Effect of forest fragmentation on Lyme disease risk

Forest destruction and fragmentation in the United States recently have been shown to reduce mammalian species diversity and to elevate population densities of white-footed mice ( Peromyscus leucopus ). One potential consequence of reduced species diversity and high mouse density in small fragments is an increase in human exposure to Lyme disease. Increased risk of exposure to this disease is expected because of the role of the white-footed mouse as the principal natural reservoir of the Lyme bacterium, Borrelia burgdorferi . Blacklegged ticks ( Ixodes scapularis ) feeding on mice have a higher probability of becoming infected with the bacterium than do ticks feeding on any other host species. We hypothesized that small forest patches ( <2 ha ) have a higher density of infected nymphal blacklegged ticks, which is the primary risk factor for Lyme disease, than larger patches ( 2-8 ha ). In the summer of 2000, we sampled tick density and B. burgdorferi infection prevalence in 14 maple-dominated forest patches, ranging in size from 0.7 to 7.6 ha, in Dutchess County of southeastern New York state. We found a significant linear decline in nymphal infection prevalence with increasing patch area and a significant exponential decline in nymphal density with increasing patch area. The consequence was a dramatic increase in the density of infected nymphs, and therefore in Lyme disease risk, with decreasing forest patch size. We did not observe a similar relationship between the density of larval ticks and patch size. These results suggest that by influencing the community composition of vertebrate hosts for disease-bearing vectors, habitat fragmentation can influence human health.