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A 3-to 5-year cycle of vole abundances is a characteristic phenomenon in the ecology of northern regions, and their explanation stands as a central theoretical challenge in population ecology. Although many species of voles usually coexist and are in severe competition for food and breeding space, the role of interspecific competition in vole cycles has never been evaluated statistically. After studying community effects on the population dynamics of the gray-sided vole (Clethrionomys rufocanus) in the subarctic birch forest at Kilpisjärvi, Finland, we report statistical results showing that both interspecific and intraspecific effects are important in the direct year-to-year density dependence. However, interspecific effects are not detectable in the 2-year delayed density dependence that is crucial for generating the characteristic cycles. Furthermore, we show that most of the competition takes place during the winter. The results are evaluated against two models of community dynamics. One assumes that the delayed effects are caused by an interaction with a specialist predator, and the other assumes that they are caused by overgrazing food plants. These statistical results show that vole cycles may be generated by a species-specific trophic interaction. The results also suggest that the graysided vole may be the focal species in the birch-forest community, as field voles may be in the taiga and as lemmings may be on the tundra.Ever since the 3-to 5-year vole cycles were first described by Elton (1), their mechanism has been hotly debated (2). The most popular current hypothesis is that the cycles are caused by an interaction with specialist predators (3-5), but overgrazing (6) and various intrinsic factors (7, 8) have also been suggested. The dynamics of cyclic voles usually are described well by simple second-order autoregressive models (9,10,47). In these models, density dependence is distributed about equally between a direct effect acting within 1 year and a 2-year delayed effect. Current research aimed at understanding the vole cycle has focused on developing models that can explain this pattern of density dependence or other associated aspects of the second-order autoregressive dynamics (10)(11)(12)47). Almost all of this work has focused on predation by weasels as the mechanism generating the delayed density dependence. This focus is not surprising, because predator-prey interaction is a simple and ecologically plausible mechanism that generates cyclic dynamics, which tend to show up as second-order autoregressive dynamics in time-series analyses (13).

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