Populations of large wildlife are declining around the world, with cascading effects on ecosystem structure and function. One such function critical to the health of both humans and wildlife is control of tickborne disease. Directly, large wildlife can serve as hosts for ticks, maintaining disease transmission cycles. However indirectly, they can modify habitat and suppress abundance of intermediate hosts and ticks, thereby decreasing disease risk. While numerous studies have attempted to correlatively link wildlife loss to changes in disease, few manipulative studies exist, and these have been done at small spatial scales that may inadequately capture indirect pathways. Even fewer studies are replicated across climatic gradients, even though it is well established that climate can strongly mediate the effects of wildlife on plants and other taxa. Furthermore, wildlife losses are frequently accompanied by livestock additions; however the degree to which livestock may alter transmission cycles or 'compensate' for wildlife remains largely unexplored. To address these gaps, we used a large-scale replicated exclosure experiment at Tejon Ranch, Kern Co., to empirically examine the effects of large wildlife loss and cattle additions on ticks and tickborne disease transmission in a California oak system.
Results/Conclusions
Our results demonstrate that removal of wildlife and cattle can impact both tick abundance and disease, but site climatic conditions modulate response magnitude. Responses varied between plots accessible to wildlife and cattle and plots accessible to wildlife only, likely due to differences in total large mammal densities among the two treatments. Removal of all large mammals led to 1) increases in total cover and biomass of understory vegetation, 2) increases in survivorship rates of juvenile ticks, 3) modest changes in intermediate host densities (rodents), and 4) an increase in the density of questing ticks. Effects of wildlife and cattle on all response variables were weakest at the most mesic of the three sites, suggesting that maintaining high densities of wildlife or livestock may be an effective tool for tickborne disease control only in drier/warmer climates. We provide the first experimental support for the theory that interactions among changing climatic conditions and changing large mammal assemblages can have significant impacts on transmission of tickborne disease, and future studies should consider these interactions to improve predictions of disease risk in this era of rapid global change.