Concurrent density dependence and independence in populations of arctic ground squirrels

No population increases without limit. The processes that prevent this can operate in either a density-dependent way (acting with increasing severity to increase mortality rates or decrease reproductive rates as density increases), a density-independent way, or in both ways simultaneously(1-3). However, ecologists disagree for two main reasons about the relative roles and influences that density-dependent and density-independent processes have in determining population size(4,5). First, empirical studies showing both processes operating simultaneously are rare(6). Second, time-series analyses of long-term census data sometimes overestimate dependence(7,8). By using a density-perturbation experiment(9-12) on arctic ground squirrels, we show concurrent density-dependent and density-independent declines in weaning rates, followed by density-dependent declines in overwinter survival during hibernation. These two processes result in strong, density-dependent convergence of experimentally increased populations to those of control populations that had been at low, stable levels.