Museum collections, species distributions, and rarefaction

Biological specimens in museums and herbaria are sometimes used to compare the geographical distribution of different species. In doing so, it is necessary to account for differences in the numbers of specimens. We show how rarefaction can be used for this purpose. Rarefaction is a simple mathematical method originally designed to compare species richness in communities that differed in the number of sampled individuals. We present an example involving two Phragmipedium orchid species. In this case, rarefaction suggests that the apparent difference in range can be explained by the difference in the numbers of specimens.