Regionally nested patterns of species composition in granivorous rodent assemblages

Many island communities exhibit a highly nonrandom pattern of species composition in which smaller biotas contain successive subsets of the species in richer ones. Here, we examine the composition of continental communities to see whether this 'nested subset' pattern holds and, if so, to determine its spatial scale and environmental correlates. Granivorous rodent assemblages at 202 sites in Great Basin, Mojave, Sonoran and Chihuahuan deserts were analysed collectively. Separate analyses were also conducted on sites within each desert, habitat (desert scrub, desert grassland, sand dunes, shrub-steppe), and on desert-habitat combinations. Nested subset patterns of species composition were found to characterize the entire assemblage of 202 sites, all Great Basin sites, all Sonoran sites, and various habitat groupings of sites within deserts. However, groupings of sites by habitats among deserts did