RESUMO
Most of accumulation curves tend to underestimate species richness, as they do not consider spatial heterogeneity in species distribution, or are structured to provide lower bound estimates and limited extrapolations. The total-species (T-S) curve allows extrapolations over large areas while taking into account spatial heterogeneity, making this estimator more prone to attempt upper bound estimates of regional species richness. However, the T-S curve may overestimate species richness due to (1) the mismatch among the spatial units used in the accumulation model and the actual units of variation in ß-diversity across the region, (2) small-scale patchiness, and/or (3) patterns of rarity of species. We propose a new framework allowing the T-S curve to limit overestimation and give an application to a large dataset of marine mollusks spanning over 11 km2 of subtidal bottom (W Mediterranean). As accumulation patterns are closely related across the taxonomic hierarchy up to family level, improvements of the T-S curve leading to more realistic estimates of family richness, that is, not exceeding the maximum number of known families potentially present in the area, can be considered as conducive to more realistic estimates of species richness. Results on real data showed that improvements of the T-S curve to accounts for true variations in ß-diversity within the sampled areas, small-scale patchiness, and rarity of families led to the most plausible richness when all aspects were considered in the model. Data on simulated communities indicated that in the presence of high heterogeneity, and when the proportion of rare species was not excessive (>2/3), the procedure led to almost unbiased estimates. Our findings highlighted the central role of variations in ß-diversity within the region when attempting to estimate species richness, providing a general framework exploiting the properties of the T-S curve and known family richness to estimate plausible upper bounds in γ-diversity.
RESUMO
Taxonomic distinctness has been applied successfully for the exploration of biodiversity patterns, yet its relevance in environmental impact assessment is far from being unquestioned. In this study, we assessed the potential of taxonomic distinctness to discern perturbed and unperturbed sites by analysing Mediterranean nematode assemblages. Geographic and habitat-related effects on the performance of the index were also explored. Above all, our findings do not corroborate the conjecture that taxonomic distinctness could be largely unaffected by natural variability, habitat features, and biogeographic context, casting doubts on potential generalization concerning its application as an indicator of environmental stress. Taxonomic distinctness represents an excellent metric to identifying taxonomic properties of ecological systems but, as for other ecological indices, it should be viewed as a complementary tool in environmental impact assessment, due to its sensitiveness to specific environmental features of systems being investigated.
