Intrinsic and climatic determinants of population demography: The winter dynamics of tundra voles

The relative impacts of intrinsic factors (e.g., density dependence) and extrinsic factors (e.g., climate) on winter demography may be critical for the generation of different population dynamic patterns (including cyclicity) in northern vole and lemming populations. However, little is known about winter demography because studies with temporal and spatial replication at the population level and an adequate sample of individuals with known fates, within each population are rare. In this study, we monitored the winter demography of 48 local tundra vole populations introduced to experimentally enclosed plots the preceding spring for four years in Norway. The rate of population change over the winter (November-May) was density dependent due to recruitment. However, the large variation in the rate of change between the different winters was due to a density-independent, and most likely a climatically driven, variation in survival rate. In particular, mild weather that led to the formation of ice on the ground. seemed to be detrimental for winter survival. We predict that if increased frequency of such events arose, due to climate change, normal cyclic dynamics of northern small rodent populations would be disrupted. We found support for the hypothesis that voles adjusted their body mass toward a certain mean during the winter so as to maximize winter survival. The survival rate of males was lower than that of females, possibly due to their larger body mass, and this resulted in female-biased-population sex ratios in the spring. This result suggests a link between sexual selection (responsible for the sexual size dimorphism) and natural selection (operating though size-dependent winter survival) with implications for the demographic structure of the population.