Assessing Vegetational and Climatic Influences on the Distribution of Dermacentor andersoni in Western Montana

Abstract

Both on the micro and macro levels human risk to the host-seeking adult stage of Dermacentor andersoni is poorly understood (Eisen et al. 2007). Due to the difficulties associated with identifying, quantifying, and evaluating the many ecological variables that determine an organism’s ideal niche, there is some contradiction among the primary literature, as to a reliable, conclusive point regarding seasonality and distribution of a given species of tick (Schaalje & Wilkinson, 1985). D. andersoni is the most important North American tick in regards to disease transmission (Mail, 1942). In brief, D. andersoni is the principle vector of Rocky Mountain Spotted Fever, Colorado Tick Fever, Tularemia, and Tick Paralysis (Mail, 1942). Due to the life cycle of the Ixodid tick, its overwintering habits, and its variance in required vertebrate hosts/appropriate vegetative substrates, the most plausible factors which might indicate elevated probability of tick presence are soil type, ground cover (leaf litter), brush cover, and mammal presence. Eisen et al. (2007) found climatic factors to be better overall determiners of tick presence than either topographical or vegetational features. Variables found to be most informative in reporting tick presence were mean annual values for minimum temperature, mean annual values for maximum temperature, base 10 °C growing degree-days, and median length of annual freeze free period, while the uninformative variables were mean annual values for precipitation, snowfall, and relative humidity (Eisen et al. 2007). Flag sampling was conducted throughout the Helena valley and various sites in western Montana. Climatic variables and landscape ecology were assessed through a combination of field-testing and GIS provided data in an effort to determine whether or not there are any easy identifiers that can serve as tools to ascertain the presence or absence of ticks in a given location.