Seascape biodiversity patterns along the Mediterranean and the Black Sea: lessons from the biogeography of benthic polychaetes

The purpose of this paper is to investigate seascape biodiversity patterns along the Mediterranean and the Black Sea through the study of the benthic polychaete biogeography of the region. A set of non-parametric multivariate analyses and recently developed diversity indices were performed on the benthic polychaete inventories of the areas of the Mediterranean and the Black Sea. Both the numbers of species and the multivariate analyses demonstrate a west-east zoogeocline, shown by the decreasing number of species and by the multivariate similarity pattern of the areas. The performance of the 'second-stage' multi-dimensional scaling (MDS) shows, when species information is aggregated to genera, results belonging practically to the same similarity pattern, independent of the similarity coefficients utilized. The same similarity pattern is derived when species information is aggregated to the zoogeographical categories of amphi-Atlantic, Atlanto-Mediterranean, cosmopolitan and endemic species. BIO-ENV analysis reveals a large number of geographic, climatic and trophic variables to be highly correlated with the similarity pattern derived from the various taxonomic/zoogeographical categories. The synergy, however, of the environmental variables is best reflected in the case of the endemic Mediterranean species. The latter category is considered as the critical zoogeographic category with respect to providing information on the evolutionary history of the taxon in the region. The application of the average taxonomic distinctness and the variation of taxonomic distinctness indices resulted in a diversity ranking of the areas, which appears to be independent of the number of species hosted in each area. However, this is not the case for the phylogenetic diversity (PD) index. Additionally, the former 2 indices indicate that the benthic polychaete species-pool, hosted in the Mediterranean and the Black Sea areas, may well serve as a useful basis for future comparisons in environmental assessment studies. Finally, results from the application of the area-diversity formula show that the equilibrium model can be applicable for the endemic benthic polychaetes of the region: the number of endemic species can be considered as a function of the degree of isolation of the area from the source region against the degree of within-area isolated habitats.