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 simultaneously1,2,3. However, ecologists disagree for two main reasons about the relative roles and influences that density-dependent and density-independent processes have in determining population size4,5. First, empirical studies showing both processes operating simultaneously are rare6. Second, time-series analyses of long-term census data sometimes overestimate dependence7,8. By using a density-perturbation experiment9,10,11,12 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.
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We thank M. Evans, E. Gillis, E. Gold, L. Hofer, J. Hoyle, H. Jewell, J. Lindell, C. McColl, C. Riley, W. Ruscoe, C. Walasek and J. Wilson for field assistance, and D. S. Hik, C. J. Krebs and J. R. Rosenfeld for comments and suggestions on the manuscript. This study was funded by a Northern Studies Training Program grant (Department of Indian and Northern Affairs) to T.J.K., a Natural Sciences and Engineering Research Council (NSERC) grant to R.B. and a NSERC collaborative special project grant to the Kluane boreal forest ecosystem project. T.J.K was supported by a fellowship from the University of Toronto.
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