Measuring changes in taxonomic dissimilarity following species introductions and extirpations

Species extirpation and non-native species introduction induce changes in compositional dissimilarity among communities. This major component of the current biodiversity crisis has been measured using different metrics since a methodological consensus is still lacking. Here, we provide a consensual framework to assess and discuss the changes in taxonomic dissimilarity.Using a complete mathematical formalism of how extirpation and introduction processes affect changes in taxonomic dissimilarity, we compare the ability of the two most commonly used indices (Jaccard's and beta-sim) to detect the effects of these changes in species composition and richness. Simulations showed that the two indices indicate opposite direction of changes in more than 14% of the cases studied and that in most of the remaining cases the two indices show a discrepancy of more than 10%.By returning to the definition of the dissimilarity concept we demonstrate that the Jaccard index is the most appropriate to measure changes in taxonomic dissimilarity whereas the beta-sim index only measures species turnover. Finally, the changes observed in Jaccard's dissimilarity can be decomposed into changes in taxonomic turnover and changes in taxonomic nestedness. Under the context of global change, the framework we propose will be useful as a toolbox to measure and predict human impact on biodiversity.