LIMITATION OF COLLARED LEMMING POPULATION-GROWTH AT LOW-DENSITIES BY PREDATION MORTALITY

Populations of the collared lemming (Dicrostonyx kilangmiutak) and the tundra vole (Microtus oeconomus) have been at consistently low densit ies, and non-cyclic, at Pearce Point, Northwest Territories, Canada, f or six yr. In most summers population densities decline, or only incre ase marginally, despite ongoing reproduction. We investigated the hypo thesis that predation mortality is sufficient and necessary to curtail lemming population growth in summer. To test predictions of the hypot hesis, we compared lemming demography, using mark-recapture and radiot elemetry, on a population from which predators were excluded (PE), usi ng a perimeter fence and aerial mesh of nylon (11.4 ha), with demograp hies of three control populations (18-25 ha). Predation was the proxim ate cause of the majority of adult and neo-natal mortality, and was no t replaced in a compensatory fashion by any other mortality factor in the PE. Significantly fewer adult lemmings died in the PE, and consequ ently survival inside the PE was significantly enhanced. Recruitment o f juvenile lemmings to the adult population was significantly higher i n the PE on a per unit area basis. The lemming population in PE follow ed a significantly different trajectory than the control populations i n 1990 and 1991, remaining fairly stable while controls declined. Howe ver, the protected population did not grow, apparently because of juve nile dispersal: telemetered juveniles dispersed at an average rate of 53 m/d within the first ten d after weaning. We believe that the prote cted area was too small to encompass such dispersal, and that emigrant s were not replaced by immigrating juveniles since the latter faced he avy mortality outside the exclosure. In 1992, numbers on PE and all co ntrols grew, in conjunction with a regional absence of rough-legged ha wks (Buteo lagopus) and a scarcity of red foxes (Vulpes vulpes), the m io principal microtine predators. Tundra vole population growth was al so limited by predation mortality, but to a lesser extent. We conclude that predation mortality is sufficient and necessary to limit summer population growth in these microtine species.