While pathogens are often assumed to limit the growth of wildlife populations, experimental evidence for their effects is rare. A lack of food resources has been suggested to enhance the negative effects of pathogen infection on host populations, but this theory has received little investigation. We conducted a replicated two-factor enclosure experiment, with introduction of the bacterium Bordetella bronchiseptica and food supplementation, to evaluate the individual and interactive effects of pathogen infection and food availability on vole populations during a boreal winter. We show that prior to bacteria introduction, vole populations were limited by food availability. Bordetella bronchiseptica introduction then reduced population growth and abundance, but contrary to predictions, primarily in food supplemented populations. Infection prevalence and pathological changes in vole lungs were most common in food supplemented populations, and are likely to have resulted from increased congregation and bacteria transmission around feeding stations. Bordetella bronchiseptica-infected lungs often showed protozoan co-infection (consistent with Hepatozoon erhardovae), together with more severe inflammatory changes. Using a multidisciplinary approach, this study demonstrates a complex picture of interactions and underlying mechanisms, leading to population-level effects. Our results highlight the potential for food provisioning to markedly influence disease processes in wildlife mammal populations.

Department of Biological and Environmental Science University of Jyväskylä Jyväskylä Finland

Department of Veterinary Biosciences University of Helsinki Helsinki Finland Department of Infection Biology Institute of Global Health University of Liverpool Liverpool UK

Natural Resources Institute Finland Suonenjoki Finland

Natural Resources Institute Finland Suonenjoki Finland Department of Biological and Environmental Science University of Jyväskylä Jyväskylä Finland

Natural Resources Institute Finland Suonenjoki Finland Department of Botany and Zoology Masaryk University Brno Czech Republic

Natural Resources Institute Finland Vantaa Finland

Production Animal and Wildlife Health Research Unit Finnish Food Safety Authority Evira Oulu Finland

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