Regime Shifts DataBase

Large persistent changes in ecosystem services

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.