Using species co-occurrence patterns to quantify relative habitat breadth in terrestrial vertebrates

The breadth of habitats that a species uses may determine its vulnerability to environmental change, with habitat specialists at greater risk than generalists. To test that hypothesis, we need a valid index of habitat specialization. Existing indices require extensive data, or ignore the magnitude of differences among habitat categories. We suggest an index based on patterns of species co-occurrence within each of the 101 habitat categories recognized by the International Union for Conservation of Nature. Using this metric, a species is allocated a quantitative score based on the diversity of other taxa with which it co-occurs: A generalist species occurs in a range of habitat categories that vary considerably in species composition, whereas a specialist species is found only in habitats that contain a consistent suite of other species. We provide data on these scores for 22,230 vertebrate species and show that habitat breadth varies among Classes (amphibians>birds>mammals>reptiles). Within each Class, generalist species are less likely to be in decline or threatened with extinction. Because our index is continuous, based on biologically relevant parameters, and easily calculated for a vast number of taxa, its use will facilitate analyses of the evolution and consequences of habitat specialization.