Explaining bank vole cycles in southern Norway 1980-2004 from bilberry reports 1932-1977 and climate.

Correlations between mast fruiting of bilberry Vaccinium myrtillus and peak levels of Clethrionomys-voles have been reported from both Norway and Finland, but there has been a discussion whether this is a bottom-up or a top-down relationship. In a multiple regression model, 65% of the variation in a bilberry production index calculated from game reports from southern Norway 1932-1977 could be explained by the berry index of the two preceding years and climate factors acting during key stages of the flowering cycle. High vole populations in previous years did not contribute to explain the fluctuation in berry production. I used the selected model and climate data to predict bilberry production for the period 1978-2004. Predicted berry indices of the current and previous year explained 38% and the total amount of precipitation in May-June explained 16% of the variation in a log-transformed snap-trapping index of bank vole Clethrionomys glareolus 1980-2004. The vole index was not related to any of the climate variables used to predict berry production. This pattern supports the hypothesis that vole cycles are generated by changes in plant chemistry due to climate-synchronized mast fruiting.