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

We have monitored population fluctuations of microtine rodents since 1977 i n two habitat complexes in Finnmark, northernmost Norway - a low arctic pla ins landscape, with patches of willow scrubland embedded in lichen-dwarf bi rch 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-Eke density fluctuations. This pattern is consis tent with time trajectory of prey in a predator-prey Emit cycle. Autoregres sion analysis implies that the prey pattern is cleanest in the most product ive plains habitats, while dynamics in the prevailing heath and bog habitat s are governed by two significant lags, implying that even vole-plant inter actions count. In the highlands, lemmings had two outbreaks, characterized by J-shaped growth curves, and separated by long periods of low density. Th e fluctuation pattern of lemmings in highlands was consistent with the pred icted time trajectory of a predator. The implications of time trajectories are corroborated by direct evidence on microtine impacts upon the vegetatio n and on spatial patterns in predator activity. Even the strong dispersal t endency of lemmings during population peaks is consistent with the conjectu re that they are adapted to play the role of a predator in a sustained pred ator-prey cycle. As a whole, the pattern supports T. Oksanen's modification of the hypothesis of exploitation ecosystems, where both local productivit y and the structure of the landscape are taken in account.