| 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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