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Population ecology of arctic ground squirrels in the boreal forest during the decline and low phases of a snowshoe hare cycle Byrom, Andrea Elizabeth


I examined food and predation as factors limiting arctic ground squirrel (Spermophilus parryii plesius) populations during the decline and low phases of a snowshoe hare cycle (1992 - 1995). Food and predator limitation were tested experimentally with large-scale (36-ha) experimental manipulations: two food-supplemented areas, a predator exclosure, and a food-supplemented treatment where predators were also excluded. Predator removal doubled population density, while addition of food resulted in a four-fold density increase. Removal of predators and addition of food together resulted in a 10-fold increase in arctic ground squirrel population densities. Population densities and adult survival rates were lower in 1992 and 1993 (two years after the snowshoe hare population decline) than in 1994 and 1995. Food supply and predation interact to limit arctic ground squirrel population densities in the boreal forest during the decline and low phases of the snowshoe hare cycle. Supplemental food did not affect dispersal distances or dispersal frequency of 172 radio-collared juveniles of either sex in any year. Juveniles that moved farther from their natal burrow were more likely to die. Males moved farther than females and died more frequently. Dispersal tendency was unrelated to population density in males. Females increased their tendency to disperse only on a study site with population densities 17x those of control populations. Male arctic ground squirrels probably disperse to avoid inbreeding, while females may disperse in response to resource limitation at very high densities. Philopatric females had higher fitness than females that dispersed, particularly if survival during dispersal was taken into account. As population density increased from 1992 - 1995, home range overlap of adult females also increased, as daughters survived to reproductive age in contact with their mother's home range. A stage-based simulation model of the annual cycle of activity and hibernation in arctic ground squirrels was strongly sensitive to adult and juvenile female survival. Arctic ground squirrel populations in the boreal forest can sustain a positive rate of population increase during the low phase of the snowshoe hare cycle, facilitated by flexibility of adult female home range overlap and high adult female survival.

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