Effects of acorn production and mouse abundance on abundance and Borrelia burgdorferi infection prevalence of nymphal Ixodes scapularis ticks.

Risk of exposure to Lyme disease is a function of the local abundance of nymphal Ixodes ticks that are infected with the etiological agent, the spirochete Borrelia burgdorferi. We monitored abundance of white-footed mice (the principal B. burgdorferi reservoir in the eastern and central United States) and acorns (a critical food resource for mice), and Ixodes scapularis ticks, as well as ambient temperature (cumulative growing degree days) and growing season precipitation, in a forested landscape of southeastern New York State from 1994 to 2000. We found that acorn production in autumn strongly influenced abundance of white-footed mice the following summer and that abundance of mice in summer, when larval ticks are active, influenced the abundance of infected nymphs the following year. Consequently, the abundance of infected nymphal ticks can be predicted from acorn production 1.75 years earlier. Monitoring of natural fluctuations in acorn production thus supports results of prior acorn addition experiments that were conducted at small spatial scales. Growing degree days and precipitation either had no significant effect on density of nymphs or marginally increased the explanatory power of models that included acorns or mouse density as independent variables. We conclude that, at our study site in New York, the risk of human exposure to Lyme disease is affected by mouse density in the prior year and by acorn production 2 years previously.

Intrinsic density-dependent regulation of vole populations.

Considerable controversy exists over the role of density-dependent processes in controlling animal population size. In populations that fluctuate cyclically or erratically, for example many voles and insects, theory predicts that either density-dependence is weak, or that density-dependent responses lag behind density. One key mechanism for lagged density-dependence is a delay in regeneration of food resources following heavy exploitation. Here we show that meadow vole (Microtus pennsylvanicus) populations respond immediately to high density by reducing breeding effort and hence population growth, disproving the hypothesis that density-dependence is weak. In addition, vole populations do not show a delay in growth following marked reduction in plant biomass (their source of food and cover). We conclude that intrinsic density-dependence processes tend to stabilize vole populations, and that cyclic dynamics are not caused by lagged effects of resource exploitation.