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 landsca pe pattern on vole-plant interactions in a system with grey-sided vole s (Clethrionomys rufocanus (Sund.)) and their favourite winter food pl ant, bilberry(Vaccinium myrtillus L.). The study was conducted during a vole peak year (1992-93) in a tundra area in northern Norway. 2. Usi ng Mantel statistics we were able to separate the direct effects of th e spatial patterning of habitats and the indirect effects due to spati al aggregations of similar habitats. 3. Results indicate that knowledg e about the explicit spatial patterning of patches does not improve ou r 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-gro und shoots cut). Increasing vegetation height increased, and increasin g bilberry biomass decreased, the cutting intensity. 4. The conclusion that grey-sided voles are able to distribute themselves relative to h abitat quality was also partially supported by our estimated over-wint er 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 rel ation between vegetation height and autumn vole density. This conforms to the expectations from the theory of ideal-free habitat distributio n. 5. The cue for vole habitat choice, i.e. vegetation height, indicat es that either predation or freezing risk is important for voles when selecting over-wintering habitat.