O que ganhamos 'confundindo' riqueza de espécies e equabilidade em um índice de diversidade?

Abstract

In Community Ecology and in many applications of Conservation Biology, diversity means variety of species, which may or not include information on the relative importance of each species. Diversity is one of the most important attributes in the study of communities and, as a result, many methods are available to its measurement. Among them, non-parametric diversity (or heterogeneity) indices such as Shannon and Simpson formulae are widely employed in a range of studies. These indices are composed of (or confound) two components, species richness and evenness. Different indices can be obtained combining the two components with different weights. The lack of an objective criteria to guide appropriate weighting of each component results in an arbitrary decision to use an index and not another. Additionally, depending on the weight the indices give to each component, an index may indicate that community A is more diversified than community B while a second index may indicate the contrary. Also, diversity indices applied to samples differing in species richness and evenness may produce similar values. Such problems can be avoided using alternative methods. One of them is diversity profile, which includes not one index but many diversity indices differing in the weight given to each component. Additional alternatives include the use of species richness only, Whittaker’s diagram (or of dominance) and scatter diagrams with axes defined by species richness and an evenness index. Except by species richness, the cited alternative methods show graphically much more information than that contained in a single value produced by a diversity index. In studies requiring a response variable to be modeled in relation to predictor variables (Linear Models such as Regression and Analysis of Variance), I suggest the separate use of species richness and evenness as each one may reflect different aspect of communities.