Influences of patch size and microhabitat on the demography of two old-field rodents

We used capture/recapture methods to test the responses of two small mammal species (Peromyscus leucopus and Microtus pennsylvanicus) to small- (microhabitat) and large- (patch) scale habitat variation. Analyses examined the responses of individuals to microhabitat variation among trap stations as well as differences in the density and persistence time of adults and juveniles, and the proportion of reproductive females on experimentally created patches of three sizes (0.0625, 0.25, and 1.0 ha). With the exception of transient P. leucopus, all groups shared significant correlations with microhabitat at the scale of trap stations. By contrast, only juvenile P. leucopus exhibited a response to patch-size (i.e., higher densities on small relative to larger patches). Microhabitat differences among patches also accounted for variation in M. pennsylvanicus densities (but not P. leucopus) in analyses of covariance. Our results suggest that both individuals and populations of M. pennsylvanicus responded to habitat variation at the microhabitat scale, while P. leucopus appeared to respond to both microhabitat and patch scale habitat variation. We note that species characteristics (particularly relative dispersal ability) may prove critical in predicting the scale of habitat responses. We conclude by noting that current theory that assumes uniform responses of population to homogeneous patches is too simplistic to be of much predictive value in field tests.