LONG-TERM MICROTINE DYNAMICS IN NORTH FENNOSCANDIAN TUNDRA - THE VOLE CYCLE AND THE LEMMING CHAOS

Densities of microtine rodents in two habitat complexes in the tundra of Finnmarksvidda, Norwegian Lapland, were studied during 1977-89 by means of snap trapping (Small Quadrat Method). Predator populations were studied by mapping breeding raptors and by snow-tracking small mustelids. During 1977-85, snow-tracking was conducted only during peak and decline years, whereas during 1986-89, snow-tracking was conducted every winter (November-December) and live-trapping (in August) was used as an additional method. Lowland vole populations had regular density fluctuations with peaks in 1978-79, 1982-84 and 1987-88. Highland vole populations fluctuated less regularly and at lower over-all densities. Highland lemming populations had two outbreaks, in 1978 and 1988, ending in abrupt winter crashes. In the lowland, outbreak levels were reached only in 1978. All microtine declines in relatively productive lowland habitats were accompanied by intense activity of small mustelids, whereas avian predators were common only in 1983. Lowland declines also showed clear between-habitat asynchrony: they started in areas with an exceptional abundance of productive habitats and then spread to more barren areas, These lowland data are consistent with the hypothesis of a mustelid-microtine limit cycle, although also several other hypotheses remain unrefuted. The highland lemming data suggest a simple exploiter-victim interaction between lemmings and the vegetation.