Assessment of habitat fragmentation, roads and weather on elk harvest and elk vulnerability in the upper Bitterroot Valley, Montana

Vulnerability of elk (Cervus elaphus) to hunters is widely accepted as the consequence of many variables: habitat fragmentation, road density, number of hunters, season length, elk behavior, hunter behavior, and weather. The importance of each of these variables to elk vulnerability is not well understood. Long-term data from hunter check stations offer a continuous historical record of harvest information and are potentially a valuable source of information on elk vulnerability. This study evaluates long-term check station data from Darby, Montana, with respect to its utility to provide insight into elk vulnerability questions.; Thirty-seven years of elk harvest data obtained at the Darby hunter check station were compiled and analyzed for two hunting districts (HD 250 and HD 270) in the upper Bitterroot Valley. Concurrent information on annual changes in road density, percent of area logged, season length, and number of hunters was also compiled and analyzed. An index of winter severity (IWS) was computed for 15-day intervals across all years based on snow depth and used to examine the influence of weather on elk vulnerability. Several indices were found useful to measure changes in elk vulnerability: hunter success, distribution of the harvest, percent of total harvest occurring opening day, mean and variance of the rate of harvest (number killed/day). Statistical analysis of variables was severely constrained by problems of autocorrelation (due to time trend) and multicollinearity. Principle components analysis (PCA) of variables was conducted to create independent synthetic variables. Results of graphical and statistical analysis of data from two hunting districts revealed large increases in vulnerability occurred in the mid-sixties concurrent with periods of peak road construction and logging, but resolution of the data was insufficient to detect incremental increases in vulnerability during the most recent decade. The influence of weather on elk vulnerability differed between the two hunting districts. In HD 270, where elk migrate from large tracts of secure habitat to concentrate on highly fragmented, accessible areas, weather accounts for up to 50% of the variation in the harvest. In HD 250, where elk migration is less pronounced and elk are uniformly more accessible, numbers of hunters and road density were more important in determining vulnerability.