Grazing Impact by Small Rodents in a Steep Cyclicity Gradient

Abstract

Small rodents show a gradient of increasing cyclicity in population density northwards in Scandinavia. This has been interpreted as regulation by generalist predators in the south, with increasing snow cover giving way to food limitation in the north. Another hypothesis is that the amount of agricultural land, instead of snow conditions, decides the regulatory ability of generalist predators. The importance of food, snow and agricultural land, in contrast to social regulation, was examined by measuring vole population variations, grazing impact and food selectivity in five Swedish localities characterized by a steep increase in cyclicity. Peak population densities and effects on the habitat increased strongly northwards, with the most pronounced crash also in the northernmost locality. At the same time, peak grazing impact was at least one order of magnitude higher in the northernmost locality than in the southern area. At that high grazing intensity a much broader range of plant species, including lignoses, were attacked by the voles. The increase in densities and grazing was strongly related to snow cover but not clearly to the amount of agricultural land. There were indications that the crash was amplified by disease outbreaks. Taken together, these observations demonstrate increasing population fluctuations and a deterioration of the food situation with increasing snow cover, which cannot be explained by any intrinsic mechanism of population regulation.