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

We present a new similarity index, taxonomic similarity (Delta (S)), which can be used to measure beta -diversity. Delta (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 Delta (S) is analogous to taxonomic distinctness recently developed for biodiversity assessment at alpha- and gamma- (landscape or seascape) scales. Delta (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 Delta (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 Delta (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 beta -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 WE, 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 beta -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 beta -diversity at small spatial scales. The extent to which patterns observed are evident for other marine species groups is not well known.