Measuring b-diversity using a taxonomic similarity index, and its relation to spatial scale

Abstract

We present a new similarity index, taxonomic similarity ( S ), which can be used to measure -diversity. S utilises species presence/absence data, and incorporates both higher taxon richness and evenness concepts. It is derived from the average taxonomic distance (relatedness) of any 2 species from different sites. Therefore S is analogous to taxonomic distinctness recently developed for biodiversity assessment at and -(landscape or seascape) scales. S is a new index, although its derivation uses a concept similar to the 'optimal taxonomic mapping statistic' developed independently for quantifying structural redundancy in marine macrobenthos. Using echinoderm data, we show that S exhibits smoother behaviour and is less influenced by species richness, and hence sampling effort, than the widely used Jaccard coefficient of species similarity. We also believe S to be a more intuitive and comprehensive measure of similarity than Jaccard and other conventional indices based solely on species held in common. Taxonomic similarity between sites is computed for echinoderms examined over 3 different spatial scales: local/small-scale (<10 km), intermediate-scale (10 to 100s km) and province/oceanic-scale (100s to 1000s km). Taxonomic similarity between sites increases progressively with spatial scale, with significantly lower values and higher -diversity at small spatial scales. The same pattern is evident for species similarity, using the Jaccard coefficient. Possible explanations for this pattern centre on: (1) the large-scale oceanic area examined (Indo-West Pacific), representing a metapopulation of echinoderms for the 2 other, smaller areas examined within (Pula W, Sumatra and Lakshadweeps); (2) greater biophysical instability and unpredictability at small spatial scales. Compared with larger spatial scales, these may be characterised by greater likelihood and influence of species migrations and extinctions on a site's total species composition. Hence, species composition may be highly changeable at small scales, leading to high -diversity. These findings are based on 1 set of comparative data for 1 faunal group. Any wider conclusions drawn would be premature, although corals may also show greater -diversity at small spatial scales. The extent to which patterns observed are evident for other marine species groups is not well known.