Demography of sympatric arctic and snowshoe hare populations: an experimental assessment of interspecific competition

Geographic distributions of arctic (Lepus arcticus) and snowshoe hares (Lepus americanus) do not overlap on insular Newfoundland, where snowshoes were introduced, or in continental North America, where both species are indigenous. We conducted a field experiment to explore the role of interspecific competition in maintaining this allopatry. In September of 1984 and 1985, single- and mixed-species populations of arctic and snowshoe hares were established as "treatments" on six islands off Newfoundland's southwest coast. Island treatments differed between years. Demographic elements of each species population were monitored as potential response variables. Neither snowshoe nor arctic hares were demographically affected by the sympatry imposed in this experiment. Survival, reproduction, recruitment, habitat use, home range size, and over-winter weight change did not differ between single- and mixed-species treatments. There were, however, major demographic responses when each species was forced to occupy only atypical habitat, i.e., habitat normally used by the other species. Thus, most arctic hares confined to forested islands died from starvation in 1–3 months, after weight losses of 20–30% and marrow-fat reductions to <15%. Such responses occurred during winter and early spring, regardless of whether snow-shoe hares were present. In contrast, arctic hares at similar densities in their typical barrens habitat maintained weight and survived well. Snowshoe hares also maintained weight when restricted to barrens, but unlike those in forest habitat, were rapidly decimated by raptor predation. We believe that lack of suitable food over winter, rather than interspecific competition, has been a major constraint on the arctic hare's ability to populate forested regions of Newfoundland, whereas heavy predation has prevented snowshoe hares from colonizing mountain and coastal barrens.