Do sheep affect vole populations in alpine meadows of central Norway?

Abstract

We tested the hypothesis that domestic sheep (Ovis aries) in alpine meadows of central Norway negatively affect populations of field voles (Microtus agrestis) because grazing reduces cover and increases the concentration of secondary compounds in forage plants. We maintained 3 treatments for 3–4 years on 11 experimental plots. Treatments included fencing to remove sheep; fertilization to improve forage quality; and grazed, unfertilized controls. Within experimental plots we collected data on cover of vascular plants, forage quality (nitrogen, fiber, and secondary compounds), and indicators of performance for vole populations (summer density, winter activity, and body size). Sheep appeared to reduce the cover of vascular plants, but quality of 4 species of forage plants showed little response to grazing. Concentrations of nitrogen in forage increased on fertilized plots, as did the ratio of nitrogen/(fiber plus phenolic compounds). Voles occurred sporadically on the experimental plots, however, and indicators of vole performance differed little among treatments. Furthermore, the quality of preferred food (Carex bigelowii) of voles showed no clear response to fertilization, and the activity of voles on plots showed no significant correlation with quality of C. bigelowii irrespective of treatment. We concluded that removal of grazing by sheep in these meadows did not have a substantial short-term impact on forage quality or on vole populations, but longer-term studies are needed because the composition of vegetation may change slowly in response to exclusion of herbivores.