DENSITY EFFECTS AND SNAKE PREDATION - PREY LIMITATION AND REDUCED GROWTH-RATE OF ADDERS AT HIGH-DENSITY OF CONSPECIFICS

To test the hypothesis that snakes limit the population growth of thei r prey, we measured the density of adders (Vipera berus) and field vol es (Microtus agrestis) over several years on six small islands (1-12 h a) in the Baltic Sea. In both 1989 and 1990 we found a negative relati onship (significant in one year) between the density of adders and vol es across islands, indicating that on islands with high densities the adders exert such a high predation pressure that the voles become pred ator limited. In 1991 and 1993 we experimentally manipulated adder den sities to rule out potential island effects. A comparison between tran sferred and resident snakes revealed no negative effects of being tran sferred to an unfamiliar environment. Furthermore, vole density decrea sed more on islands where we had experimentally increased adder densit y compared with islands with reduced densities and unmanipulated contr ols, but the difference was not significant. To test the hypothesis th at adders at high densities of conspecifics decrease the vole populati on so much as to become resource limited, we calculated mean annual re lative growth rate (a size-independent measure of growth rate) of adde rs 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 e ffect 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.