REGULATION OF POPULATION-SIZE - EVIDENCE FROM COLUMBIAN GROUND-SQUIRRELS

The hypothesis that food resources regulate population size was tested in Columbian ground squirrels, Spermophilus columbianus, from 1981 to 1986 in southwestern Alberta, Canada. Two replicate populations recei ved supplemental food resources from 1981 to 1983, and were subsequent ly monitored until 1986. Two reference (unmanipulated) populations wer e monitored throughout the 6 years. During the experiment, dramatic in creases in population size of about 500% occurred. After supplementati on, spring populations declined by about 20% per year, under condition s that produced stressful shortages of food (as evidenced by significa nt decreases in body mass of ground squirrels). The demographic proces s that contributed most to increasing and decreasing experimental popu lations was production of yearlings, through changes in reproductive s uccess and survival of young. Changes in migration contributed seconda rily to changes in population size, and changes in adult survival were least important. While similarities of demographic processes were evi dent between replicates during population increases, a dramatic decrea se in survival of young contributed to population decline at one repli cate, and ''normal'' levels of reproduction and survival were sufficie nt to produce a decline in the other replicate. Demographic mechanisms may vary, but population regulation of mammalian populations appears strongly dependent on food resources.