Seven forms of rarity in mammals

Conservation biologists have identified threats to the survival of about a quarter of the mammalian species; to identify patterns of rarity and common ness of mammals, we studied a global sample of 1212 species (about 28% of t he mammals) using the '7 forms of rarity' model (in which species are rough ly divided into above and below the median for local population density, sp ecies' range area, and number of habitat types). From a niche-based hypothesis of abundance and distribution, we predicted t hat mammals would exhibit a bimodal pattern of rarity and commonness, with an overabundance of species in the relatively rarest and most common catego ries; and just such a significant bimodal pattern emerged, with over a quar ter of the species classified as exceedingly rare and a further quarter ver y common, supporting the niche-based hypothesis. Orders that include large mammals, including perissodactyls, primates, dipr otodonts, and carnivores, exhibited significantly high proportions of relat ively rare species; and tropical zoogeographic regions, especially Indomala ya, had relatively high proportions of species in the rarest category. Significant biases in the available data on mammals included under-sampling of small species like rodents and bats, and a relative paucity of data on zoogeographic regions outside of North America and Australia. Mammalian species listed as of conservation concern by the IUCN occurred in all cells of the model, indicating that even relatively common species can be listed as threatened under some conditions; but we also found that sixt y-three species were relatively rare in all three criteria of the 7-forms m odel but were not listed as threatened, indicating potential candidates for further study. Mammals may be a group of animals where rarity or commonness is a natural a spect of species biology, both confirming and perhaps partly explaining the large proportion of mammals assigned threatened status.