Resilience to stress

Resilience usually refers to an individual’s tendency to cope with stress and adversity. This concept is increasingly being used in the scientific fields of ecology and ecosystems research.

Resilience in an ecosystem

An ecosystem is a living community of plants and animals sharing an environment with non-living elements, such as water, sand and rocks in a marine ecosystem. Resilience of an ecosystem is its capacity to withstand stress and reorganise after a disturbance without losing important species of plants or animals.

In a resilient ecosystem, the process of rebuilding after a disturbance promotes new growth and a stronger, more resilient ecosystem. Without resilience, an ecosystem succumbs to the effects of disturbance. For example, pollution over time may cause a clear lake to become a murky and oxygen-depleted pool. The new state may not only become biologically impoverished, but can be irreversible.

Ecosystems are dynamic, interacting and interdependent. Resilience in such an interactive system concerns:

• how much shock the ecosystem can absorb and still remain in a desirable state
• the degree to which the ecosystem can reorganise itself
• the degree to which the ecosystem can adapt.

Stresses in marine ecosystems

Stresses in marine ecosystems may be induced naturally or by humans.
Human stressors include:

• pollution (such as oil spills, PAHsheavy metalsnutrient runoff)
• sedimentation (such as due to deforestation, earthworks)
• overharvesting (such as heavy fishing in one area, trawling).

Natural stressors include:

• sedimentation (natural wearing away of soils around coastlines)
• wave action
• predation.

The collapse of an ecosystem may be caused by gradual stress occurring over time. Stress may build up in increments until the ecosystem suddenly collapses and organisms die off. Collapse may also occur when there is more than one stress on an ecosystem. The ecosystem may be weakened by, for example, pollution. Another stressor such as sedimentation may cause the system to collapse completely. Sometimes, even if stress is reduced from an ecosystem, it may not recover.

A collapsed marine ecosystem example

A well known example of the collapse of a marine ecosystem occurred in Chesapeake Bay, USA, when overfishing and other environmental toxins depleted the oysters. Oysters played an important role as filter feeders. They filtered the water column, keeping in balance the most abundant ocean plant, microscopic algae. The depletion of oysters (there are about 1% left) caused algal blooms – sudden, enormous growth of algae. The algae then died off because it had consumed all the nutrients available. The decomposition of algae caused a depletion of oxygen, resulting in life-depleted areas known as dead zones. Chesapeake Bay’s dead zone stretches for hundreds of square kilometres.

Attempts to restore Chesapeake Bay have been unsuccessful. This ecosystem was benthic dominated – seafloor-dwelling oysters dominated the ecosystem. The oysters had a massive filtration capacity, filtering the entire water column in the Bay, so when the oysters were depleted, it fundamentally changed the way the system worked. The whole ecosystem was affected, and it was unable to recover from the shock.

Biodiversity is the key

The diversity of life in natural ecosystems (biodiversity) is believed to play a crucial role in resilience, helping ecosystems stand up to various natural and human stressors. Ecosystems seem to be particularly resilient if there are many species performing the same essential function (such as filtration or photosynthesis) and if species with the same function respond in different ways to stresses. Species can then replace or compensate for others in times of disturbance. Ecosystems are at risk if biodiversity is reduced. This can occur when humans develop monocultures, such as oysters for farming. The lack of biodiversity contributed to the collapse of the marine ecosystem in Chesapeake Bay.

Marine ecosystems in New Zealand

Marine ecosystems in New Zealand are generally healthy and in good shape. Two problems to be aware of though are sedimentation and overfishing.

Sedimentation is probably the biggest problem inshore because it’s difficult to remove sediment from an ecosystem, although the removal of harvestable species (particularly shellfish) has taken its toll as well. Sedimentation is slow and pervasive, and the effects are difficult to reverse. The mangrove expansion in some estuaries in the Bay of Plenty reflects the input of sediments that have happened over time.

Overfishing, particularly through trawling, is like bulldozing an area and stripping it of its life. This is a particular problem in offshore areas that would otherwise have very little natural disturbance (such as from waves) due to the greater depths. Some marine areas of New Zealand are trawled often, so disturbance, and therefore stress, to those ecosystems is very high. Due to lack of research, it is unknown how these benthic communities might be able to recover from this stress.