Demographic responses of arctic hares (Lepus timidus) to experimental reductions of red foxes (Vulpes vulpes) and martens (Martes martes)

This study aimed, through a field experiment, to test predictions from two hypotheses: (i) predation is a factor limiting arctic hare (Lepus timidus) populations, and (ii) predation shifts to arctic hares when voles (Microtus and Clethrionomys) decline to scarcity. Our approach was to compare demographic characteristics of hare populations on two large islands (Rånön, 2350 ha, and Bergön, 1800 ha) in the northern Baltic, where foxes (Vulpes vulpes) and martens (Martes martes) were alternately present in normal numbers or reduced by snaring and shooting over winter. Hare densities in March, as indexed by transect counts and estimated by Jolly–Seber analyses, averaged two to three times higher on each island during years with reduced numbers of foxes and martens. Rates of population increase on Rånön, and thus population trends, were correlated with annual survival of adults and yearlings, but more strongly with survival of juvenile hares from birth to March (indexed by recruitment). Survival estimates were incomplete for Bergön and could not be similarly analyzed. Hare survival rates were lowest on Rånön during the 3 years when vole populations had declined to cyclic lows, but adult and yearling survival dropped only slightly during the vole low that coincided with reduced fox and marten numbers. We conclude that fluctuations were imposed on Rånön's hare population by the vole cycle's effect on predation rates, whereas on both islands major differences in population levels between groups of years were largely due to the number of foxes and martens present. Demographic differences between hare populations of Rånön and Bergön, apparently unrelated to predation, included dispersal rates, mean body weights of females, and mean first-litter conception dates. Densities, survival rates, and natality are compared with selected arctic hare populations elsewhere.