Seasonal drivers of amplitude patterns in a population of red-backed voles (Clethrionomys rutilus)

Abstract

Northern red-backed voles (Clethrionomys rutilus) are an important species in the Interior Alaska boreal forest ecosystem, both as important herbivores and as a key food source for many mammalian and avian predators. However, they exhibit dramatic inter- and intra-annual population fluctuations, for which causes are not entirely known. Winter mortality is often very high and altered weather conditions due to climate change, such as rain on snow or delayed snowfall in autumn, may increase stress during an already difficult season. These considerations prompted this investigation into overwinter survival of northern red-backed voles in Denali National Park and Preserve, with the goal of examining a time series of population densities and assessing how weather variables influenced patterns of mortality. Using a 30-year record of mark-recapture data, I applied spatially-explicit methods to calculate density estimates for autumn and early summer trapping sessions. I also assessed cyclic behavior and used post-hoc linear modeling to examine patterns in amplitude and period of population fluctuations. I found that this microtine population appears to be cycling on a 2-4 year period, with some differentiation among sampling sites. Models of autumn amplitudes suggested a linkage between white spruce (Picea glauca) seed mast, either an important source of food during winter seasons, or as a coincidental product of the underlying multi-annual environmental triggers that promote high seed fall. I also found a negative effect of combined late snowfall and cold temperatures, a scenario that may become more prominent under future climate changes. Lastly, my models of early summer density showed an apparent negative density dependence, in which high population densities in autumn were followed by low densities the following spring. Continued monitoring of voles, alongside more thorough assessments of snow conditions, habitat, diet, and predator status would assist further attempts to cast light upon the complex population dynamics of this species and their many predators.