RESPONSES OF A PREDATOR TO VARIATION IN PREY ABUNDANCE - SURVIVAL ANDEMIGRATION OF ADDERS IN RELATION TO VOLE DENSITY

We tested for effects of temporal and spatial variation in;prey densit y on survival, growth, body condition, and emigration of a snake speci es feeding on voles. We carried out a capture-mark-recapture study (19 89-1994) of two subdivided populations of adders, Vipera berus, inhabi ting two groups of islands (Angskar and In-Fredeln) in the Baltic Sea to estimate, compare, and model survival and capture probabilities, an d to test for effects of time, locality, sex, and vole density on surv ival, using the computer program SURGE. TO quantify variation in prey availability among years and localities we trapped field voles, Microt us 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 relat ively low throughout the study period at Angskar. Annual survival prob abilities of adders varied in a nonparallel manner through time betwee n 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 female s. Survival of adders increased with population density of voles in on e of the two populations (In-Fredeln), suggesting that starvation may have been an important source of snake mortality in some years, or tha t physiological stress resulting from food shortage may render individ uals more susceptible to predators and diseases. Adders at In-Fredeln also grew faster and were in better body condition when vole density w as high than when it was low. Inter-island movement rates of adders ra nged from 0 to 29% among years but were not correlated with vole densi ty, suggesting that the relationship between snake survival and vole d ensity did not simply reflect increased emigration in years when prey were scarce.