Ungulate Population Models with Predation: A Case Study with the North American Moose

Abstract

In this article I examine moose—wolf interactions over a broad spectrum of moose densities with the primary objective to test empirically whether wolf predation can regulate moose numbers. I also present four conceptual models of moose population dynamics and outline their specific predictions. Based on 27 studies where moose were the dominant prey species, the functional and numerical responses of wolves to changing moose density were derived using an hyperbolic, Michaelis—Menten function. Per capita killing rate was strongly related (P = .01) to moose density, as was the density of wolves (P < .01). Killing rate plateaued at 3.36 moose°wolf—1°(100 days)—1 when predators were fully satiated. The asymptotic value for wolf density was 58.7 animals/1000 km2. Wolf predation rate, as derived from the total predator response, proved to be density dependent from 0 to 0.65 moose/km2, and inversely density dependent at higher moose densities. Predator: prey ratios reflected wolf predation rate poorly because they did not integrate the functional response. An empirical model based on these results suggests that moose would stabilize at 2.0 moose/km2 in the absence of predators, and at °1.3 moose/km2 in the presence of a single predator, the wolf. Density—dependent food competition creates these two high—density equilibrium conditions. If moose productivity is diminished through either deteriorating habitat quality or bear—induced early calf mortality, then a low—density equilibrium (0.2—0.4 moose/km2) is predicted. The model also suggests that when a low equilibrium develops, a “predator pit” is absent or extremely shallow, thus arguing against the appropriateness of a predation—food two—state model. Further research on the density relationship of bear predation, on the effect of alternate prey on wolf total response, and on the regulatory impact of food competition at high moose densities, is required for a full understanding of moose demography.