ARCUS 13th Annual Meeting and Arctic Forum 2001

Concurrent Density Dependence and Independence in Populations of Arctic Ground Squirrels

Tim J. Karels¹, Rudy Boonstra^2
1Division of Life Sciences, University of Toronto at Scarborough, 1265 Military Trail, Scarborough, ON, M1C 1A4, Canada, Phone 604/822-5942, karels@zoology.ubc.ca
²Division of Life Sciences, University of Toronto at Scarborough, 1265 Military trail, Scarborough, ON, M1C 1A4, Canada, Phone 416/287-7419, Fax 416/287-7642, boonstra@scar.utoronto.ca

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 rare⁶. 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.

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