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 not exhibit this structure. Results suggest that three conditions may be necessary for the development of nested subset structure: (1) a common biogeographic history, (2) generally similar contemporary environments, and (3) hierarchical organization of niche relationships. The presence of this structure in diverse continental communities indicates that it is not solely an attribute of island communities but is a more general ecologtical property. Moreover, nested subsets identify a greater role of coevolution than is recognized by current community theories. Until now, the occurrence of species in many different combinations at different sites within a region has been taken to imply spatial instability in selection pressures for resource partitioning.