The Role of Habitat Patchiness in the Population Dynamics of Snowshoe Hares

Population densities of snowshoe hares in Alaska fluctuate on a 10—yr cycle. A snowshoe hare population was studied near Fairbanks, Alaska USA from 1971 through 1977 during the peak, decline, low, and early increase phases of the cycle. The mosaic of habitat types was utilized by hares according to the amounts of available food and cover. Habitats utilized were from the most dense to the least dense during the increase phase of the cycle and subsided in reverse order during the population decline. Presaturation dispersal was the causal force for habitat expansion during the increase phase of the cycle, while predation was the primary force for habitat contraction during the population decline. Snowshoe hares tend to move from winter to summer ranges in response to availability of food and cover. A patchy environment which provides refuges like dense black spruce or willow—alder thickets in winter and open summer range allow hares to shift their use seasonally in response to changes in diet and to take advantage of changing environmental conditions. During summer, hares feed on herbaceous material and low shrubs in open areas; during winter, when herbs and shrubs are covered with snow, they move into the dense spruce thickets to feed on spruce, willow, and alder. During periods of high hare densities, browsing intensity reached 100% in all habitat types and diameter at point of browsing exceeded 10 mm. Both indices suggest that a hare—vegetation interaction can initiate cyclic decline. These conclusions are consistent with Keith's model for the 10—yr cycle of snowshoe hares. Dense spruce or willow—alder thickets play an important role in snowshoe hare cycles in that they provide protective cover from predators during a population crash. Snowshoe hares, which are thus able to avoid local extinction by seeking these refuges, provide the stock for subsequent population increase. In the northern part of their range snowshoe hare populations are cyclic but in the southern part they are not. The cycle in the north is regulated in part by the presence of obligate hare predators which exhibit a delayed density—dependent cycle with snowshoe hares and the existence of a vegetational mosaic which provides both suboptimal and optimal habitats. The discontinuous coniferous forests, the facultative resident hare predators, and the presence of at least three other lagomorphs are all proximate factors which help to maintain a stable hare population in the southern portions of its range.