COMPARISONS OF COMMUNITY STRUCTURE AMONG SMALL MAMMALS OF NORTH-AMERICAN AND AUSTRALIAN DESERTS

Data on the presence of small mammal species at 201 sites in North Ame rican deserts and 245 in arid Australia were used to quantify similari ties and differences in community structure between the two continents . In all, 41 species of rodents were sampled in North America and 26 s pecies, both rodents and marsupials, in Australia. On both continents, each species occurred throughout its geographic range with many other species and as a member of many different combinations. The number of species with which each species coexisted was positively correlated w ith its area of geographic range, although most species occurred at le ss than half the sites within their ranges. The median number of speci es at a site was three in North America and two in Australia, but the numbers of species per site on the two continents were not significant ly different. Despite the relatively small numbers of species per site , numerous different combinations of species were observed (North Amer ica, 158; Australia, 108). Communities in North America were dominated by granivores and in Australia by omnivores; insectivores were also p rominent in Australia. These results support conclusions of an earlier study of granivorous rodents in North America, in suggesting that the re is substantial spatial variation in the composition of small mammal communities. The magnitude of this variation indicates that species a nd communities tend to be individualistic in spatial distribution, suc h that communities are not tightly restricted to particular specific c ombinations. There is evidence from North America that deterministic p rocesses play a limited role in determining community structure; no su ch analyses have yet been undertaken for communities in arid Australia . Trophic structure of communities is markedly different on the two co ntinents. Not all trophic differences can presently be explained, but they may be due to differences between the continents in temporal patt erns of food supply.