Multiannual vole cycles and population regulation during long winters: An analysis of seasonal density dependence

The small-rodent community in northern Fennoscandia shows regular multiannual fluctuations in abundance. We analyze time series of spring and fall abundances covering 44 yr from the vole community at Kilpisjarvi, northern Finnish Lapland, which is dominated by the gray-sided vole (Clethrionomys rufocanus). Treating spring and fall abundances as variables in a bivariate time-series model, we study seasonal aspects of population regulation and thereby utilize more of the available data than in most previous analyses of rodent fluctuations. The analysis demonstrates substantial density dependence of population growth during the winter. This includes a strong direct effect of previous fall density and a strong 2-yr delayed effect of spring density. Population growth during the summer shows evidence of delayed density dependence but is generally weaker than that during the winter season. Winter regulation appears to be a crucial element in the production of multiannual cycles in this community, and we suggest that long and severe winters coupled with delayed density dependence may be a direct cause of the vole fluctuations in northern and high-elevation areas.