27th Annual Meeting and Symposium of the
Desert Tortoise Council, March 22-24, 2002
Abstracts

Feasible Demography Analyses: Consequences of Additional Adult and Juvenile Mortality on a Stable Population of Desert Tortoises

Justin D. Congdon¹, Harold W. Avery², and James R. Spotila^2
1Savannah River Ecology Laboratory, Aiken, SC, 29802; 
²School of Environmental Science, Engineering, and Policy, Drexel University, Philadelphia, PA 1910

In populations of long-lived organisms, the negative impact to populations caused by increased mortality of adults is greater than similar levels of impact on eggs or juveniles. A cohort model for a stable population of Desert Tortoises was generated using data on clutch size (4.9 eggs), reproductive frequency (1.5 clutches per year), and adult survivorship of 0.985 from the population on the upper Ivanpah Valley study population. Age at maturity was estimated at 15 and 20 years and average nest survivorship at 0.30. Combined with an average juvenile survivorship (ages 1 - 14) of 0.75 resulted in a stable population. If adult survival rate was reduced by 1% and 2% the population would be halved in just less than 100 yr and 50 yr, respectively. A similar reduction in juvenile survivorship would have approximately half the negative impact on the population. Two components of risk associated with paved roads (traffic volumes and vehicle speeds) have the potential to have substantial impact on populations. Mortality associated with roads may be higher in adults because they represent a larger portion of the population, they move more frequently, and move greater distances than do young juveniles. However, mortality of juveniles may be higher than their representation in the population because they are smaller and thus more difficult for drivers to see. In areas with high tortoise densities that border paved roads with high traffic volumes and speeds, fencing represents an important tool for reducing additional mortality of Desert Tortoises and other terrestrial turtles.