DENSITY-DEPENDENT PROCESSES IN MEADOW VOLES - AN EXPERIMENTAL APPROACH

Patterns of population dynamics are determined by the interaction betw een density-dependent and density-independent processes. To examine th e responsiveness of various demographic and behavioral processes to po pulation density, we maintained replicate meadow vole (Microtus pennsy lvanicus) populations at low, medium, and high densities inside field enclosures in southeastern New York. Density was manipulated by re mov ing subadults during biweekly live-trapping sessions. Under higher den sity conditions voles exhibited: (1) a truncated breeding season; (2) reduction in growth rates of individuals; (3) increase in age (but not size) at sexual maturity; and (3) reduction in movement distances of individuals. Extremely large-bodied votes were most frequent in low-de nsity enclosures and least common in high-density ones. Survival rates of adults were delayed density dependent. Reproductive rates during t he middle of the breeding season and survival rates of juveniles and s ubadults were not affected by vole density. We conclude that vole popu lations are stabilized by density-dependent reductions in reproductive rate and recruitment, but destabilized by density-independent surviva l rates of young, and by density-dependent compression of individual m ovements, which accomodates population growth. The time delay in the e ffects of density on survival of adults may provide a mechanism for vo le cycles in which lagged density-dependent mortality plays a central role.