What is the Impact of Stressors on Ecosystems?

Here a clear distinction between point and non-point sources is necessary before analyzing the po­tential effects of stressors on the structure and function of ecosystems.

Such stressors which impinge on small parts of the land­scape can be taken as point sources. Acid rain, diseases or regionally- dispersed chemicals or pollutants are non-point stressors. These latter may pervade through the entire ecosystem, causing physical destruction of cer­tain important structural components of the ecosystem.

Ecosystem homoeostasis is generally achieved through interactions among functional groups of organisms and abiotic components.

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The effect of stress on ecosystems is best exemplified by eutrophication which is caused by forced input of nutrients into water. Increased primary production causes oversaturation of water with oxygen but even then the oxygen content is often insufficient for complete decomposition of the or­ganic matter produced and a part of the latter is deposited on the bottom sediment.

Some oxygen also escapes into the air. In the hypolimnion, denitrification results in escape of some nitrogen to the atmo­sphere, whereas the increase in water pH, resulting from the assimilatory activity of phytoplankton, precipitates inorganic phosphate. Thus the conse­quences of ecosystem fertilization are partly counteracted by some of the available elements being temporarily driven out of the cycle.

McCormick (1978) made a comparative study of tropical and temperate ecosystems with regard to their response to stressors, and proposed three ways in which these systems tend to recover following perturbations. The most common pattern involves the temporal and spatial replacement of species. Such replacement is generally faster than in typical succession.

Secondly, some species can persist by altering rates of physiological pro­cesses in keeping with changes in environment. The third recovery strategy is resistance which can be exhibited by certain species or even individuals that are so well established that perturbations do not markedly harm them.

One general characteristic of most terrestrial ecosystems seems to be that there is more dead than living organic material. Similarly, most of the flux through heterotrophic respiration in the ecosystem is accomplished by decomposers.

This energy flux through decomposer microbes, plants and animals is a fairly new appreciation of ecosystem dynamics. Ecosystems tend to allocate a significant amount of their total energy resources to internal recycling and in this respect differ from most man-modified sys­tems where energy resources are exported.