Biodiversity evaluation of the macroalgal flora of the Gulf of Trieste (Northern Adriatic Sea) using taxonomic distinctness indices

Recently a new index has been proposed for the evaluation of biodiversity: taxonomic distinctness. One of the positive features of this index is that it is neither sampling-effort nor sample-size dependent. Until now, its application has been limited to the assessment of zoobenthos and fish biodiversity. The main objective of this paper was to test the applicability of this index to the macroalgal flora of the Gulf of Trieste (Northern Adriatic Sea). For this purpose the flora recently censused in this area was compared with a checklist of the entire region compiled from the literature. Two indices were mainly used for this study: average taxonomic distinctness based on presence/absence data (AvTD), and variation in taxonomic distinctness (VarTD). Their relationship with species richness was also assessed. The distinctness was compared with statistically significant limits estimated using randomisation tests made on the local master species list. On the same data set multivariate analysis based on a taxonomic similarity index was performed, and a 2nd stage MDS was used to compare results at four taxonomic levels. The results confirmed that statistical over-threshold situations can be highlighted only by one index (AvTD or VarTD) or only by their relationship (AvTD vs VarTD) and not necessarily at the same time by both indices and their relationship. While the average distinctness (AvTD) did not show values significantly different from the expected ones, the variation (VarTD) in one site showed values significantly exceeding confidence limits. This situation has already been described for zoobenthic communities and explained as a consequence of a decrease in habitat diversity. In this case it might be the result of the reduced presence of hard substrata suitable for macroalgal colonisation. The joint analysis of both parameters (AvTD vs VarTD) revealed one site exceeding the 95% confidence limit, which was not identified by analysing only one parameter at a time. This significant over-threshold pattern in the relationship of the distinctness indices could be explained by a relative increase of Rhodophyceae, attributable to the intensive grazing of the sea urchins at this site. The analysis of taxonomic distinctness indices at each site compared with the 95% probability funnels or ellipses derived from the regional species pool gave results consistent with the ones obtained using multivariate analysis. The results of this study suggest that the method may have more general validity.