Research Spotlight: New Study Identifies Effective Under-Road Passage Designs for California Tiger Salamanders

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A new study by USGS researchers and partners investigated how California tiger salamanders in Stanford, California interacted with different types of barrier fencing while migrating to their annual breeding site and whether they were able to find and use an under-road tunnel passage system.

A salamander with light-colored spots walks slowly over a camera trap threshold

A California tiger salamander moves through a tunnel over a camera trigger threshold.

(Public domain.)

The results provide science-based guidance for environmental planners to help California tiger salamanders and other migrating amphibians more safely cross the road.

Many reptile and amphibian species must cross roads to reach essential breeding and foraging habitat and are slow moving or are too small for drivers to see and avoid. These behaviors put them at high risk of vehicle collisions. Under-road tunnels have been used around the world to help amphibians pass under roads safely, and barrier fencing and turn-arounds along roadsides aim to direct amphibians towards the passages. However, the effectiveness of these tunnel and barrier systems has not been well studied.

The new study sought to determine how far migrating California tiger salamanders (Ambystoma californiense) would move along the barrier fencing before either finding the passage or “giving up”. Using cameras placed near the passages and along barrier fencing, the researchers observed how far and how quickly adult salamanders moved along barrier fencing made of different materials and whether they reached an under-road passages.

They found that the probability of a salamander reaching a passageway decreased rapidly as distance from the tunnel increased, suggesting that passageways should be located fairly close together for most salamanders to encounter them. They estimate that with a maximum of 12.5 meters between passages, approximately 90% of adult salamanders are likely to encounter road crossings. The materials that the barrier fences were made of also made a difference: the salamanders moved more efficiently along solid fencing than along mesh fencing without a visual barrier.

These considerations can assist those seeking to design effective road mitigation for California tiger salamander and other migratory amphibians.

Management and Planning Considerations:

  • The probability of a salamander reaching a passageway decreases rapidly as distance from the tunnel increases.
  • A maximum of 12.5 m between passages along California tiger salamander migration routes should allow approximately 90% of adult salamanders to encounter road crossings.
  • Use of solid fencing or a visual barrier on mesh fencing may help to lead salamanders to passages most efficiently.


This research spotlight refers to:
Brehme, C.S., Tracey, J.A., Ewing, B.A., Hobbs, M.T., Launer, A.E., Matsuda, T.A., Adelsheim, E.M.C. and Fisher, R.N.,2021. Responses of migratory amphibians to barrier fencing inform the spacing of road underpasses: a case study with California tiger salamanders (Ambystoma californiense) in Stanford, CA, USA. Global Ecology and Conservation, 31,p.e01857.

Click here to download a PDF version of this research spotlight.

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