Survival of Snowshoe Hares at a Geographic Range Boundary

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Abstract
The southern limit of snowshoe hare (Lepus americanus) distribution passes through central Wisconsin [USA]. Reproduction does not decline at this range boundary, hence lower survival must prevent additional dispersal and colonization. During February 1982-January 1983 we monitored survival of radiocollared hares to test the hypothesis that predation is the major immediate cause of death and is heaviest on dispersers and individuals within habitats having little understory cover. We stimulated dispersal by transplanting snowshoes throughout the year onto study areas with and without good understory cover, inside and outside currently occupied range. A control cohort of residents (not transplanted) was also monitored year-round. Predators killed 87% of the 67 radio-collared hares that died. Decreased survival was strongly associated with the hare movements and factors tending to increase movement: viz., dispersal, poor understory cover, presence of resident hares, and fragmented habitat. Survival was apparently enhanced by cover that concealed hares or obstructed predators. Poor physical condition and small body size were linked to increased rates of loss from predation. This study supports the view that predator-caused mortality determines the geographic limit of snowshoe hare distribution in Wisconsin, and losses are most markedly influenced by factors which induce movement. Dispersal beyond established home ranges may be relatively uncommon in these marginal populations due to heavy predation on individuals moving between patches of suitable habitat within relatively large home ranges. The high incidence of trap deaths among hares captured in bogs and the frequency of low bone marrow fat in live-trapped and predator-killed samples suggest that these snowshoes may compromise their access to an adequate food supply by seeking protection from predators in dense conifer cover.