Mechanisms of density dependence in fluctuating vole populations.

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It is essential to analyze the relationship between seasonal and annual dynamics in order to understand multi-annual cycles observed in small mammal populations. Based on recent advances in time-series analyses of ecological dynamics using statistical and mathematical models, we analyzed the seasonal and annual density dependence structure in 189 populations of the gray-sided vole (Clethrionomys rufocanus Sundevall, 1846) in Hokkaido, Japan. Annual direct density dependence was strong in almost all populations. In contrast, delayed density dependence was generally weak, although clear delayed density dependence was detected in some of the studied populations. Although seasonal density-dependence was observed both in winter and summer, direct density dependence was much more profound during winter, thus, winter density dependence contributed most to the overall annual direct density dependence. We found no correlation between the seasonal components of annual delayed density dependence ; however corresponding seasonal components for annual delayed density dependence were positively correlated. We conclude that winter conditions influence the strength of annual direct density dependence most profoundly and that direct density dependence during summer and winter may be generated by different mechanisms, whereas delayed density dependence seems to be generated by a common mechanism.Annual density dependence was stronger in north-eastern populations than in south-western ones for both direct and delayed density dependence. The temperature of Hokkaido becomes lower toward north-east, because Hokkaido Island is heavily influenced by warm (west) and cold (east) sea currents. Thus this geographic gradient that populations in areas with longer winter exhibit stronger density dependence well corresponds to the above results that density dependence is stronger in winter than in summer.