Ecological thresholds and regime shifts
What are ecological thresholds?
Ecological thresholds have been defined as the tipping points, where a change in environmental conditions causes an abrupt shift in ecosystem state (structure) and function. Such an abrupt shift is called a regime shift. Persistent gradual change in environmental conditions can bring an ecosystem close to a threshold where a small perturbation can produce large responses in the ecosystem (Groffman et al., 2006; Scheffer, 2009). These kind of dramatic shifts have been documented for many systems; from rapid eutrophication of coastal waters to structural changes in fish communities. Trespassing of thresholds may have serious economic consequences (Barbier 2007; Taylor et al., 2006).
|Table 1: EXAMPLES OF DOCUMENTED SHIFTS IN STATES IN AQUATIC ECOSYSTEMS|
|(Modified from Folke et al., 2002)|
|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|
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 or biological systems are typically complex dynamic systems characterized by nonlinear dynamics, with possibilities for alternative stable state, thresholds (tipping points) and regime shifts, which may preclude a return to the previous state. In nonlinear mathematical models, regime shifts are associated with critical or bifurcation points, and are referred to as critical transitions (Scheffer, 2009). 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). See also the article Resilience and resistance.
In models, it is possible to determine the exact threshold, but regime shifts in nature occur within ranges of pressures and not at an exactly specifiable point (Huggett, 2005). Another important characteristics is hysteresis, which means that the state of a system depends on its history. 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. 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. Regime shifts in coastal areas can be triggered by processes in the open sea and by processes in the water shed. 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 trespassing of 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).
Climate-induced regime shifts
The case of the sudden transition of the Sahara from a vegetated wet land to a dry and barren desert some 5500 years ago is a typical example of a regime shift (Foley et al., 2003).
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). This in turn affected different components of North Sea marine ecosystems. Model simulations provide some evidence that sustained CO2 emissions and global warming will cause a regime shift in the oceans with oxygen levels dropping drastically and many species failing to survive (Petrovskii et al., 2017; Baroni et al., 2020).
Regime shifts in coastal waters
While there are well-documented regime shifts in coastal waters, the phenomenon is probably more common and will likely become even more frequent as ecosystems face increasing pressures (Walker and Meyers, 2004). 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). A threshold of light attenuation of 0.27-1 m has been observed, setting a depth for seagrass in the Mediterranean Sea (Duarte et al., 2007). The Black Sea provides examples of changes that clearly have been caused by human pressures (Daskalov et al 2007).
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). 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, indicating a shift between two alternative states of the macroalgal community.
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). 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.
Implications for management
Regime shift and thresholds can have major implications for ecosystem management (Petersen et al., 2005). Regime shifts linked to human pressures are generally viewed as a change from a healthy to an unhealthy state of the ecosystem (Rapport, 2007). 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 site and as a Special Bird Protection Area. Several bird species used to forage on the water vegetation, which has decreased dramatically. Return to the previous turbid green state would be an obligation from the perspective of bird protection, but would not be admissible under the European Union Water Framework Directive. Complex interactions with different consequences and management implications have also been documented for coral reefs (Knowlton and Jackson, 2008) and fisheries (Burgess et al., 2003). Resolving conflicts between conservation goals and the livelihoods of local communities is a major challenge for reef management today.
- Resilience and resistance
- Disturbances, biodiversity changes and ecosystem stability
- Biodiversity and Ecosystem function
- Thresholds and Marine Policies
- Sustainability indicators
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