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

The small-rodent community in northern Fennoscandia shows regular multiannu al fluctuations in abundance. We analyze time series of spring and fall abu ndances covering 44 yr from the vole community at Kilpisjarvi, northern Fin nish Lapland, which is dominated by the gray-sided vole (Clethrionomys rufo canus). Treating spring and fall abundances as variables in a bivariate tim e-series model, we study seasonal aspects of population regulation and ther eby utilize more of the available data than in most previous analyses of ro dent fluctuations. The analysis demonstrates substantial density dependence of population growth during the winter. This includes a strong direct effe ct of previous fall density and a strong 2-yr delayed effect of spring dens ity. Population growth during the summer shows evidence of delayed density dependence but is generally weaker than that during the winter season. Wint er regulation appears to be a crucial element in the production of multiann ual cycles in this community, and we suggest that long and severe winters c oupled with delayed density dependence may be a direct cause of the vole fl uctuations in northern and high-elevation areas.