An improved environmental index based on the relative abundance of oligochaete species

Abstract

Howmiller & Scott (1977) presented an ‘Environmental Index’ based on thorough knowledge of the ecological demands of species of freshwater oligochaetes frequently found in North American waters. These authors emphasized the fact that common diversity measures do not utilize information about the ecological attributes of the constituent species but rather about community structure. A modification of the ‘Environmental Index’ is here presented with the aim of increasing its sensitivity in Europe and possibly also in North America. Four ecological groups are here preferred to the original three, and the ecological valence of the ‘key-species’ Tubifex tubifex is discussed at some length. Whereas most environmental and diversity indices remain little affected by changes in the total abundance of oligochaetes, the modified index is sensitive to such changes. Comparisons are made with two of the most popular diversity indices applied to specific situations, i.e. the Shannon-Weaver index and Simpson's index.

In spite of recommendation in pollution manuals, indices of diversity may be inadequate in correctly characterizing situations of nutrient enrichment of freshwater environments. In the present study it is stressed that low diversity is usually found at both ends of the trophic scale and high diversity, which is supposed to characterize a clean and unstressed environment, is generally obtained in zones of transit between oligotrophic and eutrophic conditions — a likely result of the ‘nonequilibrium state’ described by Connell (1978) from coral reefs and rain forests in the tropics.