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Many amphibians are either too small or too slow to avoid an oncoming car. For some populations of the Federally threatened Yosemite toad (Anaxyrus canorus), this has meant increased mortality from vehicle strikes in addition to other threats from disease, drought, and habitat loss.

toad on the road
A rare Yosemite toad (Anaxyrus canorus) rests in the middle of a forest road. (Credit: Cheryl Brehme, USGS Western Ecological Research Center. Public domain.)

Biologists from the USGS Western Ecological Research Center have helped launch a new project with the U.S. Forest Service (USFS) to test whether erecting low bridges over roadkill hotspots can protect Yosemite toads and other sensitive amphibians from vehicle strikes. The study is using state-of-the-art wildlife camera systems to determine how toads move along barrier fencing leading to the elevated road segment and whether toads will successfully travel underneath to reach habitat on the other side.

“We are testing a completely new strategy. We hope that our findings will inform the management and recovery of pond breeding amphibians and other small animal populations that may need to regularly cross roads to persist,” says Cheryl Brehme, USGS biologist and lead scientist on the project.

The USFS began by identifying two roads that were particularly deadly for the Yosemite toad on the Sierra National Forest, CA. Both cut through important stretches of land linking the toads’ upland habitat to the wetlands they rely on to breed, putting these animals at risk as they attempt to migrate from one habitat to another over the summer.

To address the problem, the USFS teamed up with USGS to install a short, elevated road segment over one of the roadways. Black fencing extends from the ramp on each side, to funnel toads toward the refuge provided by the low bridge. Ideally, cars will pass overhead while toads hop safely beneath the bridge to habitat on the other side.

Elevated road segment
Scientists work along elevated road segment established for cars traveling along a rural highway while the road is closed. This low bridge provides safe passage for a rare species of toad as well as vehicles. (Credit: Stephanie Barnes, U.S. Forest Service. Public domain.)

Road crossings built for amphibians and reptiles elsewhere are typically narrow, underground tunnels only a few feet wide. While not all amphibians reach these small tunnel openings, elevated road segments can be any length. This first prototype, for example, provides a crossing space nearly 100 feet wide and allows light and rain to pass through, mimicking the natural paths that toads take to cross from one habitat to another. The long, elevated segment also potentially allows for a higher chance of safe passage during the spring, when lingering snow could easily block the smaller entrance to a tunnel crossing.

View under an elevated toad bridge
View of the underside of an elevated road segment constructed for the threatened Yosemite toad. (Credit: Cheryl Brehme, USGS Western Ecological Research Center. Public domain.)

“With this project, we’re really hoping to learn ways to mitigate road impacts for this extremely rare toad as a model for other amphibian species in decline,” says Robert Fisher, Principal Investigator overseeing the study.

The USGS and USFS plan to check regularly on the wildlife cameras to determine whether Yosemite toads are crossing beneath the new “toad bridge.” Already, they’ve had a snapshot of success: toads captured on camera, using the new passageway during a thunderstorm.

Toad crossing beneath an elevated road segment
A toad caught on webcam, crossing beneath an elevated road segment in the Sierras, CA. (Credit: USGS Western Ecological Research Center. Public domain.)

“This is a very exciting project that has involved a lot of coordination and support from the Forest Service, USGS, U.S. Fish and Wildlife Service, partners, and volunteers. We need to develop a viable solution to our road mortality issue that also supports the multiple uses which occur on Forest Service roads. We are hoping the findings from the study will provide the answers we need,” says Stephanie Barnes, the USFS aquatic biologist for the project on the Sierra National Forest. “We want to be able to successfully share the road with the toad.”

Meanwhile, in collaboration with the California Department of Transportation (Caltrans), the scientists are also testing how well fencing will help guide toads toward the one place where they can cross safely — or whether toads will simply give up and turn back after a point. If fencing works, what types of fencing may be most effective?

“Caltrans is required to mitigate for adverse impacts arising from the construction, maintenance, and operation of the state transportation system to listed and other sensitive amphibians and reptiles. This project will develop the necessary information and guidance for Caltrans to plan and implement roadway passages for vulnerable amphibians, and construct sustainable highways within budget and time constraints,” says Simon Bisrat, an environmental planner with Caltrans.

Brehme and colleagues are currently conducting similar studies with salamanders and other species of toads in Stanford and southern California.  The studies are in collaboration with Caltrans and the Western Transportation Institute to inform the construction of efficient reptile and amphibian road crossings. Recently, the team published a comprehensive paper that assessed more than 160 native Californian reptiles and amphibians in terms of their risk of vehicle strikes or habitat fragmentation from roadways. Read the WERC news story here, or access the publication online.

Acknowledgements: The study authors would like to thank EMTEK® Matting Solutions, LLC for their contributions to building and installing the elevated road segment; ERTEC® Environmental Systems for donating and shipping fencing materials; Animex®, and Hobbs Ecology for the HALT camera trapTM technology. 

Yosemite toad team
Together, Cheryl Brehme (USGS Western Ecological Research Center; USGS), Stephanie Barnes (U.S. Forest Service; USFS) and colleagues are studying how building low bridges over roadways affects the numbers of amphibians struck by cars. (Public domain.)  From left to right first row: Stephanie Barnes (USFS), Denise Clark (USGS), Brittany Idrizaj (USGS), Cheryl Brehme (USGS), Tristan Edgarian (USGS), Devin Adsit-Morris (USGS).Second row: Cassie Vaughan (USFS), Wesley Burton (USFS), Tony Borelli (USFS), Jennifer Kingston (USGS), Alonso Ruiz (USFS),  Mindy Mcclurg (USFS),  Phil Mazon (USFS); not pictured Ed Schiedel (Volunteer USFS).


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