SEASONAL FORCING ON THE DYNAMICS OF CLETHRIONOMYS RUFOCANUS - MODELING GEOGRAPHIC GRADIENTS IN POPULATION-DYNAMICS

We interpret gradients in population dynamics of the gray-sided vole f rom the southwestern part of the island of Hokkaido to its northeaster n part within the framework of a phenomenological model involving the relative length of summer and winter. In Hokkaido, as in other norther n regions, both spring and fall is considered as short transition peri ods between the two main seasons - summer (the primary breeding season ) and winter (the non-reproductive or secondary breeding season). We s how that the geographic transition in dynamics may be understood as th e combined consequence of different patterns of density-dependence dur ing summer and winter, and geographically varying season lengths. Diff erences are shown to exist between summer and winter with respect to s trength of density-dependence. Direct density-dependence, in particula r, is stronger during winter than during summer. A model is presented to show how relative lengths of seasons can induce both stable and per iodically fluctuating population dynamics. The results are compared an d contrasted with what is otherwise known about the gradient in rodent dynamics in Fennoscandia.