Effects of environmental stress on species rich assemblages

Abstract

Selye's widely used model of responses of individual organisms to a stressor (1973, American Science, 61: 692 699) is not appropriate for describing effects at the population or community level. At the ecosystem level a number of functional responses have been suggested by Rapport, Regier & Hutchinson (1985, American Naturalist, 125: 617–640) but detailed analysis shows that, in general, functional responses are not sensitive to the early detection of impending ecosystem damage. Three clear changes in community structure occur in response to stressors. These are reduction in diversity, retrogression to dominance by opportunist species and reduction in mean size of the dominating species. Statistically significant reductions in diversity occur rather late in the sequence of increased stressor impact. The first stages of impact are clearly shown by moderately common species yet most attention has concentrated on the common species. Species which dominate in heavily stressed habitats are often species complexes and the possible genetic mechanisms causing this are considered. Whilst changes in the mean size of the dominant organisms can be shown in experiments there is no clear evidence that recorded reductions in the size of North Atlantic and North Sea plankton are induced by man-made stressors.