Ah. Hubbs et R. Boonstra, POPULATION LIMITATION IN ARCTIC GROUND-SQUIRRELS - EFFECTS OF FOOD AND PREDATION, Journal of Animal Ecology, 66(4), 1997, pp. 527-541
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.