Associate Professor University of Maine, United States
Land use change is a significant anthropogenic force that shapes the landscape, with consequences for disease systems. While there are correlations between land use change and vector-borne disease or pathogen transmission in many systems (e.g., deforestation and increases in malaria cases, forest fragmentation and increases in tick densities yet decreases in human incidence of Lyme disease), the underlying ecological mechanisms that explain these patterns are complex, and studies may even produce contradictory results. For example, extensive research examined the impacts of forest fragmentation on the blacklegged tick (Ixodes scapularis), the vector of Lyme disease in the northeast United States, but studies’ results are conflicting. This past work provides important insight into the relationship between landscape scale drivers and tick populations, and provides the opportunity to further explore underlying mechanisms. To fully understand the relationship between landscape fragmentation and Lyme disease, and to explain contradicting results of past studies, we must examine an exhaustive list of fragmentation metrics (e.g., total percent habitat, average patch size and isolation, the categorical matrix between patches of habitat, and connectivity). Furthermore, regarding speculation on mechanisms, there has been limited investigation into the cascading effects of fragmentation on population size and behavior of small mammals that provide blood meals for I. scapularis and are reservoirs for the causative agent of Lyme disease, Borrelia burgdorferi. Fragmentation may reduce the activity of predators of small mammals (e.g., bobcats, foxes, etc.), releasing their prey (e.g., mice, etc.) from the threat of predation, thereby increasing prey foraging, tick-host encounter rates, and thus tick densities. Additionally, fragmentation increases forest edge, a preferred browsing habitat of deer, which are important hosts for adult I. scapularis. This study tests the hypotheses that fragmentation results in higher tick densities through (1) decreased predator activity, (2) increased prey foraging, and (3) increased deer activity. We selected 20 sites, each 1km2, in southern Maine, an area with a high incidence of tick-borne disease. These study sites ranged along gradients of multiple fragmentation metrics. We used drag sampling to collect ticks and determine tick densities at each site. To quantify the activity of small mammals, their predators, and deer, we deployed trail cameras to estimate the relative abundance of predators and deer and used foraging trays to assess small mammal foraging behavior. Preliminary results suggest higher tick densities at intermediate levels of fragmentation.
