Twelve years after three species of kangaroo rats (Dipodomys spp.) were removed from plots of Chihuahuan Desert shrub habitat, density of tall perennial and annual grasses had increased approximately threefold and rodent species typical of arid grassland had colonized. These were just the most recent and drmatic in a series of changes in plants and animals caused by experimental exclusion of Dipodomys. In this ecosystem kangaroo rats are a keystone guild: through seed predation and soil disturbance they have major effects on biological diversity and biogeochemical processes.

Presence/absence data for the small-mammal species at sites in seven deserts were analyzed for evidence of similarity in community structure. The deserts studied were located in North and South America (268 and 118 sites, respectively), Australia (245 sites), Israel (54 sites), and greater Eurasia (Thar, 15 sites; Turkestan, 36 sites; Gobi, 98 sites). Patterns observed in all deserts included: (1) low a diversity (2-4 species per site); (2) high ~ diversity (species turnover between sites); and (3) local coexistence of 20-30% of the species in the regional pool. Additionally, the number of species with which a species co-occurred increased with the number of sites at which that species occurred. Although these results suggested that some features of community structure were similar across deserts, other aspects, especially trophic structure, differed widely. Deserts in the northern hemisphere possessed more granivores, and the Turkestan Desert more folivores, than other deserts. Carnivorous small mammals were most strongly represented in Australia, and omnivores in South America, Australia, and the Thar. The structure of desert small-mammal communities is strongly influenced by historical factors; different taxonomic groups with distinctive trophic adaptations proliferate in different desert regions where they are subject to some common structuring processes of community assembly.

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The contemporary distribution of biological diversity cannot be understood without knowledge of how organisms responded to the geological and climatic history of Earth. In particular, Quaternary expansions and contractions of glacial ice sheets are thought to have played an important role in shaping the distribution of biodiversity among current populations in the north-temperate region. In the central U.S., fossil and palynological data provide support for the maintenance of a large southeastern refuge during the last glacial maximum, and many temperate organisms are believed to have responded to glacial expansion by shifting their ranges to southern refugia and recolonizing northward to track the receding ice sheets. Thus, organisms are assumed to track favorable climates, and species ranges are expected to have shifted significantly. Here we present data from a deciduous forest vertebrate, the eastern chipmunk (Tamias striatus) in the central U.S., indicating the maintenance of multiple refugial sources as well as a southward expansion from a northern refugium. These results challenge the view that, during glacial maxima, organisms must have migrated south out of their ranges to track favorable climates.