Some Indices of Diversity

An index of diversity (H) based on the Shannon—Weaver formula, H = — Σn1Pi log Pi, 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 Pi in order to give a new index of community diversity which was sensitive to changes in community structure. Pi defined in terms of relative biomass (Hb) failed to improve the index substantially and Pi was then successfully defined in terms of relative productivity (Hp). An index of community diversity sensitive to changes in relative abundance of all the trophic levels, such as the index Hp, appears to be a necessary prerequisite to comparative community studies.