Niche Separation and Assembly in Four Species of Sonoran Desert Rodents

Current ecological theory predicts limits to the closeness of species packing. Increased competition from more competitors should result in greater average niche separation. This idea was tested for locally sympatric heteromyid rodents in part of the Sonoran Desert of Arizona, and the predictions were verified. As a consequence of diffuse competition, niche separation increases with species diversity, and rodent species show separation on both niche dimensions quantified in this study (seed-size collection and habitat utilization). However, when local diversity is low (Dipodomys merriami-Perognathus penicillatus), rodents are very similar in their overall utilization of both dimensions. As diversity increases, first seed-size differences then subsequently both seed-size and habitat differences are apparent. Therefore, the pattern of niche separation and diversity demonstrates niche dimensionality in this fauna. The pattern of niche separation and diversity also accounts for the occurrence of specific combinations of species (local assemblies). Of all possible pairs and triplets of species that can be constructed by computing niche separation, the observed assemblies are those showing minimum separation and hence maximum utilization of resource space. Imaginary assemblies are undersaturated. Compatibility of species does not appear to be important in determining the species composition of these heteromyid rodent assemblies.