An index to compare geographical distributions of species

We present an index for the dissimilarity/distance between geographical distributions based on reporting rates recorded on a regular lattice. Reporting rate data are common, for example, in bird atlas projects where observers fill in check lists of encountered species in a particular area. Our index is a variation of the Euclidean distance, with the contribution of each grid cell weighted by the number of checklists collected for the grid cell, and a scaling factor to ensure that the dissimilarity ranges between zero and one. Reporting rates were transformed to ordered percentile classes to make species with different mean reporting rates comparable. The index was developed for the comparison of distributions of The Atlas of Southern African Birds . We illustrate the dissimilarity index comparing distributions of whydahs and indigobirds (widowfinches), which are specialized brood parasites, to the distributions of their hosts: waxbills and other finches.