Possible impact of winter conditions and summer temperature on bank vole (Myodes glareolus) population fluctuations in Central Norway

In recent decades, population fluctuations of microtine rodents have frequently faded out in parts of northern Europe, possibly because of climate change. Weather events may affect the rodents directly, but also indirectly, by acting on factors that generate population fluctuations. However, few studies have addressed the latter aspect. A species well suited for such a study is the bank vole (Myodes glareolus), which usually increases in number after a year with high seed production of bilberry (Vaccinium myrtillus). We analysed two snap-trapping time series of bank vole from Central Norway. When controlling for the impact of seed production of bilberry and Norway spruce (Picea abies), the number of bank voles trapped in the low-altitude area was negatively related to a summer temperature index of the two previous years, possibly because of the impact of temperature on the nutritional status of food plants. In the area situated at the highest altitude, there was a negative relationship with the winter NAO-index and a positive relationship with a snow depth index. Wet and mild winters are assumed to give unfavourable snow conditions, with alternating thawing and freezing, sometimes also creating an ice-layer at ground level, whereas a sufficient thick snow cover protects the rodents in periods with critical low temperatures. We conclude that an understanding of the ultimate cause of population fluctuations is needed to reveal the disturbing effects of weather events or other factors.