29th Annual Meeting and Symposium of the
Desert Tortoise Council, February 20-23, 2004
Abstracts

Road Effect Zone for Desert Tortoises: Problems and Solutions

William I. Boarman
U.S. Geological Survey-Western Ecological Research Center 5745 Kearny Villa Road, Suite M, San Diego, California 92123

Roads are a pervasive landscape feature throughout the Mojave Desert. Their impacts on desert tortoise (Gopherus agassizii) populations are manifold and may extend well away from the road edge. The most immediate impact to tortoises is through mortality from collisions with vehicles. By one very conservative estimate, an average of 1 tortoise died annually along every 3.3 km of one highway in the western Mojave Desert. The components of the population most vulnerable to road mortality are probably adults and subadult males. Roads also cause the spread of exotic weeds, facilitate increases in local raven populations, and may introduce toxicants to the environment. Roads also aid intrusions into tortoise habitat by many human activities, some of which may be hazardous to tortoises. A zone of reduced tortoise density occurs within at least 400 m of highway edges and may extend much farther. The exact cause of this depression zone is unknown, but is likely to be the cumulative effects of the many detrimental aspects of roads. Barrier fences reduce mortality along roads, but increase population fragmentation. Passageways can help mitigate the fragmenting effects of roads and fences.