Taxon size predicts rates of rarity in vascular plants

We surveyed rarity in the vascular plants of the continental U.S.A. and Canada and the vascular plants of Hawaii to test the hypothesis that rates of rarity are independent of taxonomic group size. We demonstrated that taxonomic groups of plants with few species consistently contained fewer than the expected numbers of rare species. This pattern was apparent at the levels of genus, family, order and class. We also found that the pattern remained when we examined rates of rarity by comparing sister taxa that share a common ancestor. This pattern may arise from either differential speciation and extinction patterns or taxonomic bias in species designations (Lumping and splitting). The pattern of lineages with few species demonstrating reduced rates of rarity is opposite to that previously observed in mammals and birds. If the protection of representatives from a diversity of lineages is a conservation objective, plant conservation is facilitated by the fact that relatively few species-poor lineages contain rare species.