Populations of large mammalian herbivores are declining around the world, with widespread effects on ecosystem structure and function, including disease transmission. Simultaneously, stocking of livestock is increasing; the extent to which these changes in large herbivore density can impact disease are unknown. Directly, large herbivores host arthropod vectors, including ticks; however indirectly, they may impact vectors by modifying critical habitat characteristics—including plant cover, litter accumulation, and temperature—and reducing survivorship when vectors are off-host. Coupled with concurrent global changes, such as climate change, this may have profound effects on vector-borne disease in the Anthropocene.
We used a large-scale herbivore exclosure experiment in southcentral California to investigate the potential indirect effects of large herbivores on larval tick survivorship rates, focusing on two tick species of infectious disease concern which differ in their tolerance of thermal stress. We deployed a total of 6480 caged larvae across 27 1-ha plots during the larval questing season. Plots were established along a climate gradient, and fenced to excluded different combinations of herbivores (“high”, “moderate”, or “low” herbivore density). We used linear mixed effect models to evaluate changes in understory vegetation and microclimate, and examined differences in tick survivorship using cox proportional hazard models.
Results/Conclusions
Herbivore density, site aridity, tick species, and their pairwise interactions were all predictive model factors. Larvae of both species survived significantly longer in plots which excluded all larger herbivores (2-4 weeks depending on the site). Such an extension in survival rates may expand the window of opportunity in which ticks can successfully find hosts, potentially increasing disease transmission rates. At high densities, large herbivores removed a substantial amount of understory biomass, reducing vegetation cover, and increasing direct exposure to solar radiation. This resulted in a mean temperature difference of 1.8 C from both high to moderate, and moderate to low density plots. Removal of understory biomass by herbivores likely accounted for the observed differences in larvae survivorship. Effects were strongest at the most arid of the three sites, indicating that herbivore-induced habitat changes may be particularly important in more arid systems where ticks are most vulnerable to dessication. This provides the first large-scale experimental support indicating that large herbivores can affect tick survival during a critical life stage, important for pathogen transmission to humans. Herbivory by large mammals and reduction of shading by understory vegetation may be an important component of tick-borne disease transmission.