AN ESTIMATOR OF SPECIES OVERLAP USING A MODIFIED BETA-BINOMIAL MODEL

A new species overlap measure is defined as the probability that, give n a randomly selected species is present in at least one of the two co mmunities, it is present in both communities. This measure can be thou ght of as a community analog of the Jaccard index for species overlap between two samples. Standard maximum likelihood methods are used to e stimate the parameters of a modified beta-binomial distribution. This leads directly to estimates of the community overlap measure. In an ex ample from decapod communities on coral reefs. it is shown that this n ew estimate of community species overlap is 50% larger than the common ly used Jaccard measure of species overlap.