The role of pathogen infection in field vole (Microtus agrestis) population cycles

Models demonstrate that parasites and pathogens may regulate host abundance in some circumstances. However, only a few studies of wildlife species have provided empirical support for this hypothesis and the majority have focussed on epidemic pathogens causing high levels of mortality. Most pathogens are endemic, persist in host populations and show relatively small fluctuations in prevalence. To a large extent, the effect of such pathogens on the dynamics of wild hosts remains unknown. This study examines both the effect of microparasite infection on host demographic rates and the impact of these effects on host dynamics, using 6 years worth of longitudinal data from cyclic field vole (Microtus agrestis) populations in northern England. Pathogens may have an important role in affecting cyclic host dynamics, providing an alternative to other explanations for multi-annual cycles. Pathogens with the potential to drive host density cycles must adversely affect host survival or reproduction and also exhibit delayed-density dependence. Several microparasites exhibit delayed density dependence in these field vole populations. Here, we consider a range of microparasites (Bartonella spp., Babesia microtii, Anaplasma phagocytophilum, Mycobacterium microti and cowpox virus) and investigate whether infection by one microparasite or a combination of microparasites affects survival probabilities during a population crash.