Reptile and Amphibian Road Ecology

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USGS is working with many partners to help reptiles, amphibians and other animals cross roads safely, improving access to essential habitat.

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 WERC research is generating data to inform the designs of road crossing and barrier systems for sensitive herpetofauna and other species.

Major highways of CA colored by the number of species at risk due to that road, with greatest risk along the SoCal coast.

Density of High and Very-High Risk Reptile and Amphibian Species across the State Highway System. Note: California Highway Numbers are in Black.

(Credit: Elise Watson, USGS. Public domain.)

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 WERC used a novel multi-tiered system to rank and identify reptile and amphibian species in California that may be most at risk.  The ranking was based upon 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, reproductive rates, and species distribution and conservation status.  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. Results from this comprehensive ranking system were largely consistent with local and global scientific literature in identifying high risk species and groups.

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

A Yosemite toad looks through a mesh fence, holding itself up on its back legs

A Yosemite toad looks through mesh fencing alongside a road used to mitigate negative road impacts and guide amphibians towards safe passages.

(Credit: Cheryl Brehme, USGS Western Ecological Research Center. Public domain.)



Research to Inform Design of Road-Crossing & Barrier Systems

Researchers and managers have developed a variety of structures that aim to help amphibians, reptiles, and other small mammals cross roads safely. Passage 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 as to how effective they are. In collaboration with state and federal partners, USGS WERC is working to fill these gaps by studying how amphibians and reptiles use these structures to help inform their construction and implementation in California.

Video Transcript

This video shows a California tiger salamander finding its way to an underpass designed to help amphibians and reptiles cross the road safely, with the help of a "turn-around" that helps guide the salamander back in the right direction as it travels away from the underpass. 

(Credit: USGS Western Ecological Research Center. Some images used with permission.)

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

  • Spacing of underpasses for migrating amphibians
  • 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 Best Management Practices and Technical Guidance document based on this research is available here.


Vehicles cross over a amphibian passage on a forest road via an elevated road segment

Vehicles drive on an elevated road segment that allows amphibians and reptiles to pass safely beneath and cross the road.

(Credit: Cheryl Brehme, USGS Western Ecological Research Center. Public domain.)

In another project, USGS researchers in collaboration with the US Forest Service designed and installed a 100 foot long novel elevated road passage system to decrease road mortality and increase permeability for the endangered Yosemite toad. Department of Transportation Pooled Funds are currently being used to evaluate the elevated road passage system and engineer similar concept designs that can be made to any length for use on primary roads and highways. Learn more about this project here.

USGS WERC is also studying whether adding internal structures and ledges to large wildlife underpasses will increase their use by reptiles, amphibians and small mammals for DOT partners.  At the request of 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 2 highways in Riverside and Orange Counties.



Brehme, CS and RN Fisher. 2020. Research to Inform Caltrans Best Management Practices for Reptile and Amphibian Road Crossings. USGS Cooperator Report to California Department of Transportation, Division of Research, Innovation and System Information, 65A0553.

Chapters in the above report include:

  • Watson, E and CS Brehme. 2020. Spatial Mapping-California Essential Habitat Connectivity Lands, Highways, and High-Risk  Species.
  • Brehme CS, JA Tracey, BAI Ewing, MJ Hobbs, A Launer, E Adelsheim, and RN Fisher. 2020. Movement of California Tiger Salamanders Along Barrier Fencing and Underpasses in Stanford, CA
  • Brehme CS, S Barnes, JA Tracey, BAI Ewing, C Vaughan ,and RN Fisher. 2020. Movement of Yosemite Toads Along Barrier Fencing and a Novel Elevated Road Segment in Sierra National Forest, CA
  • Brehme CS, JA Tracey, J Kingston, JB Sebes, TK Edgarian, and RN Fisher. 2020. Effect of Fence Opacity on the Movement of Reptiles and Amphibians and the Effectiveness of Two Jump-out Designs in Brehme CS and RN Fisher
  • Brehme CS, JA Tracey, J Kingston, JB Sebes, TK Edgarian, and RN Fisher. 2020. Effectiveness of Turnarounds in Changing the Trajectory of Reptiles and Amphibians in San Diego, CA in Brehme

Brehme CS, JA Tracey, B Idrizaj, MJ Hobbs, A Launer, E Adelsheim, and RN Fisher. 2019. Movement of California Tiger Salamanders along barrier fencing and underpasses in Stanford, CA. Presented at the International Conference of Ecology and Transportation, Sacramento, CA, September 22-26, 2019.

Brehme, CS, S Barnes and RN Fisher. 2019. A New Road Crossing Structure for Small Animals: Case study with the Yosemite Toad. Presented at the International Conference on Ecology and Transportation, Sacramento, CA, September 22-26, 2019.

Brehme, CS, SA Hathaway, and RN Fisher. 2018. An objective road risk assessment method for multiple species: ranking 166 reptiles and amphibians in California. Landscape Ecology, 33(6), pp.911-935.

Langton, T.E.S. and A.P. 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. (produced by WTI in collaboration with Brehme & Fisher)

Mitrovich, MJ, JE Diffendorfer, CS Brehme, and RN Fisher. 2018. Effects of urbanization and habitat composition on site occupancy of two snake species using regional monitoring data from southern California. Global Ecology and Conservation, 15, p.e00427.

Hobbs, MT and CS Brehme. 2017. An improved camera trap for amphibians, reptiles, small mammals, and large invertebrates. PloS one, 12(10), p.e0185026.

Tracey, JA, CS Brehme, C Rochester, D Clark, and RN Fisher. 2015. The differential use of large underpasses by small animals and effects of adding structure. Presented at 2015 International Conference on Ecology & Transportation, Sep. 20-24, Raleigh, NC.

Brehme, CS, JA Tracey, LR McClenaghan, and RN Fisher. 2013. Permeability of roads to movement of scrubland lizards and small mammals. Conservation Biology, 27(4), pp.710-720.



Army Corps of Engineers

California Department of Fish and Wildlife

California Department of Transportation (Caltrans)

Department of Transportation Pooled Funds Partners

Stanford University, Conservation Program Land Use and Environmental Planning

US Forest Service

Western Transportation Institute

Transport Ecology Services (HCI Ltd.)