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Desert tortoises get help from the Marines.

Desert tortoises (Gopherus agassizii) have crawled the Mojave Desert since California's southern interior was covered with green ponds and wetlands. Millions of years have altered the landscape dramatically, turning it into an arid expanse dominated by wind, rocks, and sand. The desert tortoise has adapted to major geological and climate change and continues to dig burrows there, waiting out the harshest periods of the year in safety under ground.

In recent decades, a new tenant has arrived on the scene: the Department of Defense. In 1952, the DoD found that the Mojave Desert's wide open spaces provided an ideal backdrop for Marines to practice war fighting. The Marine Corps moved some of its units from Camp Pendleton on the California coast to what is now the Marine Corps Air Ground Combat Center near Twentynine Palms, California. A 596,000-acre (240,200-hectare) spread of rugged landscape directly north of Joshua Tree National Park, the base has evolved into the Corps showcase for large scale live-fire training.

The desert tortoise is an amazingly adaptive animal. However, despite the species' remarkable longevity, its survival is now in peril. In the early 1980s, human migration to the Mojave Desert rose and so did the incidence of trash scattered throughout the landscape. Benefiting from increased food (from human trash) and water, populations of the common raven, a prolific omnivore, skyrocketed. Unfortunately, the raven became one of the main predators of young tortoises. For this and other reasons, including disease, the U.S. Fish and Wildlife Service listed the Mojave population of the desert tortoise in 1990 as threatened.

For every 15 clutches of eggs laid (each clutch typically numbers 3 to 10 eggs), only one individual is likely to live to maturity. Once a desert tortoise has reached adulthood, its prospects for a long life are promising. Its shell is hard enough to protect it from all native wild animals except the mountain lion. However, during its first three to seven years of life, the reptile's shell is soft, and it fails against a wide variety of predators, most significantly the raven. Other creatures that take their toll on eggs and immature tortoises are foxes, dogs, bobcats, and badgers.

For tortoises that survive the elements and predators, there is yet another threat: upper respiratory tract disease (URTD). The primary pathway for UTRD bacteria is direct nose-to nose contact. While there is some question to whether URTD-causing bacteria are native or introduced to the Mojave Desert, the release of diseased pet tortoises does appear to exacerbate the condition in the wild. Rather than killing the tortoise directly, URTD depresses the immune system. A tortoise can survive URTD in a year when food and water are plentiful. In a bad year, however, the disease can be the straw that breaks its back, allowing death by malnutrition, predators, or other diseases.

DoD Takes Action

Two military bases within the native range of the Mojave Desert tortoise population have already acted to overcome the effects of the exploding raven population and respiratory disease. Edwards Air Force Base and Fort Irwin, in concert with the University of California at Los Angeles (UCLA), were first to open captive-breeding pens for the tortoise. Now, the Marine Air Ground Task Force Training Command at Twentynine Palms is kicking off its own effort. The Tortoise Research and Captive Rearing Facility is a 2.25-acre (1-ha) protected enclosure located a few miles from the main base in an area that carries a high tortoise population. Its mission is to protect tortoise nests, hatchlings, and juveniles for the first three to seven years of life. The base environmental staff has been the main proponent for building the captive rearing facility. The Marine Corps recognizes the expertise of UCLA, and it is paying the university to manage the tortoise rearing facility and to provide personnel and equipment.

The much-anticipated program began operating in March 2006. UCLA staff locates female tortoises in the training area surrounding the rearing facility. With a transportable x-ray machine, tortoise handlers check tortoises to determine if they are carrying eggs. If so, staff will take them to one of three large enclosures inside the facility to lay eggs, afterwards returning them to their original location. The eggs will hatch on their own as they would in the wild. (In the wild, adult tortoises do not provide parental care.)

To prevent transfer of the URTD bacteria, personnel keep the tortoises separated in the rearing facility. Biologists wear latex gloves, disinfect equipment between uses, clean their shoes after working in the disease pen, and take other preventative measures.

Hatchlings will live in protection for two to seven years, waiting until their shells have hardened sufficiently to resist predation. New tortoises will be brought into the enclosure in coming years so that a variety of ages are represented. Once released into the wild, the tortoises will be tracked for at least one year to determine their location and overall welfare.

The captive rearing facility also provides a laboratory for scientists to study such topics as tortoise disease transmission, genetics, paternity, and diet. Because rainfall in the Mojave Desert is fickle, the rearing facility will be supplemented with irrigation when necessary to encourage growth of native plants for forage and shelter.

Efforts by Edwards Air Force Base, Fort Irwin, and now the Marine Air Ground Task Force Training Command are coordinated with those of UCLA, the U.S. Fish and Wildlife Service, and tortoise protection groups. All of these agencies and organizations want to see the desert tortoise return to a secure status, making Endangered Species Act protection no longer necessary. These captive-rearing projects will not only contribute directly to recovery by increasing tortoise numbers, but augmented populations will also provide the basis to evaluate other management efforts on the landscape, thus contributing to a comprehensive recovery strategy.

by Captain Aaron Otte, U.S.M.C.

Captain Aaron Otte is assigned to Headquarters Marine Corps, Navy Annex, in Arlington, Virginia (telephone 703-695-8302; email aaron.otte@usmc_mil.)
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Author:Otte, Aaron
Publication:Endangered Species Update
Geographic Code:1U9CA
Date:Jul 1, 2006
Words:1000
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