Responses of a predator to variation in prey abundance: survival and emigration of adders in relation to vole density

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Abstract
We tested for effects of temporal and spatial variation in prey density on survival, growth, body condition, and emigration of a snake species feeding on voles. We carried out a capture–mark–recapture study (1989–1994) of two subdivided populations of adders, Vipera berus, inhabiting two groups of islands (Ängskär and In-Fredeln) in the Baltic Sea to estimate, compare, and model survival and capture probabilities, and to test for effects of time, locality, sex, and vole density on survival, using the computer program SURGE. To quantify variation in prey availability among years and localities we trapped field voles, Microtus agrestis, the primary prey of adders, every year from 1989 to 1993 on the islands where we captured snakes. We found that vole densities showed large fluctuations among years at In-Fredeln but remained relatively low throughout the study period at Ängskär. Annual survival probabilities of adders varied in a nonparallel manner through time between localities and ranged from 0.36 to 1.00, with a mean of 0.72 across years and localities. Survival did not differ between males and females. Survival of adders increased with population density of voles in one of the two populations (In-Fredeln), suggesting that starvation may have been an important source of snake mortality in some years, or that physiological stress resulting from food shortage may render individuals more susceptible to predators and diseases. Adders at In-Fredeln also grew faster and were in better body condition when vole density was high than when it was low. Inter-island movement rates of adders ranged from 0 to 29% among years but were not correlated with vole density, suggesting that the relationship between snake survival and vole density did not simply reflect increased emigration in years when prey were scarce.