We present a new similarity index, taxonomic similarity (∆ S ), which can be used to measure β-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 α-and γ-(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 β-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 Wé, 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 β-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 β-diversity at small spatial scales. The extent to which patterns observed are evident for other marine species groups is not well known.KEY WORDS: Biodiversity · β-diversity · Taxonomic similarity · Echinoderms · Indo-West PacificResale or republication not permitted without written consent of the publisher

We examine biodiversity in Tarut Bay, Saudi Arabia, around the effluent of a refinery that has operated since 1945. The spatial study involved quarterly assessment of diatom biodiversity in 1976/77 at 24 sites. Taxonomic distinctness decreased significantly towards the refinery outfall, in contrast to the pattern for number of species, Simpson's index and Margalef's species richness index. Ammonia-nitrogen concentrations and primary productivity (mg chlorophyll-a cm−1 d−1) also increased significantly towards the effluent, probably due to nearby sewage/nutrient enrichment. The spatio-temporal assessment involved comparison of echinoderm biodiversity at eight sites in 1945 through 1947 and 1977. No significant temporal differences were observed in number of species, taxonomic distinctness or beta-diversity, nor was significant correlation detected between biodiversity and proximity to the refinery outfall. The absence of obvious impact is confirmed by statistical comparison of taxonomic distinctness for echinoderms in Tarut Bay with values for the whole Gulf. Periphyton in Tarut Bay may be perturbed towards the refinery outfall, ‘positively‘ when measured by Simpson's and other ‘absolute’ biodiversity measures, but ‘negatively’ according to taxonomic distinctness, a measure of ‘average’ properties. Hence, diatom persistence stability appears moderate to low. In contrast, the echinoderm fauna may exhibit high stability, explained by either high persistence (withstanding perturbation), or high resilience (perturbation experienced, but system reverted or acclimated to ‘normal’ Gulf state) over a time scale of 30 y.

Analysis of echinoderm datasets for the Arabian Gulf and Red Sea region reveals lower species richness in the Gulf at 4 different spatial scales: Point, Sample, Large area and Biogeographical province. Species richness correlates positively with spatial scale in both regions, as does endemism, although levels of the latter are comparable for both regions. Values of taxonomic distinctness (Δ*), a relatedness measure based on average properties, are very similar for the Gulf and the Red Sea over all spatial scales. This might seem surprising, given that (i) the Gulf's environment is highly stressful, and (ii) the Red Sea is one the world's top 10 coral reef high diversity areas, and the habitat associated with most echinoderm records for this region, but not for the Gulf. These findings may partly reflect the behaviour of Δ* to ‘saturate’ (reach maximal values as sampling/spatial scale increases) much sooner than species richness, endemism and other ‘species’ measures. Our results suggest the Gulf may not be the lowspot of biodiversity generally acclaimed, especially if a broad set of measures is utilised. Biodiversity is an important constituent of ecosystem health and criterion for determining where conservation attention should be targeted. Until the functional/ecological significance of Δ* is better understood, management implications of our study's findings remain unclear. Further attention should also be given to β-diversity, the change in species composition along a gradient. We also stress that any conclusions drawn from the study are tentative, pending compilation and analysis of datasets for other groups.

We examine biodiversity in Tarut Bay, Saudi Arabia, around the effluent of a refinery that has operated since 1945. The spatial study involved quarterly assessment of diatom biodiversity in 1976/77 at 24 sites. Taxonomic distinctness decreased significantly towards the refinery outfall, in contrast to the pattern for number of species, Simpson'sindex and Margalef'sspecies richness index. Ammonia-nitrogen concentrations and primary productivity (mg chlorophyll-a cm−1 d−1) also increased significantly towards the effluent, probably due to nearby sewage/nutrient enrichment. The spatio-temporal assessment involved comparison of echinoderm biodiversity at eight sites in 1945 through 1947 and 1977. No significant temporal differences were observed in number of species, taxonomic distinctness or beta-diversity, nor was significant correlation detected between biodiversity and proximity to the refinery outfall. The absence of obvious impact is confirmed by statistical comparison of taxonomic distinctness for echinoderms in Tarut Bay with values for the whole Gulf. Periphyton in Tarut Bay may be perturbed towards the refinery outfall, ‘positively’ when measured by Simpson's and other ‘absolute’ biodiversity measures, but ‘negatively’ according to taxonomic distinctness, a measure of ‘average’ properties. Hence, diatom persistence stability appears moderate to low. In contrast, the echinoderm fauna may exhibit high stability, explained by either high persistence (withstanding perturbation), or high resilience (perturbation experienced, but system reverted or acclimated to ‘normal’ Gulf state) over a time scale of 30 y.