Effectiveness of wildlife crossing structures to minimize traffic collisions with mule deer and other wildlife in Nevada.
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Effectiveness of wildlife crossing structures to minimize traffic collisions with mule deer and other wildlife in Nevada.

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      Maintenance of movement corridors is a fundamental component of conservation of biological diversity, and is especially important for terrestrial species that migrate extended distances. Movement corridors for large‐bodied species present unusual challenges, since body size is related to broad spatial needs. Highways and Interstate freeways not only fragment corridors, but result in increased mortality from collisions with vehicles. The observed level of mortalities of mule deer and other wildlife species on U.S. highways appears to have negative consequences on wildlife populations. Studies in other states indicate that more than 50% of the deer‐vehicle collisions nationwide are not reported and there are no records for deer-vehicle collisions that occurred in remote areas although records for collisions within or near urban areas may be reported. Wildlife crossing structures are an important tool in multiple ecosystems to allow safe passage for wildlife across roadways. Indeed, crossing structures have been used extensively in Europe to reconnect fragmented habitats for numerous species. Few projects, however, have documented responses to more than one structure type simultaneously, and fewer have provided information on successful crossings versus retractions (retreat rather than cross). We used mule deer (Odocoileus hemionus), a widespread species across diverse bio‐regions in western North America to test hypotheses about efficacy of different types of crossing structures in Eastern Nevada. The 6.6 km (4.1‐mile) study section of highway is along the U.S. 93 Highway north of Wells, NV, with 3 miles of fencing on both sides of the highway. We documented responses and success of overpasses and underpasses at allowing safe passage across roadway for mule deer. Our metrics to evaluate success included the number of animals that crossed each structure, percentage of successful crossings versus retractions, and mortalities during multiple migrations. We employed an Empirical Bayes approach to compare the ‘before’ and ‘after’ changes in collisions between wildlife and vehicles. A benefit‐cost analysis of the wildlife overpass is also included to identify its effectiveness considering factors like number of mortalities, deer‐vehicle collisions threats to human injuries or fatalities, and the construction costs. Crossing structures were immediately successful and mortalities declined with each subsequent migration, independent of population size. Although all of the crossing structures were successful to some extent; we observed substantially more successful crossings at overpasses than underpasses. Relatively few retractions strongly indicated preference for overpasses by migratory ungulates. These wildlife crossing structures successfully enhanced connectivity by allowing safe passage across highway for mule deer. The benefit‐cost analysis indicated that construction and maintenance of the overpass was economically supported. Importantly, those structures succeeded in making roadways safer for both wildlife and motorists.
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