Some Indices of Diversity

An index of diversity (H) based on the Shannon—Weaver formula, H = — Σ n 1 P i log P i , fails to reflect significant changes in a community's structure because it is only sensitive to changes in relative abundance of a few of the trophic levels of a community. Over two thirds of the species encountered in a typical plankton sample had no significant effect on the index of diversity (H), calculated for that sample. To overcome this, the Shannon—Weaver formula was twice altered by redefining P i in order to give a new index of community diversity which was sensitive to changes in community structure. P i defined in terms of relative biomass (H b ) failed to improve the index substantially and P i was then successfully defined in terms of relative productivity (H p ). An index of community diversity sensitive to changes in relative abundance of all the trophic levels, such as the index H p , appears to be a necessary prerequisite to comparative community studies.