Relative abundance and demographic structure of Agassiz's desert tortoise (Gopherus agassizii) along roads of varying size and traffic volume

Roads are recognized as important contributors to wildlife population declines and are thought to pose greatest risk to vagile species with large home ranges and long generation times. We examined variation in the relative abundance and demographic structure of Agassiz's desert tortoise (Gopherus agassizii) near roads that varied in traffic volume. We found that the abundance of tortoise sign (scat, tracks, pallets, burrows, and live and dead individuals) varied with traffic volume and distance from the road depending on traffic volume. The relative abundance of tortoise sign was greatest along roads with low traffic volume (<1 vehicle/day) compared to roads with intermediate (30-60 vehicles/day) and high (320-1100 vehicles/day) traffic volumes. Additionally, tortoise sign had lower relative abundances at least 200 m from roads with the highest traffic volumes. We found that the frequency of live tortoise encounters decreased with increasing traffic volumes. Tortoise size also correlated significantly with traffic volume, such that tortoises near the highest traffic volume road were smallest. Finally, along the highest traffic road we found greater proportions of juvenile tortoises than along either of the other traffic volume roads. Our results indicate that roads may decrease tortoise populations via several possible mechanisms, including cumulative mortality from vehicle collisions and reduced population growth rates from the loss of larger reproductive animals. Here, we provide evidence that a reptile with a slow life history is susceptible to road presence and that the effect increases with traffic volume.