Do Predators Synchronize Vole and Grouse Fluctuations?: An Experiment

Synchronized fluctuations in vole and grouse populations in boreal Fennoscandia have been explained as the result of shifting predation pressure: the alternative prey hypothesis. We tested this hypothesis by supplying additional food to predators in only one of two 615 ha areas during the crash in vole density in 1985. As predicted from the hypothesis the proportion of young grouse in autumn was higher in the experimental area than in the control area. Moreover, the proportion decreased in the control area in comparison with the year before, but did not do so in the experimental area. Thus, with respect to synchronized short-term population fluctuations of voles and grouse, we find the hypothesis that predators take only a doomed surplus less probable. However, we cannot discard the hypothesis that grouse decline during years of strong vole decline for example because a poor quality or low quantity of food makes them more vulnerable to predators. Hence, predation is a necessary but perhaps not a sufficient factor in mediating short-term fluctuations from voles to grouse in boreal Fennoscandia.