Regime Shift - Evidence

Evidence

Empirical evidence has increasingly completed model based work on regime shifts. Early work on regime shifts in ecology was developed in models for predation, grazing, fisheries and inset outbreak dynamics. Since the 1980s, further development of models has been complemented by empirical evidence for regime shifts from ecosystems including kelp forest, coral reefs, drylands and lakes.

Scholars have collected evidence for regime shifts across a wide variety of ecosystems and across a range of scales. For example, at the local scale, one of the best documented examples is bush encroachment, which is thought to follow an smooth change dynamic. Bush encroachment refers to small changes in herbivory rates that can shift drylands from grassy dominated regimes towards woody dominated savannas. Encroachment has been documented to impact ecosystem services related with cattle ranching in wet savannas in Africa and South America. At the regional scale, rainforest areas in the Amazon and East Asia are thought to be at risk of shifting towards savanna regimes given the weakening of the moisture recycling feedback driven by deforestation. The shift from forest to savanna potentially affect the provision of food, fresh water, climate regulation and support for biodiversity. On the global realm, the faster retreating of arctic ice sheet in summer time is reinforcing climate warming through the albedo feedback, potentially affecting sea water levels and climate regulation world wide.

Aquatic systems have been heavily studied in the search for regime shifts. Lakes works like microcosms (almost close systems) that to some extent allow experimentation and data gathering. Eutrophication is a well documented abrupt change from clear water to murky water regimes; which leads to toxic algae blooms and reduction of fish productivity in lakes and coastal ecosystems. Eutrophication is driven by nutrients inputs, particularly those coming from fertilizers used in agriculture. It is an example of discontinuous change with hysteresis. Once the lake has shift to murky water regime, a new feedback of phosphorus recycling maintain the system in the eutrophic state even if nutrients inputs are significantly reduced.

Another example widely studied in aquatic and marine systems is trophic level decline in food webs. It usually implies the shift from ecosystems dominated by high numbers of predatory fish to a regime dominated by lower trophic groups like pelagic planktivores (i.e. jellyfish). Affected food webs often have impacts on fisheries productivity, major risk of eutrophication, hypoxia, invasion of non-native species and impacts on recreational values. Hypoxia, or the development of so-called death zones, is another regime shift in aquatic and marine-coastal environments. Hypoxia, similarly to eutrophication, is driven by nutrient inputs of anthropogenic origin but also from natural origin in the form of upwellings. In high nutrients concentrations the levels of dissolved oxygen decrease making life impossible for the majority of aquatic organisms. Impacts on ecosystem services includes collapse of fisheries and the production of toxic gases for humans.

In marine systems, two well studied regime shifts happen in coral reefs and kelp forests. Coral reefs are three-dimensional structures which work as habitat for marine biodiversity. Hard coral dominated reefs can shift to a regime dominated by fleshy algae; but it also has been reported to shift towards soft-corals, corallimorpharians, urchin barrens or sponge dominated regimes. Coral reefs transitions are reported to affect ecosystem services like calcium fixation, water cleansing, support for biodiversity, fisheries productivity, coastline protection and recreational services. On the other hand, kelp forests are macro-algae formations that usually host biodiversity in temperate regions of the ocean. Kelps provide services important for cosmetic industry, fisheries and host biodiversity. Such services are substantially reduced when kelp forest shift towards urchin barrens regimes driven mainly by discharge of nutrients from the coast and overfishing.

Soil salinization is an example of a well known regime shift in terrestrial systems. It is driven by the removal of deep root vegetation and irrigation; which cause elevation of the soil water table and the increase of soil surface salinity. Once the system flips, ecosystem services related with food production -both crops and cattle- are significantly reduced. Dryland degradation, also known as desertification, is a well known but controversial type of regime shift. Dryland degradation occurs when the loss of vegetation transforms an ecosystem from being vegetated to being dominated by bare soils. While this shift has been proposed to be driven by a combination of farming and cattle grazing, loss of semi-nomad traditions, extension of infrastructure, reduction of managerial flexibility and other economic factors, it is controversial because it has been difficult to determine whether there is indeed a regime shift and which drivers have caused it. For example, poverty has been proposed as a driver of dry land degradation, but studies continuously find contradictory evidence. Ecosystem services affected by dry land degradation usually include low biomass productivity, thus reducing provisioning and supporting services for agriculture and water cycling.

Polar regions have been the focus on research examining the impacts of climate warming. Regime shifts in polar regions include the melting of Greenland Ice Sheet and the possible collapse of the Thermohaline circulation system. While the melting of Greenland Ice Sheet is driven by global warming and threaten world wide coast lines with the increase of sea level; the collapse of the thermohaline circulation is driven by the increase of fresh water in the North Atlantic which in turn weaken the density driven water transport between the tropics and polar areas. Both regime shifts have serious implications for marine biodiversity, water cycling, security of housing and infrastructure and climate regulation amongst other ecosystem services.