Empirical relationships between species richness, evenness, and proportional diversity

Diversity (or biodiversity) is typically measured by a species count (richn ess) and sometimes with an evenness index; it may also be measured by a pro portional statistic that combines both measures (e.g., Shannon-Weiner index H'). These diversity measures are hypothesized to be positively and strong ly correlated, but this null hypothesis has not been tested empirically. We used the results of Caswell's neutral model to generate null relationships between richness (S), evenness (J'), and proportional diversity (H'). We t ested predictions of the null model against empirical relationships describ ing data in a literature survey and in four individual studies conducted ac ross various scales. Empirical relationships between or and differed from l og S or J' and H' differed from the null model when <10 species were tested and in plants, vertebrates, and fungi. The empirical relationships were si milar to the null model when >10 and <100 species were tested and in invert ebrates. If >100 species were used to estimate diversity, the relation betw een and log S and H' was negative. The strongest predictive models included log S and J'. A path analysis indicated that log S and J' were always nega tively related, that empirical observations could not be explained without including indirect effects, and that differences between the partials may i ndicate ecological effects, which suggests that S and J' act like diversity components or that diversity should be measured using a compound statistic .