Rodent Cycles in Relation to Food Resources on an Alpine Heath

Abstract

Populations of microtine rodents in Lapland, N Sweden, show strong cyclic fluctuations. We examine relations between the density of rodents and their food resources during two four-year cycles on alpine heath. The grey-sided vole Clethrionomys rufocanus, the most common species, varied more than 100-fold in density between peak and bottom years. Breeding ceased earlier in peak than in increase years. The voles weighed 26-44% more in peak years than during the declines or early increase phases. Breeding in C. rutilus and Microtus oeconomus followed a similar pattern. Lemmus lemmus, however, reproduced longer than the other species during peak years. Reduction in the quantity of available food through grazing was rejected as a proximate cause for the rodent cycles. During the second population peak, voles ceased breeding and began to decline while preferred food plants were still abundant. Nor was there any consistent relationship between plant nutrients (N, P, Ca, Mg, K, C) and the rodent cycle. Nutrient levels in food plants seemed to depend mainly on processes other than rodent grazing. Hence, factors other than food quantity and nutrient levels seemed to be important in the short-term causation of the cycles. Exclosure experiments showed that rodents markedly reduced the phytomass of several plants, influenced the species composition of the alpine heath, and reduced the flowering frequency of food plants.