The use of wildlife underpasses and the barrier effect of wildlife guards for deer and black bear
Allen, Tiffany Dore Holland
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Roads and traffic affect wildlife on multiple organizational scales (e.g. from individuals to populations) and different spatial scales (e.g. local patch to landscapes). Roads not only affect the natural environment, but people are also at risk when animals are on the road. As transportation agencies are incorporating mitigation measures into roadway design, more opportunities are arising to study their effectiveness. One such opportunity is along U.S. Highway 93 on the Flathead Indian Reservation in Montana, where eight reconstruction projects over 90.6 km were completed in 2010. The mitigation measures include 2.4-m fencing, crossing structures, and wildlife guards. These measures are aimed at reducing wildlife-vehicle collisions and increasing human safety, while allowing unimpeded wildlife movement and traffic flow. Within the 90.6-km reconstruction zone, two sections were completed in 2006. For this study, we focused on these two sections to answer two questions: 1) To what extent are the wildlife guards a barrier to wildlife, especially deer (Odocoileus sp.)?; and 2) How do characteristics of the underpasses, landscape characteristics, and human disturbance influence use by mule deer (O. hemionus), white-tailed deer (O. virginianus), and black bear (Ursus americanus)? To answer the first question, we monitored wildlife movements with cameras at two guards and in one culvert adjacent to a guard. To answer the second question, we used both sand tracking beds and cameras to monitor 11 underpasses for over two years. We also analyzed data on structural characteristics, landscape characteristics, and human disturbance from field measurements and a geographic information system. The guards were > or =85% effective as a barrier to deer, and 93.5% of deer used the crossing structure instead of the adjacent guard. Though the guards were not an absolute barrier, the results indicate deer were substantially discouraged from crossing, and the vast majority crossed the road using the crossing structure rather than the guard, indicating the guards are an effective means of mitigation. We found that increasing distance to cover may increase mule and white-tailed deer use of underpasses. However, we were unable to determine factors related to black bear crossings. We recommend further study for all three species.