The Desert Tortoise

Description and Ecology of the Desert Tortoise (Gopherus agassizii)

Photo by: Kristian Stahl

The Desert tortoise has a small head with no visible ears, a domed shell, and stumpy elephantine hind legs. Sizes range from 2 to 2½ inches (hatchlings) to 14 inches (adults). It is a long-lived species, slow growing with a generally low reproductive rate. Desert tortoises reach reproductive maturity from 15 to 20 years of age and when more than 7 inches; females lay 0 to 3 clutches with 4-6 eggs in each clutch. Only 2% to 5% of hatchlings are estimated to reach maturity, while the adults have a 90% survivorship. The level of reproduction as well as the physical movement and activity increases and decreases with the amount of rainfall that year. Desert tortoises are inactive during most of the year. Desert tortoise are typically found on low to midelevation sites with suitable soil for their dens and adequate rainfall. Occurs from tropical areas in northern Sinaloa, through Sonora, Mexico, in Arizona throughout the Sonoran Desert, and in the Mohave Desert in southeastern California, and southwestern Utah and southern Nevada.

Geographic and Population Changes

Desert tortoises occupy large home ranges. The loss or degradation of these habitats to urbanization, habitat conversion from frequent wildfire, or other landscape modifying activities add increased risk to the Desert Tortoise. According to Grover and DeFalco, population declines of 10% or more were reported during the 1980s in the western Mojave and in the Colorado Desert. Some of the largest declines occurred at the Desert Tortoise Natural Area in the western Mojave, where densities dropped from an estimated 150 individuals/km² in 1979 to less than 25 individuals/km² 1992. At Chuckwalla Bench in the eastern Colorado Desert, density declined from about 225 individuals/km² in 1979 to about 75 individuals/km² in 1992. 

Due to non-native grass species, fire has become a major threat to the Desert Tortoise habitat. Altering plant cover and the diversity of forage species creates short term and long term threats to the inhabitants of the land.

Listing Date and Type of Listing

Desert Tortoise (Gopherus agassizii) was listed as Threatened on April 2, 1990

Cause of listing and Main threats to its continued existence

Top: unburned Bottom: burned
photo by: Greg Carttar

Desert tortoises have been subject to a variety of elements causing habitat loss, fragmentation, and degradation. These factors include direct human-caused mortality, increased predation, disease, and habitat degradation from the accumulation of trash. Continuing with the list incorporates the threat of overgrazing, off-road vehicle use, and nonnative grass invasion. Invasive plant species have increased the frequency of wildfire and altered habitat structure, introducing the potential for a nonnative grass/fire cycle. Not only does fire affect the tortoises directly but it alters their habitat. Urbanization such as roads and highways, off-highway vehicle activity, poor grazing management and agriculture uses not only fragment and degrade the habitat but contribute to the spread of non-native species.

Desert tortoises are threatened by many, primarily human-caused, factors. These include direct human-caused mortality; increased predation; disease; and habitat degradation from the accumulation of trash; overgrazing; off-road vehicle use; and nonnative grass invasion.

Description of Recovery Plan 

Maintaining high survivorship of adult desert tortoises was identified as the key factor in recovery.

Furthermore, maintaining self-sustaining populations of desert tortoises within each recovery unit into the future. As well as maintaining well-distributed populations of desert tortoises throughout each recovery unit. And ensuring that habitat within each recovery unit is protected and managed to support long-term viability of desert tortoise populations.

1. Develop, support, and build partnerships to facilitate recovery.

2. Protect existing populations and habitat, instituting habitat restoration where necessary.

3. Augment depleted populations in a strategic manner.

4. Monitor progress toward recovery.

5. Conduct applied research and modeling in support of recovery efforts within a strategic

framework.

Photo by: Ken Naussear

6. Implement a formal adaptive management program.

Interesting side note:

"If recovery actions are implemented promptly and are effective, including continued

implementation of the current monitoring program which began in 2001, recovery criteria could be met by approximately 2025. $159,000,000 plus additional costs that cannot be estimated at this time" (Recovery Plan).

Works Cited:

Meyer, Rachelle. 2008. Gopherus agassizii. In: Fire Effects Information System, [Online]. U.S.                 Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences               Laboratory (Producer). Available: http://www.fs.fed.us/database/feis/ [2015, June 4]. 

U.S. Fish and Wildlife Service. 2011. Revised recovery plan for the Mojave population of the

     desert tortoise (Gopherus agassizii). U.S. Fish and Wildlife Service, Pacific Southwest

     Region, Sacramento, California. 222 pp.