The Relationship between Habitat Heterogeneity, Space Use, and Demography in a Population of California Voles

Abstract

The relationships between habitat heterogeneity, space use, and demography were analyzed in a population of California voles (Microtus californicus). Live-trapping was conducted on a monthly basis from 1977 to 1981 on four grids, each sampling a different microhabitat type within a meadow. High quality habitat patches (having high percent cover of the perennial grass Elymus triticoides) were characterized by higher peak densities, more strongly female-biased sex ratios, longer average persistence (= survivorship), and higher rates of juvenile recruitment than were lower quality patches (Conium maculatum and annual grasses). For females, probability of persistence varied markedly in response to seasonal (wet vs. dry) variation in resource quality, while for males, probability of persistence was lower and constant across all seasons. The study included a period of low density (1977-1979) followed by an irruption (spring 1980) and subsequent crash in vole numbers. Persistence of females did not vary with population density, but persistence of males was inversely related to density. Females tended to aggregate in high quality areas but the distribution of males was more uniform. Females co-occurred at individual trap sites within 2-3 d trapping periods significantly more frequently than did males. In addition, the frequency of co-occurrence of females was higher in better quality habitat patches than in poorer microhabitat, whereas the frequency of co-occurrence of males was spatially unvarying. Both sexes showed an increased frequency of co-occurrence with increased density; however, co-occurrence of females was often high even at low densities, while co-occurrence of males was virtually non-existent at low densities. We believe that the responsiveness of females to both spatial and temporal variation in resource quality reflects a direct reliance on resource acquisition for reproductive success, and that their tendency to overlap intrasexually indicates a lack of strong intrasexual interference. In contrast, the lack of responsiveness of males to resource variation may reflect a relative independence from resource acquisition in seeking reproductive success, while their low co-occurrence and greater sensitivity to density variation indicates the presence of strong intrasexual interference.