Influence of rare species on beta diversity estimates in marine benthic assemblages

Beta diversity measures the variability in species composition, and Whittaker's index has become the most widely used measure of β-diversity. However, on soft bottom benthic assemblages Whittaker's index is heavily influenced by the number of species recorded in a single sample (defined as rare species). This over-weighting of rare species induces biased estimates of the heterogeneity, so it becomes difficult to compare assemblages containing an high proportion of rare species. In benthic communities the unusual large number of rare species is frequently attributed to a combination of sampling errors and insufficient sampling effort. In order to reduce the influence of rare species on the measure of beta diversity, we developed an alternative index based on simple probabilistic considerations. It turns out that our probability index is an ordinary Michaelis-Menten transformation of Whittaker's index but behaves more favourably when species heterogeneity increases. Our suggested index therefore seems appropriate when comparing patterns of complexity in marine benthic assemblages.