerged to floating plant dominance

The main direct driver that leads to the shift from submerged to floating plant dominance is nutrient enrichment of the water body, typically associated with fertilizer and manure runoff from agricultural activities in the catchment. Strong water-level fluctuations associated with large rainfall events or water abstraction for irrigation and other uses can also enhance nutrient input from the shoreline.

This shift can also be caused by a change in water depth (associated with rainfall events and water abstraction) or turbidity (resulting from pollution, sediment runoff, and algal growth). These both directly affect the availability of light, which in turn affects photosynthetic activity and therefore the growth of submerged plants. The deeper the water body the darker it gets with increasing water depth. The more turbid the water body the less light available for photosynthetic activity in the deeper water layers. Floating plants are therefore in a better position to compete for in situ light than submerged plants. Hence, the deeper or more turbid the water body is, the more likely a shift to a floating plant dominated system. Moreover, the floating plant regime shift can be caused by the introduction of invasive species. Invasive floating plant species might grow very rapidly and aggressively outside their natural environment with the consequence that they might take over an entire water body by reducing the incidence of light, which in turn lead to a decrease of submerged plants. A famous invasive floating plant is Eichhornia crassipes which causes substantial problems in many parts of the world.

When nutrient concentrations in the water are realtively low, nutrients settle onto the sediment floor and are taken up by submerged plants. Submerged plants can grow at low nutrient concentrations in the sediment, and their growth removes nutrients from the system, keeping the nutrient concentration in the system low.

However, once the nutrient concentrations in the water exceed a certain threshold due to excessive nutrient input from eg fertilizer runoff, it becomes possible for floating plants to establish and grow in the water body. The roots of floating plants are not connected to the sediment pool, and are therefore dependent on the availability of nutrients in the water column in order to grow. 

The growth of floating plants in the water column decreases the amount of light that reaches the deeper water layers, and therefore reduces photosynthetic activity of submerged plants, eventually leading to their death. The loss of the submerged plants in turn destabilizes the nutrients trapped in the sediment floor, which become resuspended in the water column, further increasing the nutrient concentration in the water column and enhancing the growth of the floating plants. Once the system has shifted to a floating plant regime, the dark and anoxic conditions under floating plants leave little opportunity for submerged plant life, and it may be very difficult to restore the system to a submerged plant regime. 

Floating plant dominated systems decrease fisheries and plant life due to the dark and anoxic conditions under the leaf surface. They can also have a negative impact on navigation in lakes and canals, water purification and recreation opportunities. Moreover, floating plants can clog water supply pipes for households, ditches or even a river mouth. They can also complicate fishing. In addition, there is a greater risk of pathogens that can negatively affect human health in floating plant dominated systems. 

The main management option to enhance the resilience of the submerged plant regime is to control the nutrient levels in the system, and prevent the establishment of significant numbers of floating plants through a mechanical or chemical removal.

A drastic harvest of floating plants in a shallow water body that has some submerged plants and a not too high nutrient level can shift to a floating plant-dominated regime back to submerged plant-dominated regime (see also the Dutch Ditches Case Study). The amount of harvest needed for a shift is predicted to increase with the water nutrient level. Harvesting the floating plants allows light to penetrate to the deeper water layers and enables submerged plants to re-establish. Once re-established, the submerged plants can help reduce nutrient levels by absorbing nutrients for their growth and trapping suspended nutrients in the sediment.

This regime shift entails extensive change in biodiversity and soil structure in the Arctic tundra due to increased climate warming. To date shrub invasion has been the main indicator of a potential regime shift.

Arctic tundra regime

This regime is characterized by low atmospheric temperatures that enable the formation of a layer of permanently frozen subsoil (permafrost) consisting mostly of gravel and finer material. The ecosystem has a low rate of primary production due to impeded mineral nutrient cycling. The short growing season in summer influences the vegetation and its root systems by limiting vertical expansion. Only plant species (lichens, liverworts, mosses) that are adapted to sweeping winds and soil disturbance can survive in these conditions. The atmospheric cold and existing permafrost restrains the plant growth ensuring that their roots are shallow and surface parts are low growing due to the frozen ground underneath the permafrost. Snow does not accumulate keeping the soil temperature low thus not allowing for permafrost to thaw and release carbon into atmosphere keeping the atmosphere cold.

Boreal forest regime

This regime is characterized by flora that mostly consists of cold-tolerant evergreen conifers, such as the evergreen spruce (Picea), fir (Abies), and pine (Pinus), and the deciduous larch or tamarack (Larix). Before these species are established in a region, there is an increased canopy of early-successional species such as aspen and paper birch (Frelich et al. 1995). Dwarf shrubs (e.g., Vaccinium uliginosum, Ledum decumbens, Betula nana and Cassiope tetragona) and low shrubs (e.g., Betula glandulosa, Salix glauca) act as pioneers that colonize the tundra before trees are able to establish (CAVM Team 2003). The shift to boreal forest can occur over a long time period varying from several decades to centuries depending on the migration time lag for each species and the occurrence of favorable conditions for seedling establishment.

The main external driver of this regime shift is CO₂ emissions that increase climate warming (IPCC 2007), leading to changes in the composition and abundance of arctic vegetation, animals and soil structure (Olofsson et al. 2009). Anthropogenic activities that elevate atmospheric greenhouse gas concentrations are generally considered to be the primary cause for this driver. Carbon release from anthropogenic sources is projected to continue and increase during the coming decades (IPCC 2007). This global driver is well established and could be looked as irreversible in the scale of next hundred years.

The tundra regime is maintained by the low atmospheric temperatures that ensure low soil temperature and the existence of permafrost. This determines the lack of microbial activity and scarce nutrient availability therefore ensuring species composition that are only able to exist in such circumstances.

A shift from the tundra regime to boreal forest is initiated by the atmospheric warming that is caused by continuing increase of CO₂ emissions. The atmospheric temperature increases the period of time when microbial activity and thus nutrient availability is sufficient enough for shrubs and boreal forest pioneer species (birch etc.) to withstand the winter conditions.

As a result the increasing number of shrubs ensure that snow drifting creates deep drifts that surround and extend downwind from shrub patches. These drifts increase the insulation of the soil below. This insulation elevates the soil temperature thus further allowing microbial activity to continue during the frigid arctic winter. The elevated soil temperatures under the snow lead to permafrost degradation that release carbon trapped in tundra soils, thereby further contributing to climate warming.

Shift from Tundra to Boreal Forest

Tundra provides biodiversity for different animal and plant species that characterize tundra ecosystem. The shift from tundra to boreal forest is projected to occur over large geographic areas throughout the tundra zone, with substantial impacts on the landscape and biodiversity (Bonan 1992). Changes in vegetation are also likely to affect composition of foraging mammals and birds (Hinzman et al. 2005). In addition to modifying wildlife habitat, increased woody shrub abundance will make traversing tundra more difficult for caribou, subsistence hunters and communities that rely on caribou for food, as well as hikers. Herbivores such as reindeers and microtine rodents can also be influenced by the shift towards woody vegetation as they prefer lichens, dwarf shrubs graminoids and deciduous shrubs over tall woody shrubs (Sturm et al. 2005). Increased expansion of woody vegetation may therefore result in tundra species extinctions or radical decrease in numbers. The loss of the climate regulating ecosystem service is a major concern as permafrost thawing is reducing the carbon sink service.

For local populations the tundra provides traditional recreation opportunities and contains their knowledge and educational as well as spiritual and religious values, which would be lost with the regime shift.

Shift from Boreal Forest to Tundra 

The change in biodiversity could provide new ecosystem services such as timber production and different wild animals and plant foods. This would benefit loggers as they would increase their income from tree cutting and local communities from goods that come from forest.

In order to prevent or reverse this regime shift it is essential that the input of carbon to the atmosphere is reduced in order to prevent further climate warming. The suggestion of a planetary boundary of 350 ppm for CO₂ levels is a first step in this direction (Rockstrom et al. 2009). Increased understanding of the influence of climate change on the complexity of arctic systems is essential for informing the adaptation of social, economic, and cultural systems to the changes taking place in the Arctic (Hinzman et al. 2005). Technology transfer could be a good initiative from developed countries as they can provide more advanced technological solutions to developing countries to help accelerate the learning curve on GHG emissions. A global response particularly from developed nations that are using the majority of the world's resources on a per capita basis should be in place to deal with such complex systems. Nevertheless, discussions about desirable management strategies and outcomes will be essential as there will be stakeholders who stand to lose from this regime shift (e.g. caribou herders) while others may see the change as desirable and beneficial (e.g. timber production, game hunting).

Forest regime
Forests are ecosystems typically dominated by trees, perennial plants taller than 5 meters. Tropical forest includes moist and dry forests (MA, 2005). A mature tropical forest contain at least four layers: emergent layer up to 45 - 80 meters tall, the canopy among 35 - 45 meters tall, the understory layer and the floor layer. Such structure gives a variety of habitats that host roughly half of the known plants and animals biodiversity (MA, 2005).


Savanna regime
Savannas, on the other hand, are drylands dominated by a mixture of grasslands and shrublands. The canopy in savannas never closes, and the floor layer is dominated by grasses, especially C4 species. Savannas, dry forest and shrublands conform 40% of the world’s land area and host up to 42% of human population (Reynolds et al. 2007, Falkenmark and Rockström 2008). About 25% of drylands, including savannas, are covered by croplands and sustain 50% of world’s livestock (MA, 2005).

Shift from forests to savannas

The most widely recognized driver is deforestation and consequently fragmentation of forest landscape, which reduces rainfall and increases surface temperature (Da Silva et al. 2008, Nobre et al. 2009). Reduction of forest cover accelerates albedo effect, loss of evapotranspiration and roughness length (Sternberg 2001), activates fire feedback, changes ocean circulation and warms up sea surface in the Amazon case, and ultimately changes the spatial vegetation structure.

Deforestation and forest degradation is driven by a complex, case specific interaction of social and economic drivers. The most important reported drivers include agriculture expansion, infrastructure development, the logging industry and fast population growth (Geist and Lambin 2002, MA, 2005). For example, in the Amazon, illegal logging is a critical threat that besides its damage to the forest, bring with them secondary effects like expansion of hunting areas, slash-and-burn farms, mining, the establishment of new road networks and therefore more logging facilities. The MA (2005) reports that 70 countries have problems with illegal logging leading to national income losses of $5 billions and total economic losses of about $10 billion. By 2001, Laurance and Williamson (2001) reported that 80% of brazilian logging activity were illegal; however, government counterintuitively sponsored colonization through cattle ranching projects.

Climate change and global warming are expected to enhance the regime shift; and the loss of forest areas are expected to exacerbate climate change (Laurance and Williamson 2001, Bonan 2008). While Laurance and Williamson (2001) report that forests like Amazon apparently change from carbon sinks to carbon sources during ENSO events; Nobre et al. (2009) suggest that deforestation of Amazon may actually increase ENSO variability; and Bonan (2008) confirms that deforestation would enhance global warming by decrease of evaporative cooling and release of carbon dioxide.

Shift from forest to savanna

The expansion of agriculture, increase in population as well as deforestation reduces rainfall and increases surface air temperature. This has a direct affect on plants, reducing evapotranspiration and photosynthesis and decreasing the supply of water vapour. Cattle pastures and regrowth forest areas become increasingly prone to frequent fires, which plays a fundamental role in the shift from forest to savanna since it is a feedback that actually maintains savanna state.

When tropical forests are replaced by less vegetated cover like savannas, or ultimately by sand in deserts, net radiation at the top of the atmosphere decreases inducing subsidence that inhibits precipitation (Oyama and Nobre 2004). While in the tropics land clearing affects the water balance and as consequence warms up the climate, in boreal forest such clearing affects mainly albedo and as results cools down climate (Foley et al. 2005). The albedo feedback is strengthened by changes in land cover, typically induced by deforestation for logging or agriculture activities. Warmer temperature and drier atmospheres, such as in savanna regime, result in an increase in lifting condensation level feedback that reduces the opportunity of cloud formation and therefore the likelihood of rainfalls (Pinto et al. 2009). Plants that do not have enough water responds by reducing transpiration and photosynthesis, interrupting the supply of water vapor that contribute to the recycled component of precipitation (Oyama and Nobre 2004, Saleska et al. 2007). Less evapotranspiration blocks the inland propagation of cold fronts responsible for precipitation, increasing the dry season length (Oyama and Nobre 2003, Pinto et al. 2009).

Savanna vegetation is better adapted to dry environments. Grasses usually have C4 photosynthesis type, a chemical pathway that reduces water consumption and helps to cope with nitrogen or CO2 limitations. Evapotranspiration depends on soil moisture and biomass. Thus, for instance, droughts frequency or grazing reduce biomass, weakening in turn the feedback effect (Dekker et al. 2007, Dekker et al. 2010). In addition, the spatial distribution of rainfall is affected by both the land-cover type and topography. Cattle pastures and regrowth forest areas become increasingly prone to frequent fires. In such zones fire can be produced after few days of dry conditions. On the regional scale, fire smoke may reduce rainfall by trapping moisture and inhibiting raindrops formation (Laurance and Williamson 2001). Fire plays a fundamental role in the shift from forest to savanna since it is a feedback that actually maintain savanna state (Laurance and Williamson 2001, Hutyra et al. 2005)

Forest provides a wide range of ecosystem services. Besides being hot spots of biodiversity, forest provides soil and water protection, it prevents soil erosion, floods and landslides. For example, soil erosion may be 10-20 times higher on areas cleared of forest (MA, 2005). Depending on soil conditions, at the local scale forest can also regulate the hydrological cycles by increasing precipitation and decreasing evaporation. They regulate below grown runoff and smoothing seasonal extreme events: heavy rainfalls or dry spells. Due to its regulating function in the water cycle, forested watersheds provides water supply to one third of the world’s largest cities (MA, 2005). 

Forest sustain about 200 million people belonging to indigenous groups, who depend on forest not only as source of resources (food, fiber, fuel) but also their culture and religious traditions (MA, 2005). Forest also maintain the agroforestry industry which, including temperate forests, produces 3.3 billion cubic meters of wood (MA, 2005). 

Managerial options for the forest to savanna regime shifts requires targeting the main drivers: deforestation and landscape fragmentation. Controlling illegal logging and implementing sustainable logging plans are part of the strategy. Sustainable logging needs to take into consideration reducing fragmentation and allowing deforested patches to regrow. Likewise, the expansion of agricultural frontier and grazing areas needs to be controlled and when unavoidable, it needs to be planned in order to prevent fragmentation. The fire frequency feedback accelerates the shift from forest to savanna regime. Laurance and Williamson (2001) suggest fundamental changes in prevailing land-use practices and development policies to avoid wildfires. Such changes include the management of logging and grazing areas in order to reduce fragmentation and therefore it would reduce the fire risk. Hence, fire and fragmentation management need to be coupled strategies.

Kelp forests are marine coastal ecosystems dominated by macroalgae typically found in temperate areas. This group of species forms submarine forest with three or four layers, which provides different habitats to several species. Kelp ecosystems are important to maintaining important industries such as, lobster and rockfish fisheries, tourism based on fishing, recreational diving and kayaking, as well food and pharmaceutical products derived from kelp (e.g. tooth paste).

Three different self-reinforcing regimes can be identified:

Kelp forests are highly productive ecosystems dominated by canopy-formed algae in cold-water rocky marine coastlines. Among the biota associated with kelp forest are marine mammals, fishes, crabs, sea urchins, mollusks, other algae and biota that live on the kelp themselves (Steneck et al. 2002). At least 4 trophic levels are found in kelp forests. Apex predators such as sea otters, cod, pollock, hake, and haddock are common. These predators regulate populations of species at lower trophic levels. In particular the regulation of sea urchins populations, who are an important consumer of kelp, is important to maintain kelp forests.

Urchin barrens are an alternative regime in which there is no kelp forest and substantial populations of urchins on the sea bed. This regime has low or no populations of apex predators and large populations of herbivores that keep macroalgae at very low population levels, preventing the regrowth of the kelp forest, an important habitat for apex predators (Steneck et al. 2004). Ecosystem services related to apex predator fisheries and recreational activities are substantially reduced. However populations of bottom dwelling species can increase. In some places urchins have themselves become a valuable fishery. While in others, populations of other harvested species such as lobster and crab have increased.

Turf-forming algae regime is similar to urchin barrens in that there is no kelp forest, however rather than dominance by urchins the seabed is dominated by turf forming algae. This regime also lacks apex predators (Gorman et al. 2009). Ecosystem services related to fisheries and recreational activities are substantially reduced. However, populations of invertebrate species such as lobster and crab can be higher. This regime shift is associated with a change in habitat in shallow marine coastal ecosystems. 

This regime shift is driven by two key direct drivers: reducing populations functional groups (Steneck et al. 2002; Steneck et al. 2004) and input of nutrients (Gorman and Connell 2009). Reductions in populations of key functional groups is due to the hunting of sea otter and the fishing of large apex predator fish species. Overfishing usually diminish the controlling function of top predators, and in severe cases, the trophic levels may be simplified. Nutrient input is often due to the output of wastewater from urban settlement and agriculture in nearby coastal areas. However it can also be due to naturally occurring offshore upwelling events. Nutrients favors the growth of turf algae over kelp. Strong rain events and floods can produce shock events for kelp ecosystems by providing a pulse input of nutrients and turbidity, which can stimulate the growth of turf algae.

Other drivers can also contribute to this regime shift. Pollution discharges and sedimentation may play a synergetic role as stressors. In Tasmania for example, global warming has favored the reproduction of urchins which acting in synergy with lobster fishing has reduced kelp resilience (Ling et al. 2009). El Niño events and global warming can generate water stratification. As a consequence, nitrogen concentration declines and kelp forest growth becomes limited by the availability of nitrogen (Steneck et al. 2002).

Shifts from kelp canopies to other regimes like urchin barrens and turfed landscapes have occurred in many regions worldwide (Steneck et al. 2002; Gorman and Connell 2009; Ling, et al. 2009). This regime shift is associated with a change in the biophysical structure in shallow marine coastal ecosystems. Kelp forest dynamics often include cycles of disturbance-recovery. However, increasing human-dominated coastlines modify environmental conditions that facilitate other regimes like turfs or urchin barrens, which in turn inhibits the reestablishment of kelp forests.

Overfishing and the input of nutrients are the main drivers of the regime shift. At least 4 trophic levels are found in kelp forests, and apex predators such as sea otters, cod, pollock, hake, and haddock are common. Overfishing usually diminish the controlling function of top predators, and in severe cases, the trophic levels may be simplified. In addition, urchins are prevented from grazing in the kelp forest through the effect of kelp foliage sweeping over the rocks due to its flexibility and the force of waves (Steneck et al. 2004). 

Overharvesting of sea otter, cod and haddock can release their prey populations in particular sea urchins from their predatory controls (Steneck et al. 2002; Steneck, et al. 2004). Large populations of urchins form grazing fronts that graze on kelp forest to its elimination (Lauzon-Guay et al. 2009). These populations of urchins can survive in adverse conditions by feeding on turf-forming algae. In some cases the removal of apex predators allows populations of other species to reach sufficient population sizes to exert some control on urchins, for example lobsters and crabs in the North Atlantic (Steneck et al. 2002). Therefore, in some cases the formation of urchin barrens open the opportunity for new fishing industry to establish, like urchin, lobster and crabs. In addition, declines in predatory fish have created a market for urchins, especially in Japan, establishing a new human induced control on the herbivore. In some cases, harvesting of urchins for this market has led to the re-establishment of kelp forest, despite urchin fishery being prohibited (Steneck et al. 2002; Scheffer 2009).

On the other hand, the turf-forming algae regime is maintained by its ability to persist under conditions of elevated nutrients, frequently attributed to coastal urban settlements, inhibiting the recruitment of kelp species (Gorman and Connell 2009). The loss of kelp dominated areas undermine kelp's ability to reestablish in disturbed areas (Gorman and Connell 2009). While deforested areas surrounded by kelp patches are more likely to return to the kelp regime, isolated kelp disturbed patches are more likely to stick in the turfed regime.

Shift from kelps forest to urchin barrens or turfs

The main ecosystem impact of the loss of kelp forests is the loss of habitat complexity. Kelp is a three-dimensional structure that offers shelter and food for many species; urchin barrens and turfs do not have such characteristics. This loss is associated with the reduction of the food web complexity and loss of functional groups (Steneck et al. 2004), with varying effects on fisheries. Some valuable fish species may diminish since kelp forests provide nursery areas. Invertebrate species such as lobster and crab can increases in population (Steneck et al. 2002). Along with hosting high marine biodiversity, kelp forests provide ecosystem services related to recreation for divers. In addition, kelp supports a multi-million dollar industry of canopy-cropping for alginates (Steneck et al. 2002). This product is commercially important in pharmaceutical and chemical industry. These services can be reduced or lost by this regime shift.

The ecosystem service impacts of algae turfs are likely to be similar to those related to coastal eutrophication. Such effects include abundance of rich-nutrient environment species as shellfish, bad odors and the associated consequences for recreational and aesthetic values.

Shift from urchin barrens or turfs back to kelp forest.

The recovery of kelp forest increases habitat complexity and is expected to increase biodiversity. On the other hand, the benefits gained by urchin and lobster fisheries may be diminished, since the recovery of functional groups and increasing biodiversity will control their populations. However, other species of commercial fisheries and tourism may recover, although it is not always the case.

Options for enhancing resilience

Restoring biodiversity in functional groups in kelp forests is particularly important to maintain apex predators (Estes et al. 2011). A high profile version of this strategy has been implemented to recover sea otter populations on the west coast of North America, by protecting sea otter populations (Steneck et al. 2002). Reducing nutrient runoff from agricultural areas is essential for enhancing resilience, as well as improving water treatment and managing runoff in urban areas (Gorman et al. 2009). The removal of sea urchins along with implementing measures to maintain the abundance of predators like cod, sea otters and sheep-head through fishery controls may provide stability to kelp forest ecosystems (Steneck et al. 2002; Steneck et al. 2004).

Options for reducing resilience to encourage restoration or transformation

Transplantation of kelp-forming species has been proposed as a restoration strategy in highly fragmented seascapes. This strategy is likely to increase the supply of kelp propagules to colonize new areas and maintain free substratum for canopy recruitment (Gorman and Connell 2009). However this strategy has not been tested.

Bivalve mollusks play an important role in aquatic ecosystems by filtering and sequestering nutrients. When bivalve abundance changes, it can have substantial ecosystem impacts, creating two different, self-reinforcing regimes.

Bivalve mollusk reef regime

When aquatic ecosystems have high bivalve mollusk density, they form reefs that include many bivalves and are shallow, nearing the water's surface. The filtering activity of the bivalves results in clear water with high levels of dissolved oxygen. Plankton populations are limited in these conditions. The bivalves themselves operate as reinforcing feedbacks in that their abundance ensures sufficient filtration to maintain this clear water regime. The clear water in turn reinforces bivalve abundance by maintaining hydrodynamics that are conducive to bivalve health (Scheffer 2009). Large, shallow bivalve mollusk reefs encourage biodiversity by providing habitat and filtering nutrients.

Isolated, low density bivalve mollusk regime

When aquatic ecosystems have low bivalve mollusk density, the reefs are small and deep. Water is turbid, with low levels of dissolved oxygen. Plankton and filamentous algae flourish under these conditions. Poor reef size and water filtration limit biodiversity. The low bivalve abundance state is reinforced as bivalve health and fecundity is weakened by turbid water conditions. The turbid water conditions are in turn reinforced by low bivalve abundance(Weijerman et al. 2005, Powell et al. 2008).

The regime of low bivalve abundance is expected to reduce fish populations, water cleansing and the ability of the ecosystem to maintain biodiversity. Such effects would have an impact on fisheries, human health, and recreation. 

Mechanized anthropogenic over-harvesting of bivalve mollusks is the main driver of the shift from high to low abundance (Thrush and Dayton 2002).  High inputs of nutrients from agricultural or urban sources can act as a slow driver that weakens bivalve health by creating plankton blooms that the bivalves are unable to filter.  Plankton blooms can further weaken the health of bivalves by producing organic matter, whose decay, reduces oxygen availability.  A disturbance that can trigger a regime shift is bivalve disease (Powell et al. 2008).  When populations are weakened by eutrophication and anoxia, they become physiologically susceptible to diseases that can rapidly devastate remaining populations (Leniham 1999). 

Bivalves are a class of mollusks containing over 30,000 species which include scallops, clams, oysters, and mussels. Bivalve mollusks, like corals, are often referred to as "ecosystem engineers" because they engineer the physical and hydrodynamic structures of the ecosystems they occupy in a way that produces conditions that support their own population growth. Bivalves live on the sediment surface, where they form reefs that provide increased surface area and habitat for a variety of species. They filter nutrients from the pelagic zone and transfer it to the benthic zone, maintaining the system in a clear water state. If bivalves are not able to maintain clear water, due to declines in bivalve populations or increases in low water quality, the ecosystem can shift to a turbid water state. The main drivers of this shift is direct anthropogenic over-harvesting of the bivalves, followed by a gradual increase of nutrients discharge from agricultural activities and coastal urban settlements.  Once mollusk abundance is reduced, plankton and filamentous algae populations can substantially increase. As result, oxygen and light available in the water column is reduced, while nutrients levels accumulate due to the lack of filtration.  These processes produce conditions which favour algae over bivalves.  If nutrient inputs are substantial, eutrophication and areas of hypoxia (low oxygen) are likely to appear in areas dominated by plankton instead of mollusks. 

Shift from high to low bivalve mollusk abundance regime

The loss of abundant bivalve mollusk reefs threatens diverse ecosystem services. The most direct impact is the loss of valuable shellfisheries. A secondary, but potentially substantial impact is the loss of the filtering service provided by bivalves.  In urbanized estuaries, it can necessitate the installation of costly synthetic water filtration technology (Gren et al. 2009).  In other areas it can lead to declines in water quality that shift populations and reduce recreation opportunities, and potentially the value of waterfront property.

The loss of bivalve habitat structure leads to lower species abundances, and to declines in species richness (Airoldi et al. 2008). Both structural and functional biodiversity is threatened by loss of bivalve abundance, with far reaching effects on marine ecosystems and human ability to exploit such systems (Worm et al. 2006).  The regime of low bivalve abundance is expected to reduce fisheries, water cleansing and the ability of the ecosystem to maintain biodiversity. Such effects would have an impact on fishing productivity and human health.

The loss of bivalve nutrient filtration can lead to other regime shifts, such as eutrophication and hypoxia, which can produce unproductive and undesirable ecosystems (see Hypoxia and Eutrophication regime shifts). 

Options for enhancing resilience

Estuaries are the world's most degraded marine ecosystems, receiving land-based pollution from crop cultivation and urbanization, and suffering from anthropogenic over-harvesting (Lotze et al. 2006).  The resulting problems are complex, impacting bivalve health and fecundity as well as many other species, with far reaching implications in both biological and social domains.

Experience has shown that management focused on one species or problem tends to be ineffective.  Newer research stresses the need to address natural resources as part of complex social-ecological systems, often with long histories of human exploitation (Jackson et al. 2001).   Rather than addressing problems on a species-by-species basis, multi -species management has shown to have synergistic effects.  For example, in Chesapeake bay, fisheries management looks at oyster, blue crab, striped bass and shad together (Boesch 2004).

Taking this approach further, Ecosystem-based fishery management efforts focus on recognizing interactions between multiple species and environmental stressors, such as low dissolved oxygen levels. Success is measured by the degree to which management efforts include ecosystem-based approaches, rather than by an assessment of fishing stocks (Lotze et al. 2006). 

Options for reducing resilience to encourage restoration or transformation

The main option exercised for preventing or reversing a regime shift regarding bivalve abundance is to import bivalve mollusks from another region for aquaculture (Van de Koppel 2008) .  Another practice commonly used is to hang bivalves from the surface or build artificial reefs to elevate bivalves in order to avoid the hypoxic conditions in deeper water (Carlsson et al. 2009). 

Savannas are systems that consist of a mixture of woody vegetation (trees or shrubs) and grasses. Savannas, dry forest and shrublands cover 40% of the world's land area, host up to 42% of the world's human population (Reynolds et al. 2007, Falkenmark and Rockström 2008), and together with drylands sustain 50% of the world's livestock (Millennium Ecosystem Assessment 2005).

At small scales (up to about 10 km²) savanna systems, especially those used for extensive cattle ranching, may stabilize in two different self-reinforcing regimes (Walker 1993, Scheffer et al. 2001, Scholes 2003):  

Open, grassy savanna regime

In this regime the landscape has a productive grass layer with few mature trees. The canopy in savannas never closes, and the floor layer is dominated by grass, especially C4 species. Most young trees are unable to establish because, while often numerous, seedlings are constantly knocked back to ground level by herbivory and fire. There is enough grass after grazing to support a fire with flame-length taller than the young saplings sufficiently often to keep them in a 'fire trap' (Dublin et al. 1990, Roques et al. 2001b, a). Open savanna systems are suitable for ranching, and are maintained by fire dynamics and grazing.

Closed, woody savanna regime

In this regime the landscape is dominated by woody shrubs or trees. Once established, woody vegetation is stable because adult trees are seldom killed by herbivory or fire. These alternate regimes can occur at a range of spatial scales. Sometimes larger areas (e.g. an entire cattle ranch) may shift from a grass-dominated to a persistent woody-dominated state (Dublin et al. 1990, Walker 1993). In other cases, the alternate regimes are expressed as a mosaic of small patches of trees or bush interspersed with patches of grass, where the respective patches are highly persistent over time (Rietkerk et al. 2004).   

Bush encroachment refers to a shift from a grassy system to a persistently woody system. It typically occurs in areas used for free-range cattle ranching, and is usually caused by a combination of grazing and fire management practices. Bush encroachment involves a change in the outcome of the competitive interaction between woody vegetation (shrubs and trees) and herbaceous vegetation (grasses and herbs), mediated by nutrients, grazing, fire, rainfall variability and use of either the woody or grassy components by humans (Anderies et al. 2002, Janssen et al. 2004, Wiegand et al. 2006). The encroachment typically occurs in episodes rather than continuously, and involves a particular set of encroaching species rather than the entire woody community.

Bush encroachment typically occurs in areas used for commercial cattle ranching (as opposed to subsistence, communal, or nomadic) and may follow episodes of sustained severe overgrazing, though not necessarily so. It may also occur under other land uses (Wiegand et al. 2006). It tends to be an episodic phenomenon, where the tree cohorts can often be linked to issues in the ranching enterprise – such as drought-induced debt or downturns in the cattle price cycle (Scholes 2003, Wiegand et al. 2006). 

There are several different hypotheses regarding the mechanism by which bush encroachment occurs. Different mechanisms (or combinations of mechanisms) may be important in different places. One proposed mechanism is based on changes in fire regime: in the sustained presence of high numbers of grazers (typically cattle) accumulation of grass fuel is reduced, leading to period without intense fire long enough for woody plants to grow beyond the fire-susceptible stage, which in turn suppresses grass production and fires, further enhancing the establishment of woody vegetation (Higgins et al. 2000, Staver et al. 2009, Staver et al. 2011). A related hypothesis notes the elimination of browsers (especially very large browsers such as elephant and giraffe, but also the more-numerous small browsers) from the system when cattle are introduced (Dublin et al. 1990). Similarly, alien species, such as Prosopis in South Africa or Acacia nilotica in Australia, both deliberately introduced, can play an important role in bush encroachment by affecting fire regimes (Poynton 1990). Another hypothesis focuses on changes in water availability based on the rooting depths of plants: grasses are thought to be more shallowly-rooted than trees, so if grass cover is reduced by overgrazing, this is more water available for trees, which promotes their growth and establishment, further suppressing grass growth (Noy-Meir 1982). Refinements of these hypotheses emphasize combinations of events, such as a multiyear drought or fireless period providing a "window" for the establishment of trees (Wiegand et al. 2006).

Yet other hypotheses focus on the role that increases in global CO2 levels may play in the observed proliferation of woody plants in many, widely-separated areas of the world during the 20th century. The underlying mechanism is still debated, but several possibilities have been proposed: i) that rising CO2 levels favour C3 (woody plant) photosynthesis relative to C4 (tropical grass) photosynthesis  ii) elevated CO2 may reduce transpiration of grasses, leading to greater water percolation and therefore favoring deeper rooted woody species, iii) faster growth of woody plants due to CO2 enrichment, and therefore faster escape of seedlings from susceptibility to fire, iv) investments in carbon-based defense compounds such as tannins, which are the main defense compounds in many encroaching trees but not in grasses (Midgley and Bond 2001, Wiegand et al. 2006). It is striking that encroachment is almost unheard of on communal land, and is not universal on commercial farms. A suggested explanation for this is that communal lands use the trees for firewood and run goats along with cattle, inhibiting the establishment of trees (Scholes 2003). Bush encroachment has been documented in East and Southern Africa (but not West Africa), South America (Uruguay/Argentina and Chile), North America (Texas, New Mexico) and Australia, but not in India, also a savanna environment. Furthermore, it did not happen simultaneously in those places, but 30-50 years after the widespread establishment of sedentary grazing management, what has been referred to as 'commercial' ranching above. This tends to disfavor the rising CO2 hypothesis, although rising CO2 may predispose the shift (Midgley and Bond 2001).  

Shift from grassy to woody savanna

Woody encroachment brings a relatively rapid change, over a decade or two, from a highly productive grass layer to a sparse and unproductive grass component. Since cattle are grass-eaters, this change substantially reduces cattle productivity (Anderies et al. 2002, Scholes 2003), with major impacts on cattle ranchers. On the other hand, encroachment increases the supply of tree-based ecosystem services, such as wood for fuel, charcoal-making and building material. This is somewhat dependent on the species involved. The increase in woody cover could potentially also have macro and micro-climatic effects through impacts on albedo and CO2 uptake, in addition to the decrease in methane emissions from cattle.

Difficulties in mustering the cattle in dense bush are a contributing factor. Therefore, wood encroachment leads to economic losses for cattle ranchers in what is frequently an economically marginal area for other agricultural uses such as croplands. 

Options for enhancing resilience

There is some agreement among researchers and extension workers that encroachment can be avoided by stocking lightly and burning frequently to prevent the establishment of trees and maintain grass crowns - the productive part of the grass that is less affected by fires (Roques et al. 2001b, Janssen et al. 2004). However, this is seldom reflected in management practice. 

Options for reducing resilience to encourage restoration or transformation

Bush encroachment is expensive to reverse, since rapid results rely on arboricides or repeated mechanical or manual clearing. A common method involves the manual removal of woody vegetation, with repeated follow-up control and the use of fire to enhance the establishment and competitive advantage of grasses (Scholes 1985, 2003). Attempts to reverse bush encroachment often have poor results, either due to the rapid resprouting of the trees or the conversion of the grass layer to less desirable species in the process. 

There are anecdotal reports of widespread mortality of near-dominant encroaching species after several decades, possibly related to disease, prolonged drought or simply old age, which provides windows for grass establishment and fuel load for intense fires.

The hypoxia regime shift involves a radical change in oxygen concentration in aquatic ecosystems such as rivers, lakes and marine ecosystems. The severity and persistence of hypoxic conditions varies. Episodic oxygen depletion represents 17% of known hypoxia cases, and occurs infrequently with several years sometimes elapsing between events. Episodic oxygen depletion is the first signal that a system has reached a critical point of eutrophication, which in combination with physical processes that stratify the water column, tips the system into hypoxic conditions (Diaz and Rosenberg 2008). Seasonal hypoxia tends to occur periodically during the summer, after algal spring blooms have sunk to the bottom and are being decomposed. It lasts from days to weeks, represents half of the known dead zones, and typically abates in the autumn (Diaz and Rosenberg 2008). Boom-and-bust cycles of animal populations are frequent. Persistent hypoxia occurs when hypoxia becomes persistent due to the build-up of organic matter in the sediments over time, particularly in systems prone to persistent stratification. Persistent hypoxia accounts for 8% of reported hypoxia cases. Anoxia occurs when DO levels fall below 0.2 ml per litre. The accumulation of organic matter is exacerbated, and poisonous hydrogen sulphide (H₂S) is released due to microbial metabolism.

Normoxia: This regime is characterized by normal levels of dissolved oxygen, typically 5 to 8 ml per liter. Most aquatic organisms are adapted to survive under these conditions. Particularly important are the benthic organisms. Benthos are a community of organisms which live in sedimentary environments at the bottom of water bodies. They constitute an important key functional group (scavengers and detritivores) in the food web responsible for the decay of dead matter. Macrobenthos facilitate bioturbation processes, in other words, they displace and mix particles facilitating chemical exchange between sediments and the water column. This allows the sediments to "capture" phosphorous and nitrogen from the water column. By removing these nutrients from the water column, bioturbation therefore reduces algal growth and helps ensure that the water remains oxygenated and suitable for the survival of benthos - creating a reinforcing feedback that maintains normoxia conditions.

Hypoxia: The hypoxic regime is reached when dissolved oxygen level falls below 2 ml per liter (Diaz and Rosenberg 2008). Hypoxia is associated with so-called "dead zones". These are areas where dissolved oxygen levels are so low that most life is not able to persist and only very few specialized microorganisms survive. In the hypoxic state, accumulation of organic matter produced by eutrophic processes favors the growth of microbes who, by decomposing the organic matter, consume the oxygen in the water column.

Anoxia: Anoxia occurs when hypoxia is exacerbated to DO levels under 0.2 ml per liter (Diaz and Rosenberg 2008). Under such conditions, benthic animals suffer from mass mortality and decay processes are carried out by microbial metabolism. As A result, hydrogen sulfide is released changing water acidity (pH) and then further favoring bacteria habitat. 

Hypoxia is driven by increasing nutrient input, both natural and anthropogenic. Natural sources come from the bottom of the ocean transported by upwellings, water currents that flow vertically driven by the interaction of winds and the temperature gradient between the ocean surface and bottom. Anthropogenic sources are related mainly with the use of fertilizers in agriculture. However, growing urban settlements in coastal areas also increase the water storm runoff and sewage.

The leakage of nutrients from agriculture is further exacerbated by rainfall variability and deforestation, since vegetation offers resistance to erosion and trap nutrients and moisture in the soil. Agriculture is driven in turn by increasing demand of food and fibers as well as population growth. Another important factor leading to hypoxic conditions is the stratification of the water column, which reduce the exchange of nutrients between the surface and the bottom. Water stratification is usually driven by anomalous increase in sea surface temperature. 

The fast variable in the hypoxia regime shift is DO, and can also include faunal composition. The slow variables are the accumulation of nutrients, mainly phosphorous and nitrogen in the water column. Large areas of the sea are oxygen minimum zones due to natural conditions. Hypoxic-prone areas generally have a semi-enclosed hydrogeomorphology and water-column stratification that restrict water exchange and reoxygenation of the deeper water layers (Diaz and Rosenberg 2008). Another natural condition that facilitates hypoxia and eutrophication is coastal upwelling, that lead to nutrient enrichment along continental margins.

The normoxia regime is usually  maintained by feedbacks that control the nutrients content in the water column and keep oxygen concentrations high. Zooplankton for example plays an important role controlling micro algae numbers, hence limiting their ability to use nutrients available and reduce oxygen through their metabolism. Macrophytes or macro algae also control micro algae by consuming the same resources. Under such conditions nutrients are kept on inorganic forms unavailable for micro algae to use.

Under the hypoxia regime the feedbacks above are weakened, allowing micro algae to escape predation and competition controls; and reducing the level of DO in water. As DO decreases, mass mortality of different organism increases the availability of organic matter in the water column, decreasing DO and promoting more micro algae growth. When DO reaches critical levels under 0.5mL per liter, hydrogen sulfide is released further decreasing pH in water and reducing biodiversity. Such extreme regime is called anoxia. 

Shift from normoxia to hypoxia and anoxia

Normoxia is associated with the provision of food for humans and wildife in aquatic systems as well as regulating services such as water purification and pest regulation. This regime shift into hypoxia, often characterised as a dead zone, has been reported in more than 400 systems affecting more than 245,000 square kilometers and including important fisheries such as the Baltic Sea, Kattegat, Black Sea, Gulf of Mexico, and East China Sea (Diaz and Rosenberg 2008). A major impact on ecosystems is a change in the flux of matter and energy through trophic levels. Consequently, fisheries and hence ecosystem services such as food production are affected. For example, Diaz & Rosenberg (2008) report that biomass in the Baltic Sea has been reduced by approximately 264,000 metric tons of carbon due to hypoxic conditions. Assuming that ~40% of benthic energy passes up the food chain, 106,000 metric tons of carbon of food energy for fisheries has been lost(Diaz and Rosenberg 2008). This implies a reduction in yields and consequent impact on employment in fisheries communities. Another example of such effects is the lobster fishery collapse in Norway (Diaz and Rosenberg 2008).

Besides its effect on fisheries and employment, hypoxia also affect the health and other cultural services of the people living near by dead zones. Decaying matter after mass mortality events create odors and risk of diseases. Recreation, aesthetic and touristic values are lost.

Shift from Hypoxia to Normoxia

When hypoxia is not severe, the system is likely to return to normoxia regime. It implies the recovery of fisheries when species hasn't been extinct locally. However, food webs not always fully recover, and usually top predators are lost. With the recovery of the system, fishing, recreational and touristic activities recover too, although not necessary to the same configuration than before the regime shift. 

Options for enhancing resilience

Since one of the main drivers of hypoxia is eutrophication, Diaz & Rosenberg (2008) recommend managing the input of fertilizers on agricultural land. They recognize the necessity of developing new methods to close the nutrient cycle on farms, in order to avoid the drainage of nutrients to water sources. Nutrient reductions can also be achieved relatively cost-efficiently by improving waste-water treatment system in regions where this is applicable (e.g. Baltic Sea).

Options for reducing resilience to encourage restoration or transformation

Only 4% of the reported cases of hypoxia have shown improvement, principally due to the reduction of organic and nutrient loading, stratification strength, and freshwater runoff (Diaz and Rosenberg 2008).

Managers could take advantage of windows of opportunity provided by these variables. For example, in the Baltic Sea the stratification of the water column is determined by input of saltwater from the North Sea (Conley et al. 2009). A policy of nutrient load reduction would therefore be more effective in years when saltwater input is low and it is a particularly rainy year. While nutrient loading is largely periodic and rain dependent, stratification and freshwater runoff rely more on physical processes and climate variability. Finally, it has been suggested that an appropriate goal is the reduction of nutrient loads to the level of the mid-1900s (Diaz and Rosenberg 2008). 

Thermohaline circulation is complex, but can be summarised as a global scale deep overturning of water masses, with sinking motions occurring in the northern Atlantic and around Antarctica and a transport of relatively warm surface waters towards these sinking regions (Steffen et al. 2004). The term thermohaline circulation thus refers to a particular driving mechanism; it is a physical, not an observational concept.

Strong Thermohaline circulation

THC regime can be characterized by the formation of deep water currents in the high latitudes of both hemispheres; the spreading of deep waters, as deep western boundary currents; upwelling (or upwards movement) of deep waters which is thought to take place mainly in the Antarctic Circumpolar Current region, possibly aided by the wind, and near-surface currents, required to close the flow (Rahmstorf et al. 2006).

Collapse of the Thermohaline circulation

This regime is characterized by weak formation of the dense and saline high latitude deep waters due to the increased influx of freshwater. The decreased salinity of the deep waters is weakening the density southwards driven water movement. The transport of less dense and saline waters from south towards north is decreased causing additional atmospheric temperature increase in tropical areas.

Increasing greenhouse gas concentration from anthropogenic sources is predicted to cause a rise in global mean temperatures (Cubasch et al. 2001). One of the most common anthropogenic greenhouse gases is carbon dioxide (CO2). It is considered to be the main driver for this regime shift and is typically operating in global scale due to the global input of CO2 in the atmosphere. Increasing CO2 levels in atmosphere increases the mean atmospheric temperature. This results in weakening ice-albedo mechanism for the initial regime as a result of increased atmospheric temperatures and changes in albedo.

The increasing freshwater influx from sea-ice loss is accelerated by freshwater-overturning and precipitation-river runoff. This leads to an in increased evaporation, river runoff, and changes to water salinity and density. The burning of fossile fuels such as coal and natural gas also drives this regime shift. This occurs in a regional scale but has global impact and is well established in the literature.

In the regime of strong THC, the most simple explanation of the main variable that maintains the regime would be the high-latitude cooling. In the deep ocean, the predominant driving force of movement of currents is differences in density, caused by salinity and temperature (the more saline the more denser and the colder the water, the more dense). The ocean's density distribution is itself affected by currents and mixing. Thermohaline and wind-driven currents therefore interact in non-linear ways and cannot be separated by oceanographic measurements. Changing the wind stress will alter the THC; altering thermohaline forcing will also change the wind driven currents.

The collapse of THC occurs due to increasing CO2 levels in atmosphere that slowly enhance the mean atmospheric temperature. As a result the high latitude cooling is weakened, increasing freshwater influx from the loss of ice, both from the Arctic, Greenland and possibly the north of Canada. This strengthens two main mechanisms for the new regime: freshwater-overturning mechanism and the precipitation-river runoff mechanism. Both increase the fresh water inflow into the ocean. Therefore new mechanism such as increased evaporation, river runoff, changes in water salinity and density are activated possibly reinforcing the new regime. Once the system has shifted, the warm conditions in Northern latitudes maintain a weak formation of the dense and saline high latitude deep waters due to the increased influx of freshwater.

Most ocean–atmosphere models predict a reduction of the THC for a warmer climate in the near future. Recent observational evidence provides some support to the model projections that the North Atlantic THC circulation could weaken or shut down later this century (Steffen et al. 2004). Lenton et al. (2008) report that the IPCC models predict the regime shift to happen after 2100, however, their models do not include freshwater runoff which is expected to accelerate the process. The models that project the collapse of THC under greater perturbations identify that the collapsed state can last for centuries and might be irreversible (Delworth et al. 2008). Nevertheless the latest IPCC report shows that it is very unlikely that the THC will undergo a large abrupt transition during the course of the 21st century (IPCC 2007). Some models show a recovery of the THC after the elimination of the freshwater perturbation whereas others do not. The mechanism for this different dynamical behavior of the THC is not completely understood (Stouffer et al. 2006).

The weakening of THC will impact global water regulation as water circulations in global scale are influenced by the changes in temperature, salinity and density. In response, climate regulation is altered and changes as the transport of heat is distributed in a different pattern. The water mass and heat circulation will no longer transport warm currents in a northly direction. This could increase the occurence of hurricanes, a southward shift of tropical rainfall belts with resulting agricultural impacts, as well as disruptions to marine ecosystems. Provisioning services such as fisheries, livestock and food crops are therefore potentially affected. Schmittner et al. (2007) show that changes in Atlantic circulation can have large effects on marine ecosystems and biogeochemical cycles, even in areas remote from the Atlantic, such as the Indian and North Pacific Oceans. Although loss of THC could affect Western Europe and result in colder climates. 

Food crisis and emigration are potential threats in the regions that are subjected to this regime shift (Barnett & Adger 2007). In addition, regional changes in sea level are predicted to rise of up to 80 cm in the North Atlantic (Knutti et al. 2002). This could affect the coastlines of the United States, Canada, and Europe causing coastal erosion and affect the security of housing and infrastructure (Delworth et al. 2008). The marine biodiversity of flora and fauna will also be affected. The change in salinity and temperature may facilitate the introduction and spread of invasive species that are able to thrive in the new conditions. This could affect the livelihoods and economic activity of local fishermen as they would have to adapt to new circumstances.  

This regime shift is mainly driven by the increase in greenhouse gases into the atmosphere at a global scale. Therefore a decrease in CO2 emissions and other greenhouse gases has to be achieved in order to prevent further climate warming leading to large influx of freshwater from the depletion of Arctic ice. In the best-case scenario, it may be possible to avert a regime change in a situation where the underlying driver(s) of change are amenable to fast manipulation through management, and where there are not substantial lags in ecosystem response. In the case of the THC neither of these factors hold: the underlying causes of global warming appear very difficult to address through rapid changes in management, and there are substantial inertias in ecosystem response to changes in the driving factors. Therefore an early warning indicators would be useful in providing advance warning of a substantial coming change in ecosystem conditions, but is unlikely to be useful in averting the regime change.

The Greenland Ice Sheet (GIS) is approximately 1.7 million km² in areas covering approximately 80%  of Greenland. It is grounded on bedrock that mostly rests near or above sea level thus would contribute to the globally averaged sea-level rise of 7.3 m if melted completely (Parizek et al. 2004, Lemke et al. 2007).  Anticipated future climate warming has the potential to permanently reduce large areas of GIS or even abate it completely. The evidence suggests nearly total ice-sheet loss may result from warming of more than a few degrees above mean 20th century values, but this threshold is poorly defined (perhaps as little as 2oC or more than 7^oC) (Alley et al. 2010).

Greenland with permanent ice sheet

This regime can be described as permanent ice body cover over major parts (~80%) of Greenland only exposing the bedrock in western and southern parts. The Greenland Ice Sheet has been more closely tied to temperature than to anything else. It shrinks with warming and grows with cooling, thus the volume and cover vary throughout the seasons, but in case of cold climate the relationship between the ice growth/decline would be approximately evenly balanced or with a slightly increased ice growth. In winter when the atmospheric temperatures decrease below freezing point and precipitation levels decline, the accumulation of the ice steadily increases (Bamber et al. 2007).

Greenland without permanent ice sheet

Due to the loss of the ice-sheet volume as a result of the warming, Greenland territory in future could become free from permanent ice sheet cover. This would happen as a consequence of the negative relation to ice growth/ice decline during the winter and summer where the lost ice volume in summer could not be reproduced in the following winter. Rising sea level as a result of warming tends to float marginal regions of ice sheets and force further retreat, so the generally positive relation between sea level and temperature means that typically both reduce the volume of the ice sheet (Alley et al. 2010).

Increasing greenhouse gas concentration from anthropogenic sources is predicted to cause a rise in global mean temperatures (Cubasch et al. 2001). One of the most common anthropogenic greenhouse gases is carbon dioxide (CO2). The influence of this driver is well established as confirmed by many studies. The indirect driver that is increasing anthropogenic CO2 levels in atmosphere is the burning of fossil fuels such as coal and natural gas. It is occurring regionally but has global impact and is well established in literature.

The initial regime would typically occur in cold climate conditions where the relationship between the ice growth/decline would be approximately evenly balanced or with a slightly increased ice growth. The two main mechanisms that maintain this regime are ice-albedo mechanism and meltwater-ice sliding mechanism.

Increasing CO2 levels in atmosphere - the key driver of the regime shift, initiates the increase of atmospheric temperatures and changes in albedo. As a result - increased absorption of solar energy promotes higher air, ice, water and land temperatures which leads towards degrading sea ice. Also the inland surface temperature increase can cause surface melting in the ablation zone that presently accounts for roughly half of the mass loss from the GIS (Parizek et al. 2004). Thus this driver indirectly is increasing drainage of meltwater feeding into crevasses close to the glacier margin resulting in higher calving rates (Murray et al. 2010). Furthermore, thinning and retreating of the glacier tongue due to these increased rates cause reduced effective pressures beneath the glacier, promoting faster flow that results in decrease of ice volume.  

The increase in surface air temperatures changes the ice-albedo feedback thermodynamics. This means that the heat exchange within the sea ice, as well as between the top and bottom of the ice is changed. This leads to a decrease in sea ice volume. The resulting increased amount of open land and water surface in summer decreases the albedo, as the dark ocean surface absorbs more solar radiation (Lindsay et al. 2005).  The annually integrated absorption of solar radiation is observed to increase when the surface albedo is relatively low (Rigor et al. 2002, Holland et al. 2006). This increasingly accumulated amount of heat on the surface reinforces the initial warming. Due to the loss of the ice-sheet volume as a result of the warming, Greenland territory in future could become free from permanent ice sheet cover. This would happen as a consequence of the negative relation to ice growth/ice decline during the winter and summer where the lost ice volume in summer could not be reproduced in the following winter. The Greenland without permanent ice sheet regime is characterized by other dominant feedback mechanisms. For example ice volume-wave action, the water temperature-density and meltwater-ice sliding velocity mechanisms.

The shift to the regime of Greenland without ice sheet will mainly result in loss of some desirable ecosystem services. The ecosystem service of desirable climate regulation could be lost as the change in movement of currents (change in thermohaline circulation) and air masses would alter the transport of heat. This could lead to increased hurricane activity, a southward shift of tropical rainfall belts with resulting agricultural impacts, and disruptions to marine ecosystems.

The loss of certain animal and plant food species as provisioning services is predicted in the future. These changes may have important consequences for food webs and could well be extremely significant for the Greenland economy, which is highly dependent on fisheries (AMAP 2007). Such cultural services like recreation and aesthetical values would also be affected. Each of those services attracts more people to see the Ice sheet thus also bringing in more tourists. Water regulation as regulating ecosystem services could be altered through the large input of freshwater in the water cycle.  The vast amount of "stored" water entering the water cycle within warmer climate would result in severe winter precipitation.

A new ecosystem service is possible as the thawing ice sheet will potentially form glacial freshwater lakes in Greenland. This will generate new recreation opportunities in summer – using lakes for different purposes from different social groups. Flora could expand deeper into Greenland and new species could be introduced as the climate warms giving the local population the chance to gain additional plant foods.

The potential options for preventing or reversing this potential regime shift mainly relates to the decrease of greenhouse gas input into the atmosphere at a global scale. This has to be achieved in order to prevent further climate warming leading to the loss of Greenland Ice Sheets. Options include reduction of deforestation, use of fossil fuels and charcoal as energy source, and cleaner economies. Geoengineering strategies has also been proposed, large scale experiments to decrease global temperature and CO2 concentration in the atmosphere. However, their applicability is debated and the usefulness is contested. As this system boundaries are set mainly around geophysical variables it is necessary to look at the social mechanisms involved to limit the influence on the main direct driver of greenhouse gas emissions. Nevertheless even if CO2 levels in atmosphere leads to atmospheric temperature increase, it is very hard to achieve from the local to regional management perspective. It does require global coordination and cooperation in order to achieve CO2 reduction targets.

The system is defined by the ice volume and the territory it covers in the Arctic Ocean and the regional/global processes that ensure the existence of ice in this area. The loss of surface area and thinning of Arctic sea ice has not occurred at a linear rate which may be indicative of a systematic change towards an alternate regime.

Arctic with summer ice

Under this regime, the Arctic Ocean has an abundance of sea ice.  It is characterized by very long and cold winters, during which the ice surface area and thickness reach their maximum. The low winter temperatures and short summer help to maximize the sea ice surface area and volume over time.

Arctic without summer ice

In this regime the surface area and volume of summer sea ice in the Arctic rapidly decreases due to atmospheric warming caused by greenhouse gases. In summer when open water surface area is greater, the albedo is reduced, which causes greater absorption of solar radiation.  This raises the temperature of the water and ice, which facilitates greater losses in sea ice surface area and volume. Several models predict that ice free Arctic conditions in summer will be reached within this century (Arzel et al. 2006). Several authors have suggested that the system has already surpassed a tipping point, but convincing evidence is lacking (Lenton et al. 2008).

The main driver of this regime shift is elevated greenhouse gas concentrations in the atmosphere causing an increase in arctic air temperatures. This global driver is well established and could be looked as irreversible in the scale of next hundred years. In regards to the loss of sea ice in the Arctic, the regime shift is generally considered to be irreversible unless the main driver (increased atmospheric temperatures resulting from climate change) is changed in the near future.

Anthropogenic activities that elevate atmospheric greenhouse gas concentrations are generally considered to be the primary driver of climate change (IPCC 2007; Kinnard et. al 2011). Carbon release from anthropogenic sources is projected to continue and increase during the coming decades (IPCC 2007). This is expected to contribute to an increase in average global temperatures and more rapid decrease in sea ice cover and thickness in the Arctic. This driver initially affects the main ice-albedo mechanism thus changing the processes that characterize its initial state. Once the main mechanism has shifted the driver and the altered ice-albedo mechanism initiates change in other parts of the system.

The Arctic with summer ice regime is maintained by permanent low surface air temperatures (SAT) that maintain the thermal balance, thus ensuring balanced heat exchange between the atmosphere, sea ice, and water. The result is maintained sea ice volume, thickness and surface area. This occurs due to ensuring high albedo (a measure of reflectance) level as the dark ocean surface absorbs more solar radiation than the sea ice (Lindsay et al. 2005). This means that the high albedo reflects more radiation avoiding surface temperature increase. Avoiding increased absorption of solar energy promotes lower air, ice, water and land temperatures which lead towards maintaining sea ice. In the end, the low temperatures further promote ice maintaining arctic conditions.

There is near universal agreement that the extent of Arctic sea ice will decline through the 21st century in response to increasing atmospheric greenhouse gas (GHG) concentrations (Zhang 2006). The resulting increase in surface air temperatures (SAT) change the thermal balance which means that the heat exchange between the atmosphere, sea ice, and water is changed. The result is a decrease in sea ice volume, thickness and surface area. The increased area of open water in summer decreases the albedo as the dark ocean surface absorbs more solar radiation than the sea ice (Lindsay et al. 2005).  Increased absorption of solar energy promotes higher air, ice, water and land temperatures which lead towards degrading sea ice volume (Rigor et al. 2002; Holland et al. 2006). In the end, the increasing temperatures and accumulated heat further promote warming arctic conditions

Changes in the summer extent of Arctic sea ice are not solely forced by SATs, but could also be affected by fluctuations of atmospheric pressure at sea level that controls the strength and direction of windsin the region. More probably these changes could be driven by a combination of these (and/or other) forcing (Kinnard et al. 2011).

Local knowledge and spiritual values might be lost as the local communities have to adapt to the new circumstances and thus their lifestyle. In addition to concerns about the security of infrastructure and impacts on human well being, ice free Arctic summers have important impacts on ecosystems. One such impact is that loss of ice cover could affect the Arctic's freshwater system and surface energy budget, and manifest in middle latitudes as altered patterns in atmospheric circulation and precipitation (Serreze et al. 2007). This presents the way how water and atmospheric circulations could be altered as ecosystem services.

Summer sea ice concentration is important for navigation, and may have implications for the transport of sediments and pollutants across the Arctic. Most of the sea ice formed in the Arctic Ocean is exported through the Fram Strait into the Greenland Sea and to the North Atlantic where the ice may affect the global thermohaline circulation (Rigor et al. 2002). Sea ice also blocks the solar flux to the water and hence is a major control factor phytoplankton to seals, walrus, and polar bears while limiting access to the surface for seals and whales (Lindsay et al. 2005).

The rapidly melting sea ice in the Arctic Ocean has increased political and economic interest in the region's resource extraction and in the potential for more accessible shipping routes. By opening the Northwest passage, shipping route through the northern Canadian waters, could result in a positive economic impact. Although this also could potentially result in ecological disasters as the possibility of oil spills and other disasters associated with development would increase.

The options for preventing or reversing the loss of summer sea ice in the Arctic primarily relate to the decrease of greenhouse gas emissions on a global scale to reduce climatic warming. As atmospheric greenhouse gas concentrations increase, it is essential to understand local and regional actions that may influence the feedback mechanisms influencing the shift to an ice free summer Arctic. Technology transfer could be a good initiative from developed countries as they can provide more advanced technological solutions to developing countries to help accelerate the learning curve on GHG emissions. A Global response particularly from developed nations that are using the majority of the world’s resources on a per capita basis should be in place to deal with such complex system.