Ecological thresholds and regime shifts

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This article should be read in conjunction with the article Resilience and resistance.

What are ecological thresholds?

Figure 1. Schematic ecological responses to increasing environmental pressure. Left: gradual response, where ecosystem state changes smoothly with pressure. Middle: threshold-like but continuous response, where a small additional increase in pressure causes a rapid change in ecosystem state, without alternative stable states. Right: response with alternative stable states, where two stable branches are separated by an unstable branch. Increasing pressure can trigger an abrupt shift at the upper tipping point, while recovery occurs only after pressure has been reduced to a much lower level; this difference between degradation and recovery thresholds is hysteresis.

An ecological regime shift is a large and persistent change in ecosystem structure and functioning (Groffman et al., 2006[1]; Scheffer, 2009[2]). A regime shift can occur when gradually changing environmental conditions bring an ecosystem across a threshold, so that a small additional change produces a large ecological response. In some systems, internal feedbacks stabilize alternative ecosystem states. The threshold between such states is often called a tipping point, and reversal may then require a much larger reduction of the pressure that caused the shift. This phenomenon is called hysteresis. A schematic is shown in Fig. 1.

Regime shifts have been documented for many systems; from rapid eutrophication of coastal waters to structural changes in fish communities. Crossing thresholds may have serious economic consequences (Olin et al., 2024[3], Blöcker et al., 2023[4]).

Despite their intuitive appeal, it is difficult to define thresholds precisely. Ecological thresholds do not just refer to sudden jumps in a time series. In a mathematical description, thresholds are related to the nonlinear response of an ecological or biological system to pressures caused by human activities or natural processes. Ecological systems are complex dynamic systems that often respond nonlinearly to environmental pressures. Such nonlinear dynamics can give rise to thresholds, alternative stable states and regime shifts.

In nonlinear ecosystem models, regime shifts can occur at critical or bifurcation points and are then referred to as critical transitions (Scheffer, 2009[2]). The notion of 'stable state' should not be confounded with 'steady state'. Due to the energy flow through the system (food, light, water motion), an ecosystem moves around stable states (so-called attractors), even without any change in external environmental conditions. It may therefore be preferable to speak of multiple attractor regimes rather than multiple stable states (Scheffer and Carpenter, 2003[5]). See also the article Resilience and resistance.

In a model the bifurcation point can be determined exactly, whereas in natural ecosystems the corresponding threshold is generally uncertain and can only be identified within a range of environmental pressure (Huggett, 2005[6]).

Hysteresis means that the ecosystem response depends on its history. If a degraded state is stabilized by internal feedbacks, simply restoring the environmental pressure to the level at which degradation started may not restore the former state; the pressure may have to be reduced much further. For instance, mesocosm experiments have shown that when changing the order in which a system is colonized from a common species pool, a different 'stable' ecosystem community may result that is resistant against colonization by other species from the pool[7]. Even when a change is not irreversible, the return path from an altered state towards the original state can be drastically different from the development that lead to the altered state.

Analyses of thresholds should also recognize the possibility of interacting ecological regime shifts at different scales. Different pressures can reinforce each other. An ecosystem weakened by one pressure may therefore cross a threshold under a second pressure that would not by itself have caused a regime shift. Regime shifts in coastal areas can be triggered by processes in the open sea and by processes in the watershed. Coastal lagoons and enclosed seas often have large drainage areas relative to the sea surface. For example, the drainage area of the Baltic Sea covers 1,700,000 km2, or more than four times the entire sea area (415,266 km2). Changes that originate in land use can cause crossing ecological thresholds in the coastal waters. Hysteresis effects mean that reductions in, for example, nutrient inputs do not necessarily induce an immediate response (Stålnacke, 2005[8]).

Ecological thresholds and regime shifts in aquatic ecosystems

Some well-documented examples of regime shifts in aquatic ecosystems are indicated in Table 1. The phenomenon is probably more common and will likely become even more frequent as ecosystems face increasing pressures (Walker and Meyers, 2004[9]). Some of the mechanisms underlying regime shifts are fairly well known, see for example Table 1. The loss of plant communities on the sea floor can be attributed to increasing nutrient concentrations that stimulate the growth of phytoplankton and epiphytic algae, and their expansion in turn shades seagrasses and macroalgae (Krause-Jensen et al., 2008[10]).

Table 1: EXAMPLES OF DOCUMENTED SHIFTS IN STATES IN AQUATIC ECOSYSTEMS
(Modified from Folke et al., 2002[11])
Ecosystem Alternative 1 Alternative 2
Freshwater systems Clear water, Benthic vegetation Turbid water, Blue-green algae
Oligotrophic macrophytes and algae Cattails, and Blue-green algae
Game fish abundant Game fish absent
Marine Systems Hard coral Fleshy algae
Kelp forests Urchin dominance
Seagrass beds Algae and muddy water

The Black Sea provides examples of changes that clearly have been caused by human pressures (Daskalov et al 2007[12]). Intensive fishing reduced predatory fish, allowing smaller planktivorous fish and jellyfish to increase and thereby alter zooplankton and phytoplankton abundance. Together with eutrophication, these trophic cascades contributed to a major ecosystem regime shift.

A threshold-like change in the relationship between seagrass colonization depth and light attenuation has been observed around a light attenuation coefficient of 0.27 m-1 (Duarte et al., 2007[13]).

Similarly, an analysis of a large dataset from Danish coastal waters demonstrates that the cover of macroalgal communities in deeper water decreases markedly along an eutrophication gradient (Krause-Jensen, 2007[14]). The analysis indicates that algal abundance initially responded slowly to increasing eutrophication, but showed a more marked response at nitrogen concentrations around 35-40 µM, suggesting a possible threshold-like response of macroalgal abundance to eutrophication.

In the Ringkøbing Fjord on the west coast of Denmark transitions that indicate thresholds are driven by sluice management that affects the salinity (Hakanson and Bryhn, 2007[15]). From 1995 to 1997, a dramatic change took place because of a change in water salinity related to the implementation of a new sluice practice. The ecosystem changed from a nutrient-driven turbid green water to a grazing-controlled clear water.

Regime shifts can also occur when a keystone species is eliminated from an ecosystem (see the article Trophic cascade) or when an alien species is introduced (see the article Non-native species invasions).

Climate-induced regime shifts

The case of the sudden transition of the Sahara from a vegetated wetland to a dry and barren desert some 5500 years ago is a typical example of a regime shift (Foley et al., 2003[16]).

In the marine environment, regimes may last for decades or even centuries and natural shifts have often been linked to changes in climatic conditions. Climatic changes and human pressures appear to be the main triggering factors causing ecosystem regime shifts. For instance, increased sea surface temperature and possibly change in wind intensity triggered a change in the location of an oceanic bio-geographical boundary along the European continental shelf in the 1970 in western European basin (Beaugrand, 2004[17]). This in turn affected different components of North Sea marine ecosystems.

Climate warming can contribute to ecological regime shifts by moving species beyond physiological limits and by altering competition, predation and habitat-forming species. Marine heatwaves can provide such acute climatic disturbances and, where they cause persistent loss of habitat-forming species or major food-web changes, can contribute to regime shifts.

Implications for management

Regime shift and thresholds can have major implications for ecosystem management (Petersen et al., 2005[18]). Regime shifts can substantially alter biodiversity, ecosystem functioning and the services that people obtain from coastal ecosystems. Whether the resulting state is considered desirable depends on the management objectives concerned. Regime shifts linked to human pressures are often viewed as a change from a healthy to an unhealthy state of the ecosystem (Rapport, 2007[19]). In some cases, matters are complicated by the fact that a change that is perceived to be adverse from one perspective may turn out to be beneficial from another. For example, the Ringkøbing fjord is now closer to many environmental objectives, even though the improvements were not caused by a reduction of anthropogenic pressures, such as nutrient discharges. However, the southern part of the lagoon is designated as a Ramsar [1] site and as a Special Bird Protection Area [2]. Several bird species used to forage on the water vegetation, which has decreased dramatically. This illustrates that management objectives can conflict because different ecosystem states provide different ecological functions and benefits. A state considered desirable according to water-quality criteria need not maximize habitat value for every protected species.

Complex interactions with different consequences and management implications have also been documented for coral reefs (Knowlton and Jackson, 2008[20]) and fisheries (Burgess et al., 2003[21]). Resolving conflicts between conservation goals and the livelihoods of local communities is a major challenge for reef management today.

No generally reliable early-warning indicator is yet available for predicting regime shifts in natural ecosystems (O'Brien et al., 2023[22]). Important management consequences of the existence of ecological thresholds therefore are:

  • Not assume ecosystem response is proportional to pressure.
  • Not manage right up to an estimated threshold: the threshold is uncertain and can vary in space and time.
  • Reduce multiple controllable pressures where possible, because climate and other uncontrollable pressures can move the ecosystem closer to a transition.
  • After a regime shift, simply returning the original driver to its former value may not restore the ecosystem because of hysteresis.
  • Preserve spatial connectivity and recolonisation sources, because these can increase resilience (Olin et al., 2024[3]).


Related articles

Resilience and resistance
Trophic cascade
Disturbances, biodiversity changes and ecosystem stability
Species extinction
Biodiversity, ecosystem functioning and ecosystem function
Thresholds and Marine Policies
Sustainability indicators


References

  1. ↑ Groffman, P., Baron, J., Blett, T., Gold, A., Goodman, I., Gunderson, L., Levinson, B., Palmer, M., Paerl, H., Peterson, G., LeRoy Poff, N., Rejeski, D., Reynolds, J., Turner, M., Weathers, K., & Wiens, J.2006 Ecological thresholds: the key to successful environmental management or an important concept with no practical application? Ecosystems 9(1):1–13
  2. ↑ 2.0 2.1 Scheffer, M. 2009. Critical transitions in nature and society. Princeton University Press, Princeton, New Jersey, USA
  3. ↑ 3.0 3.1 Olin, A.B., Bergström, U., Bodin, Ö., Sundblad, G., Eriksson, B.K., Erlandsson, M., Fredriksson, R. and Eklöf, J.F. 2024. Predation and spatial connectivity interact to shape ecosystem resilience to an ongoing regime shift. Nat Commun 15, 1304
  4. ↑ Blöcker, A.M., Gutte, H.M., Bender, R.L., Otto, S.A., Sguotti, C. and Möllmann, V. 2023. Regime shift dynamics, tipping points and the success of fisheries management. Sci Rep 13, 289
  5. ↑ Scheffer, M. and Carpenter, S. R. 2003. Catastrophic regime shifts in ecosystems: Linking theory to observation. Trends in Ecology and Evolution 18: 648–656
  6. ↑ Huggett A. 2005. The concept and utility of "ecological thresholds" in biodiversity conservation. Biological Conservation 124(3):301–310
  7. ↑ Drake, J.A., Huxel, G.R. and Hewitt, C.L. 1996. Microcosms as models for generating and testing community theory. Ecology 77: 670–677
  8. ↑ Stålnacke P. 2005. Time scale in nutrient fate: examples from Eastern Europe. In: Proceedings of International Workshop on "Where Do Fertilizers Go", ISPRA, Italy, 28-29 June 2005
  9. ↑ Walker, B. and Meyers, J. 2004. Thresholds in ecological and social–ecological systems: a developing database. Ecology and Society 9(2): 3–9
  10. ↑ Krause-Jensen, D., Sagert, S., Schubert, H. and Boström, C. 2008. Empirical relationships linking distribution and abundance of marine vegetation to eutrophication. Ecological Indicators 8(5): 515-529
  11. ↑ Folke, C., Carpenter, S., Elmqvist, T., Gunderson, L., Holling, C.S. and Walker, B. 2002. Resilience and Sustainable Development: Building Adaptive Capacity in a World of Transformations. AMBIO A Journal of the Human Environment 31(5): 437-40
  12. ↑ Daskalov, G.M., Grishin, A.N., Rodionov, S. and Mihneva, V. 2007. Trophic cascades triggered by overfishing reveal possible mechanisms of ecosystem regime shifts. PNAS 104 (25): 10518-10523
  13. ↑ Duarte, C.M., Marba, N., Krause-Jensen, D. and Sanchez-Camacho, M. 2007. Testing the predictive power of seagrass depth limit models. Estuaries and Coasts 30(4): 652–656
  14. ↑ Krause-Jensen, D., Middelboe, A., Carstensen, J. and Dahl, K. 2007. Spatial patterns of macroalgal abundance in relation to eutrophication. Marine Biology 152(1): 25–36
  15. ↑ Hakanson, L. and Bryhn, A. 2007. Goals and remedial strategies for water quality and wildlife management in a coastal lagoon: a case study of Ringkoebing Fjord, Denmark. Journal of Environmental Management 86(3): 498-519
  16. ↑ Foley, J.A., Coe, M.T., Sheffer, M. and Wang, G. 2003. Regime shifts in the Sahara and Sahel: Interactions between Ecological and Climatic Systems in Northern Africa. Ecosystems 6(6): 524-539
  17. ↑ Beaugrand, G. 2004. The North Sea regime shift: Evidence, cause, mechanisms and consequences. Progress In Oceanography 60(2-4): 245-262
  18. ↑ Petersen, J., Hansen, J., Laursen, M., Clausen, P. and Conley, D. 2005. Regime shift in a coastal marine ecosystem. Paper presented at the summer meeting of the American Society of Limnology and Oceanography (ASLO). June 19–24, Santiago de Compostela, Spain
  19. ↑ Rapport, D.J. 2007. Sustainability science: an ecohealth perspective. Sustainability Science 2(1): 77-84.
  20. ↑ Knowlton, N. and Jackson, J.B.C. 2008. Shifting baselines, local impacts, and global change on coral reefs. PLoS Biol 6(2): e54 doi:10.1371/journal.pbio.0060054
  21. ↑ Burgess, D., Mous, P., Linden, O. and Obura, D. 2003. When fishing grounds are closed: Developing alternative livelihoods for fishing communities. MPA NEWS 5(2): August 2003
  22. ↑ O’Brien, D.A., Deb, S., Gal, G., Thackeray, S.J., Dutta, P.S., Matsuzaki, S.S., May, L. and Clements, C.F. 2023. Early warning signals have limited applicability to empirical lake data. Nature Communications 14, 7942


The main author of this article is Aberra, Zelealem
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Citation: Aberra, Zelealem (2026): Ecological thresholds and regime shifts. Available from http://www.coastalwiki.org/wiki/Ecological_thresholds_and_regime_shifts [accessed on 19-08-2026]



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