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Roadway Crossings for Sensitive Amphibians and Reptiles: Phase II
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2024-10-01
[PDF-5.80 MB]
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Alternative Title:Multiple Turnarounds Increase the Proportion of California Tiger Salamanders Reaching a Road Passage System in Stanford, CA [Project title from cover page]
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Edition:Final Report
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Abstract:Many migratory amphibians make annual population-level migrations among breeding wetlands and over-wintering and/or summer foraging upland terrestrial habitats. To reduce the negative impacts from road mortality on these vulnerable populations, it has been standard practice to build safe crossings in the form of small passages connected by barrier fencing as mitigation. The permeability of crossing structures is dependent upon the proportion of migrating animals that even reach the passages. In Phase 1 of this project, California tiger salamanders (Ambystoma californiense; CTS) were shown to move an average of approximately 40m along a barrier fence before giving up (90% tolerance interval of 12.5m). CTS that came in contact with fencing and initially moved the ‘wrong’ way (away from a passage) had a very low probability of reaching the passage system. In another study, turnarounds, often placed at fence ends, were shown to be effective in changing the trajectory of amphibians and reptiles. In this Phase 2 study, we tested if multiple turnarounds along the length of barrier fencing would increase the probability reaching the passage system. At the study site in Stanford, CA, we installed turnarounds every 25m, with an additional turnaround 12.5m from the passage system. Individual CTS movements were monitored using active-trigger cameras, documenting speed, direction, use of turnarounds, and success at reaching the passage system for 3 years prior and 2 years after the multiple turnarounds were in place. Our results showed that an average of 36% of CTS initially turned in the ‘wrong’ direction. Prior to installation of multiple turnarounds, 5% of CTS that initially turned in the ‘wrong’ direction made it to the passage system. After installation of the turnarounds, 96% of CTS that initially turned in the ‘wrong’ direction interacted with one or more turnarounds and their probability of reaching the passage system increased to that of CTS that initially moved in the ‘right’ direction (mean 66% success rate); with probabilities increasing in relation to initial distance from passage. To our best knowledge, this is the first study of multiple turnarounds and their impacts on passage system permeability. In addition to increasing the number and quality of passages along migratory pathways, we believe this is promising and cost-effective method to increase overall permeability of passage-barrier systems to migrating amphibians.
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