Density effects and snake predation: prey limitation and reduced growth rate of adders at high density of conspecifics

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Abstract
To test the hypothesis that snakes limit the population growth of their prey, we measured the density of adders (Vipera berus) and field voles (Microtus agrestis) over several years on six small islands (1–12 ha) in the Baltic Sea. In both 1989 and 1990 we found a negative relationship (significant in one year) between the density of adders and voles across islands, indicating that on islands with high densities the adders exert such a high predation pressure that the voles become predator limited. In 1991 and 1993 we experimentally manipulated adder densities to rule out potential island effects. A comparison between transferred and resident snakes revealed no negative effects of being transferred to an unfamiliar environment. Furthermore, vole density decreased more on islands where we had experimentally increased adder density compared with islands with reduced densities and unmanipulated controls, but the difference was not significant. To test the hypothesis that adders at high densities of conspecifics decrease the vole population so much as to become resource limited, we calculated mean annual relative growth rate (a size-independent measure of growth rate) of adders on all islands during 1989 through 1993. In all 5 years we found a negative relationship across islands between adder density and growth rate of adders. Combining all years there was a significant negative effect of adder density, suggesting that on islands with high densities the adders suffer a reduction in growth rates that was due to intense exploitative competition.