| Summary: | I examined factors limiting population growth of collared lemmings (Dicrostonyx kilangmiutak) at low densities (<3 adults/ha), at Pearce Point, Northwest Territories, Canada. Populations were followed by mark-recapture, and radiotelemetry. They fluctuated annually, typically with summer declines, and winter increases. I tested the hypothesis that predation mortality limits population growth in summer, by comparing a population in an 11 ha predator exclosure (PE) with three control populations (18-25 ha). Predation, principally by red fox (Vulpes vulpes) and rough-legged hawks (Buteo lagopus), was the proximate cause of most adult and neo natal mortality. No other mortality factor compensated for decreased predation mortality in PE. Adult survival and recruitment increased significantly in PE. Controls declined in 1990 and 1991. PE declined less quickly. It did not grow, because weanlings dispersed long distances (53 m/d), and frequently left PP. I conclude that predation mortality is sufficient and necessary to limit summer population growth. Three other factors potentially limiting population growth in PP were investigated. Social interactions did not inhibit reproduction because neither the proportion of lemmings reproductively active, nor litter sizes, differed between PE and controls. Food availability did not limit growth because principal foods were not depleted in PP. net primary production was similar to that in regions where lemmings irrupt, and enhanced production with fertilization was not consumed by lemmings. Behavioural sensitivity to predation risk appeared limiting because growth rates of neonates were higher in PE than on controls. Lemmings bred in all winters (1987-92). Variance in rates of winter population growth was significantly explained by a combination of mean daily temperature in autumn, when lemmings change morphology, and an index of cold stress in winter. Summer predation mortality was destabilizing. Specialist and semi-generalist predators drove lemmings to densities too low for persistence of these predators. Generalist predators continued to limit lemmings in the absence of specialists and semi-generalists. The summer predator community at Pearce Point, consisting mostly of generalists or semi-generalists, contrasts with a predominantly specialist predator community at arctic sites where lemmings irrupt. A relatively diverse prey base, especially including arctic ground squirrels (Sperniophilus parryii), seemed critical for the maintenance of predators that limit lemmings in summer. Science, Faculty of Zoology, Department of Graduate
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