Bridging the Barriers Roads Pose to Wildlife

Guest Post By Rebecca Cheek, 2020-2021 Sustainability Leadership Fellow and Ph.D. Candidate in the Department of Biology and the Graduate Degree Program in Ecology

We have all witnessed the consequences of vehicle and animal collisions – a misshapen mass of fur, scales, or feathers often ignored on the roadside.
When animals meet with vehicles, the animal is usually the loser. This black bear was struck on Highway 550, North of Durango, Colorado. Photo credit: Bear Smart Durango; The Durango Herald
Warning: This image contains a deceased bear and is of a graphic nature. View at your own discretion.
Roads act as barriers to wildlife which has far reaching consequences that may be clearly observed in the DNA of affected populations1. As animals move across the landscape to find food and mates, they are transporting their genetic material across a vast and interconnected network of smaller populations. Such movement is critical in preventing local population declines through immigration, as well as maintaining the genetic variation wild populations need to stay healthy2. Roads disrupt these networks, cut off populations, and can even create diverging genetic makeups for the same species on opposing sides of major highways3. Humans’ desperate need to be more connected is actively trapping wildlife in a snare of roads and increasingly encroaching development4. When a dropped plate shatters, the pieces may be glued together, but the repaired object will never be as strong as the original. The same may be said for habitat and population connectivity5. Undisturbed habitat with well connected populations, much like an intact plate, is highly resilient. Yet once that structure has been damaged, and the number of fragments increase, the plate becomes increasingly fragile no matter how much glue may be used.
Figure from Kozakiewicz et al. (2019). Each circle in this map of Southern California represents an individual bobcat color coded by population. We can see that major highways including the 101 and 405 visibly fragment different populations of bobcats with a few rare migrants.
We have national parks and preserves. What else is needed? Parks, refuges, and open spaces are critical in the conservation of species by providing them with much needed habitat, and we must preserve and restore the dwindling wildlands we have. However, for animal populations to thrive sustainably they must have diversity6. Once populations become fragmented, the greater the likelihood that individuals within a population will become less genetically diverse over time, leaving very little variation to help buffer populations from catastrophic declines as the world becomes hotter and more volatile. Our glued together plate becomes increasingly useless when pieces are lost under the stove. Same too with adaptive genes that give individuals a competitive edge. While these critical genes may exist in one population may never extend beyond a major highway or be forever lost to a car’s bumper when “just one” bear, moose, or turtle is struck. Further, global climate change will shift suitable habitat beyond animals’ current ranges and could render many current parks inhospitable in the future. The ability to move and maintain a healthy gene pool is even more essential under rapid climate change7. If animals are to survive a rapidly changing world, they must be able to cope, adapt through their genetics, or move to new ranges with more suitable habitat. So, what can be done? Think again of that image of the bear. Try picturing it alive and healthy, gingerly stepping along a well-used game trail used by moose and deer alike as they go about their daily lives. But the trail is not in the middle of a forest or park – it’s a few dozen feet above Highway 9 near Blue Valley Ranch, Colorado.
Miles of fencing help direct animals to the wildlife overpass at Blue Valley Ranch, Colorado, complete with a layer of soil to provide a more natural feel for its four-legged users (Photo Credit: J. Richert).
The trail is a wildlife overpass, a crossing structure designed to facilitate animal movement across human-made barriers. One local story that I missed during the 2020 doom scrolling cycle was that of the Highway 9 Colorado River South Wildlife and Safety Improvement project. The multi-million-dollar project was in response to years of wildlife-vehicle collisions and human casualties along the heavily trafficked corridor between Kremmling and the ski gateway town of Silverthorne, and included multiple wildlife overpasses, underpasses, pedestrian crossings, wildlife escape ramps, and targeted barriers.
Kintsch et al. (2020) showed that the 5-year average of wildlife-vehicle collision (WVC; blue bars) carcasses of elk and mule deer was 62.8 each winter prior to construction (dashed line). There has been a 90% decrease in elk and deer carcasses observed within the project area since the Highway 9 Colorado River South Wildlife and Safety Improvement project was completed in 2018.
The success of this project, as evident by their 2020 annual report, is among several completed projects in the Western United States. An exciting Colorado project currently in development proposes building wildlife over and underpasses along Interstate 70. A heavily trafficked corridor that connects the city of Denver to popular ski resorts and mountain retreats while also bisecting migration routes of elk, and home ranges of endangered lynx. Mitigating the barrier this “Berlin Wall” poses to wildlife is a critical step forward in maintaining sustainable networks of wildlife while making roads safer for drivers and animals alike. Wildlife-minded infrastructure such as overpasses and underpasses are just one tool we can use to address the current global conservation crisis6. Further work is needed to improve the connectivity between existing parks within the United States, and across international borders if we are to repair the damage done to our shattered wildlands. The significant decrease in collisions and wildlife casualties as a direct result of the efforts along Highway 9 and elsewhere demonstrates that building wildlife bridges improve road safety while also preserving the life and genetics of countless animals.
References: 1) Balkenhol, N. and Waits, L. P. (2009), Molecular road ecology: exploring the potential of genetics for investigating transportation impacts on wildlife. Molecular Ecology, 18: 4151-4164. 2) Holderegger, R., & Di Giulio, M. (2010), The genetic effects of roads: a review of empirical evidence. Basic and Applied Ecology, 11(6), 522-531. 3) Kozakiewicz CP, Burridge CP, Funk WC, et al. (2019). Urbanization reduces genetic connectivity in bobcats (Lynx rufus) at both intra‐ and interpopulation spatial scales. Mol Ecol. 28:5068–5085. https :// 4) Travis J. M. (2003). Climate change and habitat destruction: a deadly anthropogenic cocktail. Proceedings. Biological sciences, 270(1514), 467–473. 5) Franklin, Alan & Noon, Barry & George, T. (2002). What is habitat fragmentation?. Studies in Avian Biology. 25. 20-29. 6) Allendorf, F.W. (2017), Genetics and the conservation of natural populations: allozymes to genomes. Mol Ecol, 26: 420-430. 7) Scheffer, M., Carpenter, S., Foley, J.A., Folke, C., & Walker, B. (2001). Catastrophic shifts in ecosystems. Nature, 413, 591-596 8) Kintsch, J., Cramer, P., Singer, P., Cowardin, M., Phelan, J. (2020), State Highway 9 Wildlife Crossings Monitoring Year 4 Progress Report. ECO-Resolutions. Colorado Department of Transportation

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