IENE 2016

Successful wildlife crossing structure designs in the United States mountain west were documented in multiple studies of ungulate and carnivore use of wildlife crossing structures and existing bridges and culverts. Results from studies in Montana, Idaho, Utah, and Colorado illustrate the optimal wildlife crossing structure types and dimensions for mule deer (Odocoileus hemionus), white-tailed deer (Odocoileus virginianus), elk (Cervus canadensis), moose (Alces alces), and carnivores. The objectives of these studies were to determine structural attributes and fencing characteristics most important to deer and other ungulates in their use of the structures to pass beneath roads. From 2007 through 2016, the studies used remote camera traps to monitor wildlife reactions to 37 wildlife crossing structures and 27 existing culverts and bridges placed for other purposes. These structures were placed on roads ranging from two to six lanes, with daily traffic volumes ranging from 2,000 to 40,000 vehicles per day. The surrounding landscapes varied from wild lands to suburban settings. The studies generated data on over 60,000 wildlife events at structures. Statistical analyses found that the openness of structures was the most important overall predictor of mule deer and white-tailed deer success rates through the structures. In Utah the length of the culvert or bridge, meaning the distance the animal moved beneath the road, was the most important structural dimension in predicting mule deer success rates: the shorter the length, the greater the success rate. The width of the passage or the span of the structure was the second most influential dimension, with wider structures having greater mule deer success rates. The height of the structure was the least important of the three dimensions, but still important to mule deer and was also correlated with white-tailed deer success in Montana; structures under 2 meters did not have as high deer success rates as higher structures. Increased culvert and bridge width, increased openness, and decreased length increased white-tailed deer success rates while decreasing parallel rates and the rate of repellency. Though elk use of structures was quite low in all studies, the most highly used structures were a wildlife overpass and a pair of wildlife crossing span bridges underneath an interstate in Utah. Moose were most often photographed using one culvert in northern Utah. Moose use was extremely low at other individual structures, but in general they were photographed using wildlife crossing bridges that conveyed water. Fencing to structures was found to have mixed results. Fencing played an important role in motivating migrating mule deer to use existing culverts and bridges, but was not a consistent motivating source for animals to use such structures. Length of fencing was not found to be a statistically important factor for white-tailed deer use of crossing structures in a suburban setting. The information from these studies has already helped create cost savings by helping agencies to build structures with the most optimal dimensions for the target species, and to show funding partners how mitigation can pay for itself in savings from reduced wildlife-vehicle collisions.