Microhabitat heterogeneity and population ecology of an herbivorous rodent, Microtus californicus.

Existing models of microtine population regulation and life history evolution rely on data derived from empirical studies which assume spatial homogeneity of the sampling unit, and infer that population dynamics can be described adequately by aggregation of individuals differing in sex, age and social status. These assumptions were not supported in the present study, and their application would have obscured relationships of ecological and evolutionary importance. From 1975 to 1981 a population of Microtus californicus exhibited seasonal fluctuations in numbers along with two extreme peaks in abundance. Floristic richness changed under the impact of heavy grazing by M. californicus, but floristic spatial pattern showed little interannual variation. All parameters of female survival and reproduction were correlated with floristic micropattern. Voles performed better in vegetation dominated by the native perennial grass Elymus triticoides. Female dispersion was more restricted than that of males. There are no reasons to believe the microhabitat preferred by females affords superior nutrition or protection from predation during peaks of abundance. However, it should provide higher probability of survival during summer, the time of greatest stringency in central California. The implications of these results for current theory are discussed, and a hypothesis developed which emphasises the importance of improved conditions in the most unfavourable season for peaks in abundance.