Determinants of Small Rodent Distribution and Abundance in a Shrub-Steppe Ecosystem: Influences of Seeds, Ants and Shrubs

This study had two major objectives; firstly to test the hypothesis of ongoing competition for seeds between small rodents and harvester ants, Pogonomyrmex occidental is, in a cold desert ecosystem; secondly to elucidate how seed abundance and shrub cover influence the distribution and abundance of deer mice, Peromyscus maniculatus. The study area was a sagebrush (Artemisia tridentata) dominated shrubsteppe in southwestern Wyoming. Responses of the small mammal community to ant removal and food addition were assessed on replicated study plots between September 1981 and September 1983. Food addition and shrub removal manipulations were continued from September 1983 to August 1984. Rodent populations were sampled by live-trapping. Movements of 40 deermice were followed by radiotracking during the winter, spring and summer of 1983-84. Ant removal elicited little response from the rodent community, although seed preference trials with native seeds in the field indicated considerable overlap in seed use. concluded that competition was not a factor affecting small rodents during the period of this study, although the area cleared of ants may not have been sufficiently large to produce a response detectable by trapping. Seed addition resulted in increased deermouse population size, prolonged breeding, decreased winter (but not summer) home range sizes, and during periods of low density, increased grid fidelity. High fall densities were followed by a classical spring decline with associated female sex ratio skew and severe injuries to both sexes. Concluded that deermice were food limited only to the point where social interactions during the breeding season limited maximum densities. The absence of shrubs prevented winter use of the 1.25 ha cleared area by deermice. At this time on the non-cleared plots shrubs were used to support nests, as foraging sites and as a means of access to the snow surface. During summer the area cleared of shrubs provided attractive foraging habitat, but most nest sites were still located within shrub cover around the edge. An important corollary of the radio tracking was the illustration of the large home range sizes of deermice in relation to the size of the 0.7 ha trapping grids. The implications of trapping bias affecting population fluctuations and measurement of other demographic parameters from trapping studies are discussed