The English Silurian fossilPlacocystites forbesianusand the ancestry of the vertebrates

Abstract

The effects of pollution by organic matter, oil or industrial waste on marine communities are remarkably similar. Diversity values fall, biomass and numbers of organisms initially rise and then fall as the pollution load is increased. Diversity indices are, however, insensitive to pollution-induced changes and have to be assessed subjectively. Departure from a log-normal distribution of individuals among species offers a sensitive and objective method of assessing perturbation effects on communities. Under severe pollution stress, the dominant species are those which have a flexible life-history ranging from direct development to a planktonic larva and the ability to undergo short-term genetic selection. Species having a somewhat less flexible life-history strategy show increased abundance under conditions of slight pollution. The increase in abundance of seven or eight neither rare nor common species, which gives the departure from a log-normal distribution, is suggested as being the most significant and the earliest detectable change caused by pollution in a community. Thus the presence of a species in a polluted area may be more a question of life-history strategy than the tolerance of adverse environmental conditions. If this hypothesis is correct, considerable doubt must be placed on the ecological relevance of data from toxicity tests.