Long-term dynamics of voles and lemmings at the timberline and above the willow limit as a test of hypotheses on trophic interactions

Abstract

We have monitored population fluctuations of microtine rodents since 1977 in two habitat complexes in Finnmark, northernmost Norway - a low arctic plains landscape, with patches of willow scrubland embedded in lichen-dwarf birch tundra, and in adjacent highlands, occupied by scrub-free heaths, snow-beds and bogs. In the plains landscape, voles were cyclic, with a period of five years, and with wave-like density fluctuations. This pattern is consistent with time trajectory of prey in a predator-prey limit cycle. Autoregression analysis implies that the prey pattern is cleanest in the most productive plains habitats, while dynamics in the prevailing heath and bog habitats are governed by two significant lags, implying that even vole-plant interactions count. In the highlands, lemmings had two outbreaks, characterized by J-shaped growth curves, and separated by long periods of low density. The fluctuation pattern of lemmings in highlands was consistent with the predicted time trajectory of a predator. The implications of time trajectories are corroborated by direct evidence on microtine impacts upon the vegetation and on spatial patterns in predator activity. Even the strong dispersal tendency of lemmings during population peaks is consistent with the conjecture that they are adapted to play the role of a predator in a sustained predator-prey cycle. As a whole, the pattern supports T. Oksanen's modification of the hypothesis of exploitation ecosystems, where both local productivity and the structure of the landscape are taken in account.