Reptile and Amphibian Road Ecology

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Many reptiles and amphibians use both aquatic and terrestrial habitat for breeding, development, foraging, and overwintering and so require high levels of connectivity within and between aquatic and terrestrial habitats. As a result, amphibians and reptiles are particularly susceptible to negative effects of roads within their habitat. Many are slow moving, do not avoid roads, and are simply too small for drivers to see and avoid. During rains many amphibians travel long distances regardless of the presence of intersecting roadways. Snakes and lizards are often attracted to roads because paved roads typically absorb and retain more heat than the surrounding habitat. Such behaviors put these animals at high-risk of vehicle collisions, leading to detrimental effects on whole populations. USGS research is generating data to inform the designs of road crossing and barrier systems for sensitive herpetofauna and other species.

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Identifying Which Species are Most at Risk

Identifying wildlife species most susceptible to road-related impacts can help wildlife managers and transportation agencies prioritize where and how to focus mitigation efforts. USGS scientists used a novel multi-tiered system to rank and identify reptile and amphibian species that may be most at risk in California. The ranking was based upon species distribution and conservation status as well as a suite of species life history and space-use characteristics associated with negative road effects, including movement distance and frequency, speed, migratory and home range behaviors, and reproductive rates. All chelonids (turtles and tortoises), 72% of snakes, 50% of anurans (frogs and toads), 18% of lizards, and 17% of salamander species in California were ranked as high or very-high risk. Our results were largely consistent with local and global scientific literature that identifies high risk species and groups.

The USGS risk assessment informs regional-scale road mitigation planning and threat assessments for special-status reptiles and amphibians. For example, for California Department of Transportation (Caltrans), USGS researchers created a spatial geodatabase tool that identifies highway segments that overlap ranges of high-risk species and lands identified as important to connectivity as part of the California Essential Habitat Connectivity Project (Caltrans, California Department of Fish and Wildlife). This approach could also be used to identify road related risks in other landscapes and for additional taxonomic groups.

Research to Inform Design of Road-Crossing & Barrier Systems

Researchers and managers have developed a variety of structures to help amphibians, reptiles, and small mammals cross roads safely. Structures, such as tunnels under roads, are intended to allow small animals to pass underneath roads without harm. Barrier fencing is designed to prevent entry onto the road and direct animals towards safe passages. Though these tools have been used throughout California to reduce road mortality for small animals, until recently there was little information about how effective they are. In collaboration with state and federal partners, the USGS is working to understand how amphibians and reptiles respond to and use these structures. This research informs design of road-crossing structures and barrier systems in California.

For Caltrans, USGS collaborated with the Western Transportation Institute to create a technical guidance document about barrier and crossing systems that support amphibians and reptiles in California. Research conducted for this document examined a variety of topics, including:

• Spacing of underpasses for migrating amphibians based on fence movement behavior
• Responses of reptiles and amphibians to different barrier materials
• Effectiveness of fence-end "turn-arounds"
• Use of escape ramps
• Use of an existing tunnel system using video and new active trigger cameras

The resulting report, USGS Research to inform Best Management Practices for Reptile and Amphibian Road Crossings, is available here.

The Measures to Reduce Road Impacts on Amphibians and Reptiles in California: Best Management Practices and Technical Guidance document based on this research is available here.

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We are also innovating with novel solutions for crossing designs. USGS researchers, in collaboration with the U.S. Forest Service (USFS), designed and installed a 100-foot-long novel elevated road segment (ERS) passage system to decrease road mortality and increase permeability for the federally threatened Yosemite toad and other small animals. The ERS (basically a low bridge design) can be removed and installed seasonally. Working with transportation engineers, similar concept designs for use on primary roads and highways have been developed and are being applied to other amphibian species, such as the California newt. In collaboration with Caltrans, U.S. Fish and Wildlife Service (USFWS), and USFS, we are also researching the efficacy of 2 new low-cost crossing structures for Yosemite toads.  In addition, our research on turnarounds and fence movement behavior of amphibians has led to innovative fence designs that better lead animals toward passages. We are working with Federal Highway Administration, National Park Service, Caltrans, and others to continue to improve the effectiveness of amphibian and reptile road passage systems.

Finally, the USGS is also helping to make large overpasses and underpasses built for large mammals also function for small animals. We are on the scientific teams to inform designs of overpasses for Highways 101 and Interstate 15. For Caltrans and Department of Transportation partners, we evaluated whether adding internal structures and ledges to large wildlife underpasses would increase their use by reptiles, amphibians and small mammals. At the request of the U.S. Army Corps of Engineers, we are also evaluating landscape connectivity for small and large wildlife species and the use of 18 culverts and underpasses across two highways in Riverside and Orange counties.

Products

See USGS reptile and amphibian road ecology publications and journal articles »»

In Review

Partner-published Technical Reports

Brehme, C.S., Tracey, J.A., Gould, P.R., Rochester, C.R., and Fisher, R.N. 2022. Internal Structures and Ledges Facilitate the Use of Large Underpasses by Multiple Wildlife Species and Groups. USGS Cooperator Report to Nevada Department of Transportation, Transportation Pooled Fund Program Project P200-20-803. https://www.pooledfund.org/details/study/610

Langton, TES. and AP Clevenger. 2020. Measures to Reduce Road Impacts on Amphibians and Reptiles in California. Best Management Practices and Technical Guidance. Prepared by Western Transportation Institute for California Department of Transportation, Division of Research, Innovation and System Information. https://dot.ca.gov/-/media/dot-media/programs/research-innovation-system-information/documents/final-reports/ca20-2700-finalreport-a11y.pdf (produced by WTI in collaboration with Brehme & Fisher)