The Associations Between Large Mammal Abundance, Elevation, and Tick Capture Rate in the Big Belt Mountains

Abstract

The Rocky Mountain wood tick, Dermacentor andersoni, is the primary tick vector of human pathogens like Colorado tick fever and Rocky Mountain spotted fever in the Rocky Mountain Region. Drag sampling was conducted to investigate the association between large mammal abundance, elevation, and tick capture rate in the Big Belt Mountains near Helena, Montana during suspected peak tick activity in May and June. Soil temperature and type, climate, humidity, aspect, slope, and the availability of hosts have been shown to be factors that determine tick distribution. Multiple factor regression, using elevation as a covariate, found that even after accounting for elevation, the relative large mammal abundance was significantly negatively associated with the number of ticks observed per hour. It seems likely that overgrazing by large mammals, such as deer and elk, causes a trophic cascade that negatively affects small mammals and, therefore, tick populations, due to a lack of vegetation. The majority of observed ticks were found at elevations between 1323-1761 m. It is unclear exactly why the majority of ticks are found within a specific elevation range, but it may be determined by climatic conditions found within a particular elevation range such as average daily maximum temperature and humidity.