The concept of ecological resilience was first introduced by the ecologist Crawford "Buzz" Holling. But why study the resilience of ecosystems?

Well it is important that we understand how an ecosystem reacts to different changes and how the organisms in it behave, so that we are able to manage our ecosystems and help them recover from disturbance, most of which are unfortunately caused by human activities. This week's article will go into a bit of depth defining the concept of ecological resilience.

Firstly, there are two definitions of ecological resilience found in the literature. The first is by Holling and basically defines ecological resilience as the ability of an ecosystem to absorb changes and still exist. However, another school of thought is that ecosystems continuously change in response to disturbance. Therefore there is no equilibrium or undisturbed state for it to return to, so that ecosystems lack the defined state of stability as put forward by Holling. Instead the ecosystem absorbs this disturbance but reorganises and changes so that it retains essentially the same function, so that there are multiple states when the ecosystem undergoes stability. It is this second definition that is currently the accepted one for defining ecological resilience.

After defining ecological resilience, one has to understand the four critical aspects of this concept which are; latitude, resistance, precariousness and panarchy. The first aspect, latitude refers to the maximum amount of change a system can undergo before it loses its ability to recover. This does not necessarily mean returning to its original state but rather being able to still carry out its ecosystem functions. The second concept which is resistance means the difficulty the ecosystem experienced in responding to the change or if the change was made easily. Then there is the concept of precariousness which refers to how close the current ecosystem is to a state of stability. Finally, there is panarchy which can be defined as "the degree to which a certain hierarchical level of an ecosystem is influenced by other levels". For example, let us look at an ecosystem with respect to the organisation of the populations of species found there. In community A the populations are isolated and little interaction with each other occurs, while in community B they interact with each other more frequently. Therefore one can say that the community level structure is influenced by the interactions at the population level.

Some disturbances that ecosystems experience include fires, floods, invasion of pest species as well as human activities such as, pollution, deforestation and the introduction of non-native or exotic species of plants and animals. To better understand how ecological resilience works, here is an example of a disturbance caused by human activity and how the ecosystem deals with it. Agriculture is a human activity that is actually good for the ecosystem in that it adds carbon from decaying plant matter and nitrogen from fertilizers into the soil, both of which provide nutrients essential to plant growth (the ecosystem's response). However, when agriculture is practised on an intensive level it puts a strain on the ecosystem.

The many plants that are being grown require extra nutrients in terms of fertilizers. These chemicals along can be washed into our watercourses causing an increase in aquatic plant growth, this is known as eutrophication - which is the ecosystem's response. Unfortunately another response is that it in turn robs oxygen from other aquatic organisms and thus a decrease in aquatic biodiversity. The presence of crops also causes an increase in certain species of insects to pest levels in response to the presence of their food source in large quantities. These pesticides enter the environment and can kill a wide variety of animals thus changing the flow of energy in food webs in terms of the animals that get eaten and those that acts as predators. So that although the organisms and their numbers in the food web of the area has changed, the ecosystem still reaches a level of stability as there are still consumers and producers.

Another example is that of bush fires, which can start naturally or be maliciously set by humans. When an area is burnt the ecosystem of course loses its biodiversity as many plants and animals were killed or as a result of habitat loss many animals have migrated out of the area to more suitable habitats. But the area also cannot perform some of the ecosystem functions it did before the fire, such as carbon dioxide absorption, holding the soil together to prevent against erosion because the vegetation is gone. However, the fire having burnt the soil actually releases nutrients which plants such as grasses readily use and grow rapidly. Therefore the ecosystem functions are not lost for long, but they are taken over by smaller plants rather than trees.

Therefore there are two important properties of an ecosystem; resilience and stability.

From the two examples we can see that humans and the environment are involved in a "tug of war" so to say as we keep disturbing the environment and the environment has to find a way to cope with it and continue existing and functioning.

But the question we have to ask ourselves is, are we pushing our environment too far and expecting it to recover?