Dégradation des Ecosystèmes

7. How do ecosystems change over time?

• 7.1 What is known about ecosystem inertia and time scales of change?
• 7.2 When do non-linear or abrupt changes occur in ecosystems?
• 7.3 How are humans increasing the risk of non-linear ecosystem changes?

7.1 What is known about ecosystem inertia and time scales of change?

The source document for this Digest states:

Figure 7.1 Temporal and spatial scales

The time scale of change refers to the time required for the effects of a perturbation of a process to be expressed. Time scales relevant to ecosystems and their services are shown in Figure 7.1. Inertia refers to the delay or slowness in the response of a system to factors altering their rate of change, including continuation of change in the system after the cause of that change has been removed. resilience refers to the amount of disturbance or stress that a system can absorb and still remain capable of returning to its predisturbance state.

Time Scales and Inertia

Many impacts of humans on ecosystems (both harmful and beneficial) are slow to become apparent; this can result in the costs associated with ecosystem changes being deferred to future generations. For example, excessive phosphorus is accumulating in many agricultural soils, threatening rivers, lakes, and coastal oceans with increased eutrophication. Yet it may take years or decades for the full impact of the phosphorus to become apparent through erosion and other processes (S7.3.2). Similarly, the use of groundwater supplies can exceed the recharge rate for some time before costs of extraction begin to grow significantly. In general, people manage ecosystems in a manner that increases short-term benefits; they may not be aware of, or may ignore, costs that are not readily and immediately apparent. This has the inequitable result of increasing current benefits at costs to future generations.

Different categories of ecosystem services tend to change over different time scales, making it difficult for managers to evaluate trade-offs fully. For example, supporting services such as soil formation and primary production and regulating services such as water and disease regulation tend to change over much longer time scales than provisioning services. As a consequence impacts on more slowly changing supporting and regulating services are often overlooked by managers in pursuit of increased use of provisioning services (S12.ES).

The inertia of various direct and indirect drivers differs considerably, and this strongly influences the time frame for solving ecosystem-related problems once they are identified (RWG, S7). For some drivers, such as the overharvest of particular species, lag times are rather short, and the impact of the driver can be minimized or halted within short time frames. For others, such as nutrient loading and, especially, climate change, lag times are much longer, and the impact of the driver cannot be lessened for years or decades.

Significant inertia exists in the process of species extinctions that result from habitat loss; even if habitat loss were to end today, it would take hundreds of years for species numbers to reach a new and lower equilibrium due to the habitat changes that have taken place in the last centuries (S10). Most species that will go extinct in the next several centuries will be driven to extinction as a result of loss or degradation of their habitat (either through land cover changes or increasingly through climate changes). Habitat loss can lead to rapid extinction of some species (such as those with extremely limited ranges); but for many species, extinction will only occur after many generations, and long-lived species such as some trees could persist for centuries before ultimately going extinct. This “extinction debt” has important implications. First, while reductions in the rate of habitat loss will protect certain species and have significant long-term benefits for species survival in the aggregate, the impact on rates of extinction over the next 10–50 years is likely to be small (medium certainty). Second, until a species does go extinct, opportunities exist for it to be recovered to a viable population size.

Source & ©: MA  Millennium Ecosystem Assessment Synthesis Report (2005),
Chapter 7, p.88

7.2 When do non-linear or abrupt changes occur in ecosystems?

The source document for this Digest states:

Nonlinear changes, including accelerating, abrupt, and potentially irreversible changes, have been commonly encountered in ecosystems and their services. Most of the time, change in ecosystems and their services is gradual and incremental. Most of these gradual changes are detectable and predictable, at least in principle (high certainty) (S.SDM). However, many examples exist of nonlinear and sometimes abrupt changes in ecosystems. In these cases, the ecosystem may change gradually until a particular pressure on it reaches a threshold, at which point changes occur relatively rapidly as the system shifts to a new state. Some of these nonlinear changes can be very large in magnitude and have substantial impacts on human well-being. Capabilities for predicting some nonlinear changes are improving, but for most ecosystems and for most potential nonlinear changes, while science can often warn of increased risks of change, it cannot predict the thresholds where the change will be encountered (C6.2, S13.4). Numerous examples exist of nonlinear and relatively abrupt changes in ecosystems:

• Disease emergence (S13.4): Infectious diseases regularly exhibit nonlinear behavior. If, on average, each infected person infects at least one other person, then an epidemic spreads, while if the infection is transferred on average to less than one person the epidemic dies out. High human population densities in close contact with animal reservoirs of infectious disease facilitate rapid exchange of pathogens, and if the threshold rate of infection is achieved—that is, if each infected person on average transmits the infection to at least one other person—the resulting infectious agents can spread quickly through a worldwide contiguous, highly mobile, human population with few barriers to transmission. The almost instantaneous outbreak of SARS in different parts of the world is an example of such potential, although rapid and effective action contained its spread. During the 1997/98 El Niño, excessive flooding caused cholera epidemics in Djibouti, Somalia, Kenya, Tanzania, and Mozambique. Warming of the African Great Lakes due to climate change may create conditions that increase the risk of cholera transmission in surrounding countries (C14.2.1). An event similar to the 1918 Spanish flu pandemic, which is thought to have killed 20–40 million people worldwide, could now result in over 100 million deaths within a single year. Such a catastrophic event, the possibility of which is being seriously considered by the epidemiological community, would probably lead to severe economic disruption and possibly even rapid collapse in a world economy dependent on fast global exchange of goods and services.
• Algal blooms and fish kills (S13.4): Excessive nutrient loading fertilizes freshwater and coastal ecosystems. While small increases in nutrient loading often cause little change in many ecosystems, once a threshold of nutrient loading is achieved, the changes can be abrupt and extensive, creating harmful algal blooms (including blooms of toxic species) and often leading to the domination of the ecosystem by one or a few species. Severe nutrient overloading can lead to the formation of oxygen-depleted zones, killing all animal life.
• Fisheries collapses (C18): Fish population collapses have been commonly encountered in both freshwater and marine fisheries. Fish populations are generally able to withstand some level of catch with a relatively small impact on their overall population size. As the catch increases, however, a threshold is reached after which too few adults remain to produce enough offspring to support that level of harvest, and the population may drop abruptly to a much smaller size. For example, the Atlantic cod stocks of the east coast of Newfoundland collapsed in 1992, forcing the closure of the fishery after hundreds of years of exploitation, as shown in Figure 3.4 (CF2 Box 2.4). Most important, the stocks may take years to recover or not recover at all, even if harvesting is significantly reduced or eliminated entirely.
• Species introductions and losses: Introductions (or removal) of species can cause nonlinear changes in ecosystems and their services. For example, the introduction of the zebra mussel into U.S. aquatic systems resulted in the extirpation of native clams in Lake St. Clair, large changes in energy flow and ecosystem function, and annual costs of $100 million to the power industry and other users (S12.4.8). The introduction of the comb jelly fish (Mnemiopsis leidyi) in the Black Sea caused the loss of 26 major fisheries species and has been implicated (along with other factors) in subsequent growth of the anoxic “dead zone” (C28.5). The loss of the sea otters from many coastal ecosystems on the Pacific Coast of North America due to hunting led to the booming populations of sea urchins (a prey species for otters) which in turn led to the loss of kelp forests (which are eaten by urchins).
• Changes in dominant species in coral ecosystems: Some coral reef ecosystems have undergone sudden shifts from coral-dominated to algae-dominated reefs. The trigger for such phase shifts, which are essentially irreversible, is usually multifaceted and includes increased nutrient input leading to eutrophic conditions, and removal of herbivorous fishes that maintain the balance between corals and algae. Once a threshold is reached, the change in the ecosystem takes place within months and the resulting ecosystem, although stable, is less productive and less diverse. One well-studied example is the sudden switch in 1983 from coral to algal domination of Jamaican reef systems. This followed several centuries of overfishing of herbivores, which left the control of algal cover almost entirely dependent on a single species of sea urchin, whose populations collapsed when exposed to a species-specific pathogen. As a result, Jamaica’s reefs shifted (apparently irreversibly) to a new low-diversity, algae-dominated state with very limited capacity to support fisheries (C4.6).
• Regional climate change (C13.3): The vegetation in a region influences climate through albedo (reflectance of radiation from the surface), transpiration (flux of water from the ground to the atmosphere through plants), and the aerodynamic properties of the surface. In the Sahel region of North Africa, vegetation cover is almost completely controlled by rainfall. When vegetation is present, rainfall is quickly recycled, generally increasing precipitation and, in turn, leading to a denser vegetation canopy. Model results suggest that land degradation leads to a substantial reduction in water recycling and may have contributed to the observed trend in rainfall reduction in the region over the last 30 years. In tropical regions, deforestation generally leads to decreased rainfall. Since forest existence crucially depends on rainfall, the relationship between tropical forests and precipitation forms a positive feedback that, under certain conditions, theoretically leads to the existence of two steady states: rainforest and savanna (although some models suggest only one stable climate-vegetation state in the Amazon).

Source & ©: MA  Millennium Ecosystem Assessment Synthesis Report (2005),
Chapter 7, pp.88-91

7.3 How are humans increasing the risk of non-linear ecosystem changes?

The source document for this Digest states:

There is established but incomplete evidence that changes being made in ecosystems are increasing the likelihood of nonlinear and potentially high-impact, abrupt changes in physical and biological systems that have important consequences for human well-being (C6, S3, S13.4, S.SDM). The increased likelihood of these events stems from the following factors:

• On balance, changes humans are making to ecosystems are reducing the resilience of the ecological components of the systems (established but incomplete) (C6, S3, S12). Genetic and species diversity, as well as spatial patterns of landscapes, environmental fluctuations, and temporal cycles with which species evolved, generate the resilience of ecosystems. Functional groups of species contribute to ecosystem processes and services in similar ways. Diversity among functional groups increases the flux of ecosystem processes and services (established but incomplete). Within functional groups, species respond differently to environmental fluctuations. This response diversity derives from variation in the response of species to environmental drivers, heterogeneity in species distributions, differences in ways that species use seasonal cycles or disturbance patterns, or other mechanisms. Response diversity enables ecosystems to adjust in changing environments, altering biotic structure in ways that maintain processes and services (high certainty) (S.SDM). The loss of biodiversity that is now taking place thus tends to reduce the resilience of ecosystems.
• There are growing pressures from various drivers (S7, SG7.5). Threshold changes in ecosystems are not uncommon, but they are infrequently encountered in the absence of human-caused pressures on ecosystems. Many of these pressures are now growing. Increased fish harvests raise the likelihood of fisheries collapses; higher rates of climate change boost the potential for species extinctions; increased introductions of nitrogen and phosphorus into the environment make the eutrophication of aquatic ecosystems more likely; as human populations become more mobile, more and more species are being introduced into new habitats, and this increases the chance of harmful pests emerging in those regions.

The growing bushmeat trade poses particularly significant threats associated with nonlinear changes, in this case accelerating rates of change (C8.3, S.SDM, C14). Growth in the use and trade of bushmeat is placing increasing pressure on many species, particularly in Africa and Asia. While population size of harvested species may decline gradually with increasing harvest for some time, once the harvest exceeds sustainable levels, the rate of decline of populations of the harvested species will tend to accelerate. This could place them at risk of extinction and also reduce the food supply of the people dependent on these resources. Finally, the bushmeat trade involves relatively high levels of interaction between humans and some relatively closely related wild animals that are eaten. Again, this increases the risk of a nonlinear change, in this case the emergence of new and serious pathogens. Given the speed and magnitude of international travel today, new pathogens could spread rapidly around the world.

A potential nonlinear response, currently the subject of intensive scientific research, is the atmospheric capacity to cleanse itself of air pollution (in particular, hydrocarbons and reactive nitrogen compounds) (C.SDM). This capacity depends on chemical reactions involving the hydroxyl radical (OH^-), the atmospheric concentration of which has declined by about 10% (medium certainty) since preindustrial times.

Once an ecosystem has undergone a nonlinear change, recovery to the original state may take decades or centuries and may sometimes be impossible. For example, the recovery of overexploited fisheries that have been closed to fishing is quite variable. Although the cod fishery in Newfoundland has been closed for 13 years (except for a small inshore fishery between 1998 and 2003), there have been few signs of a recovery, and many scientists are not optimistic about its return in the foreseeable future (C18.3.6). On the other hand, the North Sea Herring fishery collapsed due to overharvesting in the late 1970s, but it recovered after being closed for four years (C18).

Source & ©: MA  Millennium Ecosystem Assessment Synthesis Report (2005),
Chapter 7, pp.91