Ecological stability is defined as the ability of an ecosystem to bounce back or change following a disturbance. It is measured by the amount of time the ecosystem takes to return to equilibrium following the disturbance. This is known as either resilience or resistance. These disturbances can be small or large and impact upon local stability and global stability respectively. Theoretically, ecologists believe that nature is naturally stable and only becomes unstable following a disturbance. Despite this, not all systems demonstrate these characteristics of stability, with some systems being found to not return to their original equilibriums following a large disturbance, but rather find a new alternative equilibrium.
An example of this can be seen in Robert Paine’s starfish experiments. He found that when starfish were removed from an ecosystem, mussel population increased at the expense of ecosystem biodiversity. If the starfish were returned after a short period of time, the ecosystem would revert back to its original steady-state equilibrium, wherein the starfish predate upon the mussels limiting their population size. However, if removed for an extended period of time the mussels would grow to be too large in size for the starfish to predate upon them. The dominance of the mussels, therefore, continued in the ecosystem suppressing other species and moving the ecosystem to a new, less biodiverse alternate steady-state.
Similarly to models of succession, ecosystems grow in maturity, from simple to complex through time, and therefore the assessment of maturity allows for the estimation of ecosystem stability to be made. Ecological resilience, specifically in many ecosystems, is becoming increasingly important in the context of increasing global temperatures, overfishing, and deforestation.
Another example can be seen in the Goulburn-Broken Catchment in Australia. After being ‘discovered’ by European settlers in 1853, in spite of indigenous people having lived there for hundreds of years before, the bush was cleared to allow crops to be cultivated in the same manner as in Europe. In addition, extensive irrigation was put in place and large water reservoirs were created. These changes to the land led to the amount of water being added to the soil doubling causing the water table to rise. Unknown to the settlers, the water table of this area has high natural salt levels, so when the water reached the surface it led to the land not being able to sustain plant life, both wild and agricultural species. Consequently, this ecosystem is thought to have collapsed and shifted into an altered state of a complete lack of biodiversity. While solutions were attempted, they were mainly ineffective, showcasing how a lack of understanding of an ecosystem can destroy ecological stability.
Figure 1 – Diagram showing how a system can move into a new alternate state using a land-system example.
“Regime shifts in land systems”, 2014, Graphic, Müller et al. Source: https://amor.cms.hu-berlin.de/~muelleda/download/Mueller-2014-Regime%20shifts%20limit%20the%20predictability%20of%20land-system%20change.pdf
Biodiversity – The variation of life at all levels of biological organisation, from genes to species to ecosystems (see Introduction to Biodiversity article for more information)
Ecosystem – A community of interdependent organisms and the physical/chemical environment in which they inhabit.
Global stability – The ability of a system to return to its original equilibrium position following a large perturbation.
Irrigation – The application of controlled amounts of water to plants at needed intervals often used to grow crops, maintain landscapes and revert degraded soils.
Local stability – The ability of a system to return to its original equilibrium position following a small perturbation.
Resilience – The ability of a system to “bounce back” following a disturbance.
Resistance – The ability of a system to withstand the perturbation without changing.
Steady-state equilibrium – a State in which opposing forces or influences are balanced thereby keeping conditions fixed.
Succession – the process of change in the species structure of an ecological community over time.
Water Table – The upper level layer of an underground surface in which the soils are permanently saturated with water.