The influence of food resources on population dynamics in Columbian ground squirrels

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Abstract
Determining the environmental factors that limit the size of populations and examining the influence of environmental factors on population processes (immigration, emigration, survival, and reproduction) are fundamental problems for population ecologists. We conducted an experimental study of population dynamics in the Columbian ground squirrel, Spermophilus columbianus, from 1981 to 1983, in southwestern Alberta, Canada. We supplemented the food resources of two local populations of ground squirrels at different elevations and also monitored two reference (unsupplemented) populations. Population size increased by about 85% annually and population biomass increased by about 94% annually where supplemental food was provided. Reference populations varied in numbers and biomass by less than 40% annually. Because the increase in size of supplemented populations was dramatic, we concluded that food resources probably limited the size of the ground squirrel population before we began the experiments and that other possible limiting factors were insufficient to check the increases stimulated by supplemental food. Several population processes were influenced by the increased food resources. Immigration of all age-classes was higher to supplemented than to reference populations and favored females. Survival of resident young was higher in supplemented than in reference populations. Reproduction was greatly increased in supplemented populations compared with reference populations, with surviving young residents of 1982 making the greatest contribution to increases in population size. The results were generally similar in the two replicates of the experiment. Competition for food resources appeared to be stongest among yearling and adult females and among young of both sexes.