Ecosysytem stability and resilience . LECTURE 2. SABIHA SULTANA

Ecosysytem stability Stability is commonly defined as a state in which changes does not occur. But stable ecosystem change constantly; for example, population levels increase and decrease from day to day, season to season &year to year. Only fully development or mature ecosystem are stable. They are considered stable because their structure and function remain more or less the same over long periods.

Thet are in a state of dynamic equilibrium (steady state) in which things change but remail more ar less the same.For example- if we examined a mature ecosysytem each spring for an extended period-say, 20 years-we would find that (1)the total number of species was fairly constant from year to year. (2) the same species were present each year. (3) the population size of each species was approximately the same from year to year.

As a tightrope walker moves along the high wire, tilting right, the left but always staying on the wire, So a mature ecosystem undergoes small-scale fluctuations as it remain on its main course. Stability is also the ability of a disturbed ecosysytem to return its original condition.

What is resilience? Why is it important? In ecology, resilience is the capacity of an ecosystem to respond to a disturbance by resisting damage and recovering quickly. Such perturbations and disturbances can include stochastic events such as fires, flooding, windstorms, insect population explosions, and human activities such as deforestation and the introduction of exotic plant or animal species.

The concept of resilience in ecological systems was first introduced by the Canadian ecologist C.S. Holling . Resilience has been defined in following ecological literature: “Ability of a living system to restore itself to original condition after being exposed to an outside disturbance that is not too drastic”.

So,Ecosystem resilience describes the capacity of an ecosystem to cope with disturbances, such as fire, pollution, flooding, windstorms, insect population explosions, and human activities such as deforestation and the introduction of exotic plant or animal species without shifting into a qualitatively different state

A resilient ecosystem has the capacity to withstand shocks and surprises and, if damaged, to rebuild itself.

In a resilient ecosystem, the process of rebuilding after disturbance promotes renewal and innovation. Without resilience, ecosystems become vulnerable to the effects of disturbance that previously could be absorbed.

Clear lakes can suddenly turn into murky, oxygen-depleted pools, grasslands into shrub-deserts, and coral reefs into algae covered rubble. The new state may not only be biologically and economically impoverished, but also irreversible.

Human impacts on resilience As resilience refers to ecosystem's stability and capability of tolerating disturbance and restoring itself so, If the disturbance is of sufficient magnitude or duration, a threshold may be reached where the ecosystem changes state, possibly permanently. However, the elements which influence ecosystem resilience are complicated. For example various elements such as the water cycle, fertility, biodiversity, plant diversity and climate, interact fiercely and effect different systems.

There are many areas where human activity impacts upon and is also dependent upon the resilience of terrestrial, aquatic and marine ecosystems. These include agriculture, deforestation, pollution, mining, recreation, overfishing, dumping of waste into the sea and climate change.

Gradual loss of resilience can lead to unexpected collapse There is increasing evidence that ecosystems seldom respond to gradual change in a gradual way. Studies of forests and oceans also show that human-induced loss of resilience can make an ecosystem vulnerable to random events like storms or fire that the system could earlier cope with

An ecosystem with low resilience can often seem to be unaffected and continue to generate resources and ecosystem services until a disturbance causes it to exceed a critical threshold. Even a minor disturbance can cause a shift to a less desirable state that is difficult, expensive, or even impossible to reverse.

After minor changes induced by nature or human causes a stable ecosystem can ‘bounce back’ to its previous condition. Stabilty is also a property of an ecosystem that causes it to resist being changed by natural events or by human interference (inertia).

What keeps Ecosystems stable? Population growth& environmental resistance. The forces that affect an ecosystem stability are the factors that control the size of its populations. There forces can be broken down Into two group. 1.Growth factor +Reduction factors =Ecosystem Balance.

Growth factors Biotic- Reproductive rate. Ability to adapt to environmental change. Ability to migrate to new habitat. Ability to compete, hide, defend to find food. Adequet food supply.