Predation and the dynamics of the bank vole, Clethrionomys glareolus

Theoretical analyses and data suggest that the interaction between bank voles and their predators generates a locally stable equilibrium, regardless to the composition of the predator guild. The suggested primary proximate source for stability is female territoriality. Predation_per se_appears to be destabilizing. Whether or not predation regulates bank voles appears thus to be a semantic issue, depending on the operational definition of the concept of regulation. Confusion has arisen as different operational definitions have been tacitly used by different authors. The predicted intrinsic stability of bank vole-predator systems is most clearly displayed in the southern part of the boreal zone. Currently, even the dynamics of bank voles of taiga landscapes in Finnish ?apland are close to the predicted multiannual stability. In those boreal areas, where grassy and productive Microtus habitats are relatively abundant, sustained, multiannual cycles appear to be generated by the interaction between small mustelids and Microtus spp. When desities of Microtus decline, fidelity to Microtus habitats becomes suboptimal for predators. Consequently, cyclicity seems to be externally imposed on bank voles, due to changing habitat preferences of predators during the course of the mustelid-Microtus cycle. In the temperate zone, masting (seed crops of deciduous trees) increases the reproductive rate of bank voles and ameliorates female territoriality, initiating a transient increase in the numbers of bank voles and their predators. Beforethe system reaches its new equilibrium, the mast is over. Due to the combined effect of high predator numbers and normal reproductive performance, bank vole numbers then decline to the low level typicalfor post-mast years. Depending on the frequency of masting, the post-past low can be followed by a stable phase or immediately by a new mast-triggered outbreak. Although fundamentally different mechanisms appear to account for multiannual density fluctuations in temperate and boreal bank vole populations, both phenomena can be interpreted as consequences of extremal perturbations upon intrinsically stable predator-prey systems.