The Effect of Roads, Barrier Fences, and Culverts
on Desert Tortoise Populations in California, USA


1 U.S. Geological Survey
Biology Department, University of California, Riverside, CA 92421, USA
2 California Energy Commission, 1516 Ninth St., Sacramento, CA 95814, USA
3 Department of Zoology, University of Texas, Austin, TX 70712, USA

        ABSTRACT:  Roads and highways pose several direct and indirect threats to turtle and tortoise populations.  As barriers they inhibit dispersal and subsequent gene flow between subpopulations and metapopulations.  In providing access to turtle and tortoise populations, they foster such threats as development, vandalism, and collecting.  Increased diversity and productivity of vegetation, resulting from enhanced hydrological conditions beside roads, attracts tortoises, which place them at greater risk of direct mortality from both predators and motorized vehicles.
        Roadkills are a substantial source of mortality in desert tortoises, Gopherus agassizii, in California (USA) as evidenced by data on roadkills from two highways.  Desert tortoise populations are depauperate along highways and this depression may extend for at least 0.8 km or more from the road.  Our study of the movements of desert tortoises equipped with radio transmitters suggests that tortoises living near highways move considerable distances over short periods of time and that these movements may place the tortoises at great risk of traffic-related mortality.  Other studies show that common ravens, Corvus corax, predators on juvenile desert tortoises, are more common along heavily-traveled roads than away from them.
        A 24 km long tortoise-proof fence was erected along one highway in California.  The barrier fence is made of 60 cm wide, l cm mesh hardware cloth, sunk 15 cm into the ground.  The fence is supported by a 1.5 m high, six-strand wire fence.  Several storm drain culverts span the highway.  We report on a project that is now underway to monitor the effectiveness of the fence in preventing roadkills and facilitating the recovery of the local tortoise population.  We are also measuring use of the culverts by tortoises to determine whether storm drain culverts are an effective mitigation for the fragmenting effects of the fence and highway.

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