Food resources and intestinal parasites as limiting factors for boreal vole populations during winter

Processes limiting the growth of cyclic vole populations have stimulated considerable research and debate over several decades. In Fennoscandia, the peak density of cyclic vole populations occurs in fall, and is followed by a severe winter decline. Food availability and intestinal parasites have been demonstrated to independently and synergistically limit wildlife populations. The purpose of this study was to directly compare competing food and parasite hypotheses on the limitation of overwintering high-density vole populations. Moreover, we evaluated the ability of food limitation and nematode infection to interact and thereby intensify population declines. A two-factor experiment with food supplementation and antihelminthic medication was conducted on replicated, enclosed field vole (Microtus agrestis) populations in central Finland over one full boreal winter. Population abundance, survival, and demographic attributes were monitored through live trapping. Vole feces were concurrently examined for the eggs of Heligmosomidae nemadotes and oocysts of eimerian coccidians. We found that vole density declined in all treatment groups throughout winter. However, food supplementation mitigated this decline through positive effects on reproduction, and voles in food-supplemented populations were generally in better physiological condition than non-supplemented voles. Food supplementation and antihelminthic treatment reduced the prevalence of Heligmosomidae nematodes, while neither food nor medication affected the prevalence of eimerians, or infection intensity of either parasite group. Although food supplementation and antihelminthic medication aided in the clearance of Heligmosomidae nematodes, their prevalence did not influence vole population growth, and this parasite group is therefore unlikely to contribute to the cyclic winter decline of boreal vole populations. Instead food resources acting alone were the primary factor limiting vole population growth.