Winter herbivory by voles during a population peak: the relative importance of local factors and landscape pattern

1. We studied the relative role of local habitat variables and landscape pattern on vole–plant interactions in a system with grey-sided voles ( Clethrionomys rufocanus (Sund.)) and their favourite winter food plant, bilberry ( Vaccinium myrtillus L.). The study was conducted during a vole peak year (1992–93) in a tundra area in northern Norway.
2. Using Mantel statistics we were able to separate the direct effects of the spatial patterning of habitats and the indirect effects due to spatial aggregations of similar habitats.
3. Results indicate that knowledge about the explicit spatial patterning of patches does not improve our understanding of the system. Instead, two local factors, vegetation height and bilberry biomass, explained more than 50% of the variation in cutting intensity in winter (defined as the proportion of above-ground shoots cut). Increasing vegetation height increased, and increasing bilberry biomass decreased, the cutting intensity.
4. The conclusion that grey-sided voles are able to distribute themselves relative to habitat quality was also partially supported by our estimated over-winter persistence by voles in the various habitats. Vole persistence was uncorrelated with vegetation height, the important predictor of autumn vole density, but tended to correlate with the deviation from the relation between vegetation height and autumn vole density. This conforms to the expectations from the theory of ideal-free habitat distribution.
5. The cue for vole habitat choice, i.e. vegetation height, indicates that either predation or freezing risk is important for voles when selecting over-wintering habitat.