8/26/2023 0 Comments Highway underpass wildlifeCreative solutions to combat these threats are needed, especially on highways where traffic volumes are high and collisions are especially dangerous for wildlife and people.Ī solution to WVC’s that is gaining in popularity is the construction of wildlife crossing structures ( Sijtsma et al., 2020). With naturally low population densities and large home ranges, large mammal populations are especially vulnerable to the potentially exacerbating threats of wildlife vehicle collisions (WVCs) resulting in mortality ( Ceia-Hasse et al., 2017 Gunson, Clevenger & Hall, 2003 Kusak, Huber & Frkovic, 2000) and the genetic isolation of sub-populations on either side of the road ( Riley et al., 2006 Proctor et al., 2012 Sawaya, Clevenger & Schwartz, 2019). Decades of research has shown that roads can degrade and fragment habitat, create barriers to animal movement, and be a major source of animal mortality (reviewed by Fahrig & Rytwinski, 2009 Trombulak & Frissell, 2000). Roads provide essential connection corridors for people and goods across the world but can be challenging features for wildlife to cross safely and have myriad environmental consequences ( Forman & Alexander, 1998). Future studies, however, are encouraged to further explore the specific instances when underpasses and narrower overpasses present more cost-effective ecological solutions, or how these structures can complement wide overpasses in successful wildlife crossing systems. In reviewing various studies from around the world, we conclude that wide overpasses (~50 m) continue to present ecologically sound and cost-effective solutions for decreasing the barrier of roadways, especially when targeting width-sensitive species and large assemblages of mammals. We qualitatively observed that wider North American overpasses (40–60 m), in or near compliance with expert guidelines, were associated with a more diverse set of species use and had nearly twice the average crossing rates when compared to non-compliant, narrow North American overpasses. Building on previous evidence showing that wider overpasses have increased crossing rates, we examined crossing rates for multiple large mammal species across 12 overpasses located in western North America. We investigate reasons explaining the non-compliance and provide recommendations for future overpass designs. Most wildlife overpasses located in North America and Europe did not meet their respective dimensional expert guidelines. The average width of the wildlife overpasses was 34 m. Using a novel measurement technique, we analyzed the dimensions of these 120 overpasses located in North America, Europe, Asia, and Oceania. We identified 120 wildlife overpasses across the world that were constructed to reduce the negative impacts of roads. We sought to assess how these recommendations manifested in practice-where agencies use this information to design and build structures while also balancing cost and logistical challenges-and the degree to which built structures conform to current recommendations. Accordingly, experts have suggested a width to length ratio of 0.8. Optimal width also depends on structure length, with longer structures requiring additional width. Ideal dimensions of overpasses and underpasses vary by species, but scientists generally suggest that overpasses for large mammals should be approximately 50 m wide. Governments and planning agents thus strive to balance cost and benefit to build cost-effective structures with the greatest positive impact on local wildlife and motorists. Highway mitigation projects are large economic investments that remain on the landscape for many decades. Managing animal mortality and subpopulation connectivity is crucial to successful species and landscape stewardship. It is now well evidenced that wildlife crossing structures paired with exclusion fencing reduce wildlife vehicles collisions while facilitating wildlife connectivity across roadways.
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