Density‐Dependent Processes in Meadow Voles: An Experimental Approach

Patterns of population dynamics are determined by the interaction between density—dependent and density—independent processes. To examine the responsiveness of various demographic and behavioral processes to population density, we maintained replicate meadow vole (Microtus pennsylvanicus) population at low, medium, and high densities inside field exclosures in southern New York. Density was manipulated by removing subadults during biweekly live—trapping sessions. Under higher density conditions voles exhibited: (1) a truncated breeding season; (2) reduction in growth rates of individuals; (3) increase in age (but not in size) at sexual maturity; and 4) reduction in movement distances of individuals. Extremely large—bodied voles were most frequent in low—density enclosures and least common in high—density ones. Survival rates of adults were delayed density—dependent. Reproductive rates during the middle of the breeding season and survival rates of juveniles and subadults were not affected by vole density. We conclude that vole populations are stabilized by density—dependent reductions in reproductive rate and recruitment, but destabilized by density—independent survival rates of young, and by density—dependent compression on individual movements, which accommodates population growth. The time delay in the effects of density on survival of adults may provide a mechanism for vole cycles in which lagged density—dependent mortality plays a central role.