Seventy‐five years of masting and rodent population peaks in Norway: Why do wood mice not follow the rules?

Wood mouse ( Apodemus sylvaticus ) populations are expected to show a peak in autumn in the year after a mast year of sessile oak ( Quercus petraea ), because stored acorns increase winter survival. In Aust‐Agder, South Norway, only 16 of 34 mast years from 1939–2014 were followed by a year with a peak in the wood mouse population. For many of the remaining instances, there rather was a minor peak 2 or 3 years after the mast. In multiple logistic regression models, the probability of a wood mouse population peak after a mast year of sessile oak was positively related to a snow‐corrected temperature index of the previous winter and negatively to a small rodent population index of the previous autumn. The present study thus supports the hypothesis that longer periods with snow‐free ground and subzero temperatures negatively affect wood mouse winter survival. Because it may be difficult for wood mice to survive on a diet consisting of acorns alone, the negative relationship with the rodent population index of the previous year is most likely caused by an over‐exploitation of necessary alternative food resources, such as other plant seeds and arthropods. Stored acorns not utilized during one winter are assumed to benefit wood mice in a succeeding winter, giving a delayed population peak relative to the mast year.