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We examine the operation of an assembly rule to account for the structure of rodent communities in the species-rich southwestern deserts of North America. The rule specifies the functional group from which the species should come, rather than the identity of individual species in the assemblage; it specifies that each species entering a community will tend to be drawn from a different group until each group is represented, and then the rule repeats.

Here we tested for operation of the rule using three data sets: 1) for 115 sites from a small region of Nevada we analyzed 11 species of granivorous rodents distributed among three functional groups with different foraging strategies: bipedal heteromyids, quadrupedal heteromyids, and quadrupedal non-heteromyids; 2) for the same sites, we added folivores and insectivores to the three functional groups of granivores to analyze all 14 species of rodents present; and 3) for 202 sites dispersed across the southwestern U.S. we analyzed 28 species of granivores distributed among three functional groups: bipedal heteromyids, quadrupedal heteromyids, and cricetids. We used 1000 Monte Carlo simulations of random community assembly to produce frequency distributions for the expectations of the null hypothesis to test if observed communities followed the rule overall. Then we compared observed frequencies of particular combinations of functional groups with those expected by chance.

We demonstrate that the rule applies to different functional groupings of desert rodents, and to assemblages at different spatial scales. We show that the neutral model used in the simulations is robust and appropriate. We suggest that the mechanism underlying this rule is interspecific competition, which operates by affecting the probability that species in different functional groups differ sufficiently in resource utilization so as to be able to coexist. Our results demonstrate the usefulness of an approach that incorporates both deterministic and stochastic processes of community assembly.