There is considerable interest in understanding how host community diversity affects the transmission and risk of vector-borne disease. In the case of the Lyme disease, there is strong evidence that host species vary in their competence to feed and infect black legged ticks, Ixodes scapularis. Theoretical and empirical work shows that less diverse host communities, comprised largely of highly competent hosts such as white-food mice (Peromyscus leucopus) and eastern chipmunks (Tamias striatus), increase the prevalence of infection and, under the right conditions, the density of infected nymphs relative to species rich communities, which also include incompetent hosts. A key empirical question remains, however: when host species are added to a community are more ticks fed, or do they simply redistribute themselves across this more diverse community?
To address the question of tick redistribution, we analyzed tick burdens observed on mice and chipmunks in 19 small (0.63 ha - 11.9 ha), isolated forest fragments in Dutchess County, New York, U.S.A. We used GLMs to determine whether larval and nymphal burdens decline with increasing densities of mice, chipmunks, and grey squirrels (Sciurus carolinensis) as predicted if ticks were redistributing themselves among available hosts. (Other methods are being explored.)
Results/Conclusions
Nymph burdens on mice and chipmunks decreased significantly with chipmunk densities, though not with densities of mice or squirrels. Across the range of chipmunk densities we observed during the nymphal period (0 to 30 chipmunks/ha) the average burden on mice decreased by over half and burdens on chipmunks themselves decreased by 80%. In the larval period the only significant effect of small mammal densities was of squirrels on the larval burdens of mice, but over the range of observed densities (0 to 19.5 squirrels/ha), larval burdens on mice declined by over 80%. These results suggest that there may be substantial redistribution of ticks away from highly competent hosts such as chipmunks and mice as other species (e.g., squirrels) become more abundant.