Desert Tortoise

Gopherus agassizii

Description

The desert tortoise, Gopherus agassizii, has a carapace (upper shell) length of 6-14 in (15-36 cm). Males are larger than females. Both carapace and plastron (undershell) are marked with growth rings. The six-sided carapace shields are dull brown or yellowish; the plastron is yellowish and unpatterned. The head is scaly; the limbs are elephantine with blunt nails and no webbing. Desert tortoise have also been described in scientific literature as Scaptochelys agassizii and Xerobates agassizii. It is the official state reptile of both California and Nevada.

Behavior

This slow-moving tortoise is most active in the morning, except during extremely hot weather, when it emerges from its burrow only at night to avoid the heat. It builds dens by burrowing as far as 30 ft (9 m) into an earthen bank and remains there, inactive, during the colder months. They typically remain active through the spring, and sometimes emerge again after summer storms. During these active periods, desert tortoises eat a wide variety of herbaceous vegetation, particularly grasses and the flowers of annual plants, browsing on low-growing plants and freshly fallen leaves.

During breeding the male hisses and butts the female in the flank. In a nest scooped out of the ground, the female lays 4-12 round, off-white eggs that take up to four months to hatch. Shells of young turtles are soft during the first five years of life, slowly hardening as the animal matures. Sexual maturity is reached after 14-20 years. The desert tortoise can live as long as 100 years.

Habitat

The desert tortoise is found in semi-arid grasslands, gravelly desert washes, canyon bottoms, and on rocky hillsides at elevations up to 3,530 ft (1,070 m). Within the varied vegetational communities of the Mojave region, desert tortoises can potentially survive and reproduce where their basic habitat requirements are met. These requirements include sufficient suitable plants for forage and cover, and suitable substrates for burrow and nest sites. Throughout most of the Mojave region, desert tortoises occur primarily on flats and bajadas with soils ranging from sand to sandy-gravel, characterized vegetationally by scattered shrubs and abundant inter-shrub space for growth of herbaceous plants. Desert tortoises are also found on rocky terrain and slopes in parts of the Mojave region, and there is significant geographic variation in the way desert tortoises use available resources.

Distribution

Historically, this tortoise was found in the greater Mojave and Sonoran Basin deserts in southeastern California, the southern tip of Nevada, extreme southwest Utah, and western Arizona.

The species still occurs throughout its range but in greatly decreased numbers. At one time, it was estimated that in some places there were as many as 1,000 tortoise per square mile. In a few areas of southern California, the population still reaches densities of 200 per sq mi. The overall trend, however, reveals rapidly declining populations. Recent studies in California showed significant declines (up to 55%) at seven of eight tortoise study sites. The desert tortoise is considered Threatened in Utah where it occurs in low numbers on the Beaver Dam Slope in Washington County. As of 1990, an estimated 100,000 tortoises are thought to survive in the Mojave and Sonoran deserts.

Threats

The desert tortoise has suffered from loss of habitat, overcollecting, and vandalism. Though now illegal, collecting wild tortoises continues. They are killed by vehicles, shot for target practice, or deliberately tipped on their backs and left to die. Ravens, which have accompanied human development of desert habitats, are an increasing source of tortoise mortality.

Conservation and Recovery

In 1971 the U.S. Bureau of Land Management (BLM) established a 38-sq mi (100-sq km) sanctuary near California City, California, as the Desert Tortoise Natural Area. This preserve is located in Kern County, north of Edwards Air Force Base. The area is closed to vehicles, livestock grazing, and mineral exploration.

Many conservationists have long urged the BLM, which administers 67% of all desert tortoise habitat, to reduce livestock grazing on those lands. Cattle and sheep trample tortoises directly, collapse their burrows, and compete for limited food supplies on overgrazed rangeland. Local ranchers, however, have insisted that BLM keep federal lands open to grazing.

In 1985, a petition to the U.S. Fish and Wildlife Service (FWS), filed jointly by the Defenders of Wildlife, the National Resources Defense Council, and the Environmental Defense Fund requested that the desert tortoise be listed as Endangered throughout its entire U.S. range. The petition expired, but was renewed again in 1989 by the same groups. As a result, the desert tortoise was reclassified as Endangered under the emergency provisions of the Endangered Species Act in August 1989. The revised status covered the Mojave Desert populations, encompassing all of California and Nevada, and Arizona above the Colorado River. The Sonoran Desert population was not included in the reclassification.

In October 1989, the BLM declared a special quarantine that closed 37,700 acres (15,260 hectares) of the Mojave Desert, southwest of Ridgecrest, California, to human use for one year. This action was triggered by a severe epidemic of a respiratory infection that has spread through the tortoise population. The infection, which is common in domesticated turtles, clogs the tortoise's lungs and eventually causes death. It is thought to have been spread to the desert tortoise by released pets, and its effects have been worsened by recent drought conditions. This unprecedented BLM action to protect an endangered species generated strong objections among developers, ranchers, and recreationalists in California and Nevada.

The desert tortoise received much attention when the FWS took emergency action to list the Mojave population as Endangered. During the 240 day life of the emergency rule, the FWS studied the tortoise and its habitat, sought public input, and prepared a proposal to provide long-term Endangered Species Act protection to the population.

On April 2, 1990, a final rule was published in the Federal Register listing the Mojave population of the desert tortoise as Threatened. The Beaver Dam Slope sub-population in Utah, which was listed as Threatened with Critical Habitat in 1980, is included as part of the greater Mojave population under this rule and is not treated separately; the area designated as Critical Habitat in 1980 will remain in effect. The rule also treats all desert tortoises from the Sonoran population (south and east of the Colorado River) found outside their native range as Threatened due to their similarity in appearance to the Mojave tortoises.

The Mojave population is at risk, in part, because of people's love of risk-taking. One of the population's primary locations is Clark County, Nevada, home of Las Vegas, a city and its suburbs growing so quickly that between 1980 and 2020, the population is expected to triple, reaching 1.5 million. Urban development is concentrated in the Las Vegas Valley, which encompasses only 20 percent of the county but 96 percent of the population.

The slow-moving world of the desert tortoise and the fast-paced world of Las Vegas headed for conflict in the 1980s as bulldozers and backhoes steadily extended suburbia into the scrubby habitat of the desert's original dwellers. This loss of habitat, combined with habitat damage from livestock over-grazing and off-highway vehicle (OHV) use, predation of juvenile tortoises by common ravens (Corvus corax ), drought, the spread of an upper respiratory tract disease in tortoises, and illegal collection contributed to the toll on tortoise populations, which declined by as much as 90 percent in some areas.

Listing the tortoise under the Endangered Species Act slowed the rapid commercial and residential development that had come to characterize the Las Vegas area. After developers failed to overturn the listing action in court, the Southern Nevada Homebuilders Association agreed to negotiate a solution. A steering committee comprised of representatives for area communities, planning departments, state and federal agencies, land-use and conservation groups hammered out a 30-year habitat conservation plan (HCP), approved by the FWS and the Clark County, Nevada, Commissioners on July 18, 1995. The Clark County Desert Conservation Plan replaces a short-term HCP, issued in 1991 as an interim measure and amended in 1994, that allowed development of up to 30,352 acres (12,283 hectares) and the incidental take of 3,710 desert tortoises. The Clark County Desert Conservation Plan ensures that development can continue while allowing the desert tortoise to recover. While allowed incidental take of tortoises in a limited range over a 30-year period exists, recipients of incidental take permits will carry out measures designed to minimize, monitor, and mitigate the effects of this take and the associated loss of tortoise habitat. Recovery of the desert tortoise will occur mainly on federally-administered lands.

A similar HCP was developed beginning in 1991 by Washington County, Utah (which includes the city of St. George), another fast-growing area that hosts the tortoises. The final plan was submitted to the Fish and Wildlife Service in 1995. After evaluating the HCP, the FWS issued an incidental take permit to the county in February, 1996. The permit allows Washington County to take up to 1,169 desert tortoises, over the 20-year life of the HCP, incidental to otherwise lawful activities (i.e., land development and construction projects). The permit and HCP specify that the take may occur only on non-federal lands in the St. George area, outside the boundaries of a reserve to be established under the plan. Over 60 percent of the lands for the reserve are already administered by the BLM, and the remainder of non-federal lands are being consolidated through purchase and land exchange.

The 1993 draft version of the Desert Tortoise Recovery Plan for the Mojave Population has as its objective the recovery and delisting of the Mojave population. Delisting may be considered if the population is stationary at target density or increasing toward target density; the projected habitat is large enough or is intensely managed for the benefit of the tortoise; regulatory mechanisms are in place to speed recovery; and the population is unlikely again to become threatened in the foreseeable future.

In January 1997, it was announced that the FWS and Naval Air Weapons Station China Lake at Ridgecrest, California, have developed a program expediting the Navy's environmental review process and conserving habitat for the threatened Mojave population of the desert tortoise. Under this program, the Navy will limit projects in areas that support higher numbers of desert tortoises to disturbance of less than 2.5 acres (1 hectare) per project. The program also established a general set of desert tortoise avoidance and protection measures for projects throughout the facility. A review of activities conducted under this program will be completed at the end of each year.

The continued alliances of interested parties— conservationists, federal and state governments as well as land-developers, users, and owners— is having a positive impact on recovery efforts for the desert tortoise. Though some individuals are lost in the process of incidental take, for instance, the active conservation of the species in newly designated reserves and protected habitat makes such losses worthwhile in the larger scheme of long-term recovery goals.

Contacts

U.S. Fish and Wildlife Service
Division of Endangered Species
Eastside Federal Complex
911 NE 11th Avenue
Portland, Oregon 97232
http://pacific.fws.gov/

U.S. Fish and Wildlife Service
Division of Endangered Species
Denver Federal Center
P.O. Box 25486
Denver, Colorado 80225
http://www.r6.fws.gov/

References

Berry, K. H., et al. 1986. Changes in Desert Tortoise Populations at Four Study Sites in California. Report. U.S. Bureau of Land Management, Riverside, California.

Campbell, F. T. 1988. "The Desert Tortoise." In Audubon Wildlife Report 1988/1989, W. J. Chandler, ed. Academic Press, San Diego.

Ferrara, J. 1984. "Digging In." National Wildlife 22(2): 22-28.

Mathews, J. 1989. "Efforts to Save Tortoise Close Part of Mojave Desert." Washington Post October 2, 1989.

U.S. Fish and Wildlife Service. 1993. "Draft Recovery Plan for the Desert Tortoise (Mojave Population)." U.S. Fish and Wildlife Service, Portland.

Desert tortoise

The desert tortoise (Gopherus agassizii ) is a large, herbivorous, terrestrial turtle of the family Testudinidae. It is found in both the southwestern United States and in northwestern Mexico. It is the official reptile in the states of California and Nevada. No other turtle in North America shares the extreme conditions of the habitats occupied by the desert tortoise. It inhabits desert oases, washes, rocky hillsides, and canyon bottoms with sandy or gravelly soil under hot, arid conditions.

Desert tortoises dig into dry, gravelly soil under bushes in arroyo banks or at the base of cliffs to construct a burrow, which is their home. Climatic conditions dictate daily activity patterns of these tortoises, and they can relieve the problems of high body temperature and evaporative water loss by retreating into their burrows. Since many desert tortoises live in areas devoid of water, except for infrequent rains, they must rely on their food for their water.

The active period for the desert tortoise is from March through September, after which they enter a hibernation period. Nesting and egg laying activities extend from May through July. Desert tortoises lay an average of five moisture-proof eggs, an adaptation that helps retain water in its harsh environment . These tortoises reach sexual maturity at 15–20 years, and they have a life span of up to 80 years.

The desert tortoise is very sensitive to human disturbances, and this has led to the decimation of many of its populations throughout the desert southwest. The Beaver Dam Slope population of southwestern Utah has been studied over several decades and shows some of the general tendencies of the overall population. In the 1930s and 1940s the desert tortoise population in this area exhibited densities of about 160 adults per square mile. By the 1970s this density had fallen to less than 130 adults per square mile, and more recent studies indicate the level is now about 60 adults per square mile. In southeastern California at least one population reaches densities of 200 adults per square mile, but overall tendencies show that populations are drastically declining. Recent estimates indicate that there are about 100,000 individual desert tortoises existing in the Mojave and Sonoran deserts.

Desert tortoise populations are listed as threatened in Arizona, California, Nevada, and Utah. Numerous factors are contributing to its decline and vulnerability. Habitat loss through human encroachment and development, overcollecting for the pet trade , and vandalism—including shooting tortoises for target practice and flipping them over onto their backs, causing them to die from exposure—have decimated populations. Other factors contributing to their decline are grazing livestock, which trample them or their burrows, and mining operations, which also causes respiratory infections among the desert tortoises. Numerous desert tortoises have been killed or maimed by off-road vehicles , which also collapse the tortoises' burrows. Concern is mounting as conservation efforts seem to be having little effect throughout much of the desert tortoise's range.

[Eugene C. Beckham ]

RESOURCES

BOOKS

Campbell, F. "The Desert Tortoise." Audubon Wildlife Report. San Diego: Academic Press, 1988–89.

Ernst, C., and R. Barbour. Turtles of the United States. Lexington: University Press of Kentucky, 1972.
———. Turtles of the World. Washington, DC: Smithsonian Institute Press, 1989.