Climate change is predicted to alter vector-borne disease exposure and transmission through multiple pathways, but the relative importance of direct versus indirect effects are poorly understood. We are disentangling the relative influence of direct factors such as host density, from indirect factors, such as the plant community composition, on the risk of exposure to tick-borne diseases (TBDs). Specifically, we are examining the effects of fire regimes and plant invasions on abundances of ticks and wildlife hosts, with a focus on the highly problematic cogongrass (Imperata cylindrica) and the lone star tick (Amblyomma americanum) at Department of Defense (DoD) installations throughout the Southeast United States. Our overarching hypothesis is that fire and plant invasions are dominant and interrelated factors affecting exposure risk to TBDs, and that effects of climate change are mediated by these factors. In summer 2017 we sampled six DoD installations and collected data across 29 plots in primarily pine-dominated landscapes that last received prescribed fire between 0 and 11 years ago. We measured tree basal area and density, quantified herbaceous plant community species diversity and cover, and estimated densities of ticks and their wildlife-hosts.
Results/Conclusions
Despite widespread invasions of cogongrass throughout the Southeast US, management efficacy of the invader on DoD installations limited the number of untreated invaded areas we found to eight plots, with six of them burned less than three years prior. A path analysis revealed that time since the most recent fire was the dominant factor, with positive direct effects on litter cover and tick abundance. In bivariate regressions, tick density was lower in plots that were more recently burned, which also had lower litter mass, and more open over-story canopy; however, . Tick density also tended to be higher in uninvaded plots, but the small number of invaded plots in the current data set precludes a rigorous comparison. Our results provide evidence suggesting that fire and forest vegetation and structure affect TBD risk as we predicted. In addition to direct destruction of ticks, fires reduce vegetation and litter cover, resulting in a hotter and drier microclimate, which is generally less favorable for tick survival. Frequent prescribed fires is a typical management practice to maintain pine ecosystems across southeastern US landscapes, and appear to have the additional benefit of lowering TBD risk.