Interspecific and intraspecific competition as causes of direct and delayed density dependence in a fluctuating vole population

A 3- to 5-year cycle of vole abundances is a characteristic phenomenon in t he ecology of northern regions, and their explanation stands as a central t heoretical 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 evaluat ed statistically After studying community effects on the population dynamic s of the gray-sided vole (Clethrionomys rufocanus) in the subarctic birch f orest at Kilpisjarvi, Finland, we report statistical results showing that b oth interspecific and intraspecific effects are important in the direct yea r-to-year density dependence, However, interspecific effects are-not detect able in the 2-year delayed density dependence that is crucial for generatin g the characteristic cycles. Furthermore, we show that most of the competit ion takes place during the winter. The results are evaluated against two mo dels of community dynamics. One assumes that the delayed effects are caused by an interaction with a specialist predator, and the other assumes that t hey are caused by overgrazing food plants. These statistical results show t hat vole cycles may be generated by a species-specific trophic interaction. The results also suggest that the gray-sided vole may be the focal species in the birch-forest community, as field voles may be in the taiga and as l emmings may be on the tundra.