Density and Diversity in Mojave Desert Rodent and Shrub Communities

The mechanisms allowing for coexistence of animal species, and particularly the number of species coexisting in a given habitat, have been explored from many viewpoints. Evolutionary ecologists have investigated the process whereby the species has acquired, the necessary adaptations for exploitation of the habitat (e.g. Bowman 1961; Selander 1966). Biogeographical ecologists have explored the packing of species in an environment and the effects of speciation and immigration upon community composition (e.g. Pianka 1967; Brown 1973). Thirdly, community ecologists have examined species interactions within a relatively stable community to determine how resources are allocated among the species (e.g. MacArthur & MacArthur 1961; Paine 1966; Brown & Lieberman 1973). Most of these studies have concluded that the species composition of a community is the resultant of a complex interaction between all three parameters, i.e. evolutionary, biogeographical and ecological factors. Accordingly, researchers such as Pianka (1967) and Brown (1973) have chosen to simultaneously analyse the effects of all three parameters upon animal species diversity. An alternative method of community analysis, wherein one of the three parameters is observed in closer detail, requires that the remaining two factors be conclusively eliminated as variables. In this study I have chosen to focus upon the ecological mechanisms of granivorous rodent coexistence in the Mojave Desert of southern California. The study transect has been selected to range over a geographically small area of physically similar desert habit such that all rodent species of the study are (at least potentially) evolutionarily and biogeographically able to exploit all study areas. The rodent complpment of each of the study areas is therefore determined by the interactions of the ecologies of the species and, particularly, the effect of certain varying parameters (such as rainfall and seed abundance) upon each species' competitive success. Granivorous rodent species of desert habitats vary remarkably little in general behavior and morphology. The rigors of the desert environment have necessitated physiological adaptations in most species for existence with little or no free water intake. High diurnal temperatures have required that nearly all North American desert rodent species forage at night and spend the day in burrows. Additionally, anti-predatory manoeuvers in areas of sparse vegetation have necessitated adaptations in most species for rapid locomotion in open spaces. The net result of these and other adaptations for desert survival in rodents is convergence toward very similar ecologies. It is the intent of this study to analyze the abundance and diversity of rodents in several Mojave Desert communities in the attempt to uncover predictable patterns related to