A1.1.4 Ecological Stability

 

Stability can be defined in several ways, but one definition of a stable system is one having low variability (i.e. little deviation from its average state) despite shifting environmental conditions.

Ecological stability can be defined as the ability of an ecosystem to resist changes in the presence of disturbances. A precise definition would be dependent upon the ecosystem in question, the variable(s) of interest, and the overall context. For example, in the context of conservation ecology, stable populations are often defined as ones that do not go extinct.

Local stability may relate to a system that is stable over small short-lived disturbances. Global stability may relate to a system highly resistant to change in species composition and/or food web dynamics. [Please refer to section A3.4.5 Pollution Impact on Habitat & Species for further information on food webs.]

 

The Principles of Ecosystem Stability are:

  • Ecosystems dispose of waste and replenish nutrients by recycling all elements.
  • Ecosystems use sunlight as their source of energy.
  • The size of a consumer population is maintained such that overgrazing and other forms of overuse do not occur.
  • Biodiversity is maintained.

 

Factors influencing ecosystem stability are biotic potential and environmental resistance. This could be in the form of: positive and negative factors of population growth, species diversity that is highly correlated with stability, as well as climate.

Stability of an ecosystem also needs to have a resistance to change. This resistance to change has three forms:

  1. Inertia – the resistance to change.
  2. Resilience – the ability to recover from change.
  3. Succession – the replacement of species by another.

 

There is currently great concern about the stability of both natural and human-managed ecosystems. High rates of species extinction already occur worldwide. Climate change and other human-driven environmental changes will continue to cause biodiversity loss in the coming decades.

Species play essential roles in ecosystems and so local and global species losses could threaten the stability of the ecosystem services on which humans depend. For example, plant species harness the energy of the sun to fix carbon through photosynthesis. This essential biological process provides the base of the food chain for many animal consumers.

Species diversity has two primary components:

  • Species richness – the number of species in a local community.
  • Species composition – the identity of the species present in a community.

 

Species differ from one another in their resource use, environmental tolerances, and interactions with other species, such that species composition has a major influence on ecosystem functioning and stability.

The traits that characterize the ecological function of a species are termed functional traits, and species that share similar suites of traits are often categorized together into functional groups. When species from different functional groups occur together, they can exhibit complementary resource-use, meaning that they use different resources or use the same resources at different times. For example, two animal predators may consume different prey items, so they are less likely to compete with one another, allowing higher total biomass of predators in the system. In the case of plants, all species may utilize the same suite of resources (space, light, water, soil nutrients, etc.) but at different times during the growing season — for example, early- and late-season grasses in prairies.

Increasing species diversity can influence ecosystem functions (such as productivity) by increasing the likelihood that species will use complementary resources and can also increase the likelihood that a particularly productive or efficient species is present in the community. For example, high plant diversity can lead to increased ecosystem productivity by more completely, and/or efficiently, exploiting soil resources (e.g., nutrients, water).

Other ecosystem functions include decomposition and nutrient turnover, which are also influenced by species diversity and particular species traits.

 

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