The demographic basis of population regulation in Columbian ground squirrels

Environmental factors influence the dynamics and regulation of biological p opulations through their influences on demographic variables, but demograph ic mechanisms of population regulation have received little attention. We i nvestigated the demographic basis of regulation of Columbian ground squirre l (Spermophilus columbianus) populations under natural and experimentally f ood-supplemented conditions. Food supplementation caused substantial increa ses in population density, and population densities returned to pretreatmen t levels when the supplementation ended. Control (untreated) populations re mained relatively stable throughout the study period (1981-1986). Because f ood resources regulated the size of the ground squirrel populations, we use d life-table response experiment (LTRE) analyses to examine the demographic basis of changes in population growth rate and thus also demographic influ ences on population regulation. LTRE analyses of two food-manipulated popul ations revealed that changes in age at maturity and fertility rate of femal es generally made the largest contributions to observed changes in populati on growth rate. Thus, our results suggested that abundance of food resource s regulated the size of our study populations through the effects of food r esources on age at maturity and fertility rates. Our results also indicated that different demographic mechanisms can underlie population regulation u nder different environmental conditions, because lower juvenile survival su bstantially contributed to population decline, but in only one of the popul ations. Demographic analyses of experimental data, such as those presented here, offer a rigorous and unambiguous means to elucidate the demographic b asis of population regulation and to help identify environmental factors th at underlie dynamics and regulation of biological populations.