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?
This question addresses:
- Time scales of change: how long it takes for the effects of change in an ecosystem to become apparent (also referred to as lag time).
- Inertia: the delay or slowness in the response of an ecosystem to certain factors of change.
Many impacts of humans on ecosystems (both harmful and beneficial) are slow to become apparent. This may transfer the costs associated with current ecosystem changes to future generations. For example, the use of groundwater supplies can exceed the recharge capacity for some time before costs of extraction begin to increase significantly. In general, people manage ecosystems in such ways that short-term benefits are increased, while long-term costs go unnoticed or are ignored.
Different ecosystem services tend to change over different time scales, making it difficult for managers to fully evaluate trade-offs. For example, supporting services (such as soil formation and plant growth) and regulating services (such as water and disease regulation) tend to change over much longer time scales than provisioning services do. As a consequence, impacts on more slowly changing services are often overlooked.
The degree of inertia of different drivers of ecosystem change differs considerably. The speed at which a driver reacts strongly influences how quickly related ecosystem problems can be solved once they are identified. For some drivers, such as the overharvest of particular species, lag times are rather short and the impact of the driver can quickly be reduced or stopped. Nutrient loading and, especially, climate change have much longer lag times and the effects of these drivers cannot be reduced for years or decades. The extinction of species due to habitat loss also has a significant lag time. Even if habitat loss were to end today, it would take hundreds of years for species numbers to reach a new, lower, equilibrium in response to the habitat change that took place in the last centuries.
For some species this process can be rapid, but for others, like trees, it may take centuries. Consequently, reducing the rate of habitat loss might only have a small impact on extinction rates over the next half century, but lead to significant benefits in the long term. Time lags between habitat reduction and extinction provide an opportunity for humans to restore habitats and rescue species from extinction. More...
7.2 When do non-linear or abrupt changes occur in ecosystems?
Most changes in ecosystems and their services are gradual and incremental, making them, at least in principle, detectable and predictable. However, many examples exist of non-linear and sometimes abrupt changes in ecosystems. A change may be gradual until a particular pressure on the ecosystem reaches a threshold, at which point rapid shifts to a new state occur. Some non-linear changes can be very large and have substantial impacts on human well-being. Capabilities for predicting non-linear changes are improving, but in most cases science can not yet predict the exact thresholds.
- Emergence of infectious diseases: An epidemic spreads if a certain transmission threshold is crossed, that is if, on average, each infected person infects at least one other person. The epidemic dies out if the infection rate is lower. When humans live closely together and in contact with infected animals, epidemics can potentially spread quickly through the well connected and mobile world population. 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.
- Algal blooms and fish kills: Excessive nutrient loading causes eutrophication of freshwater and coastal ecosystems. While small increases in nutrient loading often cause little change in ecosystems, once a threshold is reached the changes can be abrupt and extensive, causing bursts of algae growth. Severe eutrophication can kill animal life in the water by causing oxygen-depleted zones.
- Collapse of fisheries: Collapses of fish populations have been common in both freshwater and marine fisheries. A moderate level of catch often has a relatively small impact, but with increasing catches a threshold is reached where too few adult fish remain to produce enough offspring to support this level of harvest. For example, the Atlantic cod stocks of the east coast of Newfoundland collapsed in 1992, forcing the closure of the fishery (see Figure 3.4).
- Species introductions and losses can also cause non-linear changes in ecosystems and their services. For example, the loss of the sea otters from many coastal ecosystems on the Pacific Coast of North America due to hunting led to a boom of sea urchin populations (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. Such abrupt shifts are essentially irreversible, and once a threshold is reached the change takes place within months. In Jamaican reef systems, centuries of overfishing of algae-grazing species contributed to a sudden switch leading to low diversity, algae-dominated reefs with very limited capacity to support fisheries.
- Regional climate change: The vegetation in a region influences climate through affecting the amount of sunlight which is reflected, the amount of water released by plants to the atmosphere, and the amount of wind and erosion. In the Sahel region, vegetation cover is closely linked to rainfall. When vegetation is present, rainfall is quickly recycled, generally increasing precipitation and, in turn, leading to denser vegetation. Land degradation reduces water recycling and may have contributed to the rainfall reduction in the Sahel region during the last 30 years.
7.3 How are humans increasing the risk of non-linear ecosystem changes?
Ecosystem are resilient to disturbances until a certain threshold, meaning that they are able to withstand them or to recover from them. Changes in ecosystems caused by humans may reduce this resilience and increase the likelihood of abrupt changes in the system, with important consequences for human well-being.
The species of an ecosystem belong to different functional groups. Within each group, species may contribute in similar ways to ecosystem processes and services but respond differently to environmental fluctuations. This diversity in responding enables ecosystems to adjust to changing environments and to maintain processes and services. The loss of biodiversity that is now taking place thus tends to reduce the resilience of ecosystems.
Threshold changes in ecosystems are not uncommon, but are becoming much more likely as human-induced pressures on ecosystems are growing. For example, as human populations become more mobile, more and more species are being introduced into new habitats. This increases the likelihood of harmful pests to emerge.
Once an ecosystem has undergone a non-linear change, recovery to the original state is generally slow, costly, and sometimes even impossible. For example, the recovery of over-exploited fisheries after collapse and closure is quite variable. The cod fishery in Newfoundland has been closed for nearly 13 years, but there have been few signs of a recovery (see Figure 3.4). However, the North Sea herring fishery recovered after a four-year closure after a collapse due to overharvest in the late 1970s. More...