Kelp Transitions

Feedback mechanisms

Kelps regime is maintained by a healthy food web, usually with 4 trophic levels that keep urchin population under control. Urchin regime is maintained by reduction of predators due to lack of habitat or fishing pressure. Turf-forming algae regime is maintained by an environment over enriched by nutrients either from sediments from land or upwelling nutrients from the deep ocean. Note that canopy-forming algae and turf-forming algae are functional groups competing for resources and space in the ecosystem. All feedbacks are local and well established.

Kelp Forest

  • Competition feedback (Local, well established): Canopy forming algae and turf forming algae are two set of species that compete for space, light and nutrients. The consumption of such resources reduces the availability of the resource for its own progeny and for other species. For this reason, such relationship is represented as a balancing feedback loop. There are two competition feedbacks in figure 1. The resulting ecosystem is a kelp forest when canopy-forming algae is dominating such competition, reducing in turn the possibility of turf forming algae to develop.
  •  Predation feedback (Local, well established): The relationship between predator and prey in the food web can be viewed as a feedback. In a nutshell, the more abundant the prey is, the more resource the predator has and vice-versa, producing a balancing feedback loop. Such feedback is represented here aggregated across functional groups: between apex predators and lobsters - meso-predators; between the later and urchins; and between urchin sand algae groups. Lobsters and other meso-predators help maintain kelp forests by regulating urchin populations at low densities.
  • Structure feedback (Local, well established): Canopy forming macroalgae maintain more complex habitat structure than in turns favor the presence of high biodiversity. When diverse predatory species are present in the ecosystem, these predators regulate urchin populations, which maintains kelp forests. 


Urchin barren

  • Predation feedback (Local, well established): The ecosystem is dominated by urchin barrens when the predation feedback amongst urchins and kelps is strong, while the predation feedback amongst lobster or meso-predators and urchins is weak. It may be related with changes in water temperature that favors urchin barren establishment, or fishing pressure that reduces meso-predator abundance. 
  • Structure feedback (Local, well established): As urchin barrens dominate, less structural habitat complexity is provided by kelp forest. Thus, meso-predators habitat requirements may be affected, reducing their abundance and the predation pressure on urchins.


Turf-forming algae

  • Competition feedback (Local, well established): In turf-forming algae dominance regime, the competition feedback is favoring turfs, reducing space, nutrients and light availability for kelps to develop.


Shift from kelp forest to urchin barrens or turfs


Important shocks (e.g. droughts, floods) that contribute to the regime shift include:

  • Rain and floods (regional, well established): Strong rain events and floods represent therefore shock events for kelp ecosystems given the pulse input of nutrients. Nutrients in turn unbalance the competition between kelps and turfs favoring the development of the later; which can use the excess of nutrients faster than kelps and also take advantage from the turbidity conditions generated by nutrients. 
  • ENSO (global, well established): El Niño events or global warming events may generate water stratification. As consequence, nitrogen concentration declines and kelps become nitrogen limited (Steneck et al 2002). In addition pollution discharges and sedimentation may play a synergetic role as stressors. 


The main external direct drivers that contribute to the shift include:

  • Overfishing (regional, well established): overfishing functional groups is one of the most important drivers of kelp transitions (Steneck et al 2002; Steneck, R., et al. 2004; Estes et al. 2011). Fishing pressure reduce control of mid predators on urchins favoring the formation of turfs. When fishing is strong enough on urchins, it may favor the formation of turfs as well.
  • Nutrients inputs (regional, well established): Input of nutrients is another key driver of the regime shift, both natural from deep ocean upwelling or anthropogenic runoff (Gorman and Connell 2009). Nutrients inputs increase sedimentation and turbidity, favoring conditions for turf to outcompete kelps. 


The main external indirect drivers that contribute to the shift are:

  • Population growth (global, speculative): Population growth leads to higher demand of food. 
  • Food demand (local-regional, speculative): Higher food demands usually stimulate intense agriculture, both as expansion of agricultural frontier or increase of fertilizers use to increase yield. It also increase fishing pressure on the food web. 
  • Agriculture (regional, well established): Agriculture often requires the use of fertilizers. When soils are eroded or washed, fertilizers run downstream increasing nutrients input to lakes and rivers. 
  • Urban growth (global, well established): Urban growth in coastal zones increase the production of sewage that is rich in nutrients. It also increase the water runoff on the urban landscape, which also transports nutrients into coastal water.
  • Deforestation (regional, well established): Deforestation and poor agricultural management can accelerate, in magnitude and frequency, the nutrients runoff from agricultural lands. Deforestation increase landscape fragmentation and facilitates landscape conversion to agriculture. Both reduce the capacity of the landscape to retain water in the soil, accelerating erosive processes and runoff of nutrients (Smith and Schindler 2009).
  •  Global warming (regional, speculative): Global warming is expected to increase average water surface temperature. It is also expected to increase the gradient between land and ocean temperatures, strengthening winds parallel to the coast and as result increasing upwelling of deep ocean water (Bakun et al. 2010). This could increase nutrients inputs on coastal ecosystems at the regional scale. On the local scale, global warming has favored the reproduction of urchins which acting in synergy with lobster fishing has reduced kelp resilience (Ling et al. 2009). 


Slow internal system changes that contribute to the regime shift include:

  • Kelp fragmentation (regional, well established): Gorman and Connell (2009) also report that the loss of kelp dominated areas undermine kelp's ability to reestablish in disturbed areas. 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.

Summary of Drivers

# Driver (Name) Type (Direct, Indirect, Internal, Shock) Scale (local, regional, global) Uncertainty (speculative, proposed, well-established)
1 Kelp fragmentation Internal Regional Well-established
2 ENSO Shock Regional Well-established
3 Floods Shock Regional Well-established
4 Precipitation Shock Regional Well-established
5 Nutrients input Direct driver Local-Regional Well-established
6 Fishing pressure Direct driver Local-Regional Well-established
7 Demand for food and fiber Indirect driver Local-Regional Well-established
8 Agriculture Indirect driver Local-Regional Well-established
9 Fertilizers use Indirect driver Local-Regional Well-established
10 Urbanization Indirect driver Local-Regional Well-established
11 Sewage Indirect driver Local-Regional Well-established
12 Deforestation Indirect driver Regional Well-established
13 Global warming Indirect driver Global Well-established
14 Human population growth Indirect driver Global Well-established
15 Ocean upwelling Indirect driver Regional Well-established

Summary of Ecosystem Service impacts on different User Groups

References (if available)
Provisioning Services
Food Crops
Feed, Fuel and Fibre Crops
Wild Food & Products
Regulating Services
Air Quality Regulation
Climate Regulation
Water Purification
Soil Erosion Regulation
Pest & Disease Regulation
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Cultural Services
Aesthetic Values
Cognitive & Educational
Spiritual & Inspirational