New 'biodiversity' measures reveal a decrease in taxonomic distinctness with increasing stress

We demonstrate a continuous decrease in the taxonomic distinctness of a marine assemblage along a gradient of increasing environmental contamination, in a situation where species diversity remains constant. Two indices have been employed, Delta and Delta*, the first being a taxonomic diversity index empirically related to Shannon species diversity (H') but with an added component of taxonomic separation, and the second a measure purely of taxonomic distinctness. The values of both indices appear to be rather less influenced by sample size than does H', and markedly less sample-size dependent than other common diversity measures such as species richness and evenness. It is concluded that taxonomic distinctness may be a more sensitive univariate index of community perturbation than species diversity. We also argue that Delta comes closer to a 'biodiversity' index than H', and suggest the possibility that the total genetic complement in any biome may, within limits, remain more or less constant but be partitioned differently among the hierarchy of taxonomic units, according to the age or successional stage of the assemblage.