POPULATION LIMITATION IN ARCTIC GROUND-SQUIRRELS - EFFECTS OF FOOD AND PREDATION

1. We examined the relative importance of food and predators in limiti ng Arctic ground squirrel (Spermophilus parryii plesius Richardson) po pulations in the boreal forest of the southwestern Yukon during the pe ak and early decline of a snowshoe hare cycle (Lepus americanus Erxleb en). 2. Squirrels were live-trapped from 1990 to 1992 on two control g rids and three experimental treatments (food addition, mammalian and a vian predator exclosure, and food addition plus mammalian predator exc losure). Adult squirrels were radiocollared on all areas in 1992. 3. F ood addition increased densities 3-8 times, generally increased reprod uctive traits (increased proportion of females lactating, doubled recr uited litter sizes, resulted in earlier emergence of juveniles), incre ased immigration rates (but only in 1992), resulted in heavier females though not males at emergence in spring, and resulted in more rapid g rowth rates of juvenile males, but not of juvenile females. It had no effect on active season or overwinter survival rates. 4. Exclusion of predators had virtually no effect on any demographic variable measured , except for population densities in 1991 when they were approximately double those of the control populations. 5. Food addition plus exclus ion of mammalian predators resulted in demographic changes that were c omparable to those of food addition alone. 6. Thus, it appeared that f ood, not predators, limited ground squirrel populations at this stage of the hare cycle. However, independent of experimental treatment, act ive season survival of adult squirrels declined markedly from 1990 (hi gh hare numbers) to 1992 (low hare numbers). Most of the radiocollared squirrels disappearing in 1992 were killed by predators and this was coincident with high densities of predators. In the predator exclosure s, all predation mortalities resulted from avian predators which we co uld not exclude. 7. We conclude that both food and predators interact to limit Arctic ground squirrel populations during the peak and early decline of the hare cycle.