Desert Tortoises Essay, Research Paper
Vulnerable to the harsh desert conditions, the desert tortoise spends almost all of its time waiting out extreme temperatures in its underground burrow. There, this armored, terrestrial turtle conserves its energy and stays relatively cool during summer months, and relatively warm during winter months. This turtle, California?s official state reptile, carries a high-domed tan to dark brown carapace (shell), which protects them from most predators. Yet even those shells are unable to shield the desert tortoise from such threats as the golden eagle and the automobile.
Desert tortoises commonly dig their burrows in dry, gravelly soil beneath creosote bushes, large shrubs, or beside washes in the open desert. The burrows have entrances shaped like half moons, like the tortoises profiles, and can range from one to ten feet, depending on how many animals might inhabit the burrow. They often share their burrows with lizards, snakes, rabbits, woodrats, or other desert creatures who do not pose a threat to them.
Late winter and spring are peak times to view active desert tortoises. Although sometimes a tortoise may emerge from its burrow during the early morning or late afternoon hours of the summer months, most remain underground until a late summer rainfall offers them a chance to replenish their water reserves. By October, most tortoises begin their winter hibernation.
In mid-March, rising temperatures and sprouting annual plants entice desert tortoises out of hibernation. They emerge to feast upon the tender grasses, broad-leafed annuals, and new shoots of perennials that pop up from the deserts sandy soils. Desert tortoises also like to dig shallow basins in areas with impermeable soil to catch drinking water from brief thunderstorms the desert might have. However, they can go for years without drinking any water, taking most of their moisture from the plants they feed on and storing the water in their bladder.
When not in hibernation, desert tortoises tend to court whenever the opportunity arises. There is no well-defined mating season, but much of the mating takes place in the month of April. By the middle of May until July, the female desert tortoise will scoop a nest in soft soil near, or at, the burrow entrance. Depending on her size, a female desert tortoise can lay between three and fourteen hard-shelled eggs that are about the size and shape of ping-pong balls. The eggs are then covered with soil for protection from coyotes, foxes, badgers, or other predators. During years when food is plentiful, tortoises can lay up to three clutches of eggs.
Young tortoises hatch between August and October, depending on when their clutch was laid. They are slow growers, averaging less than one inch per year. Their flexible shells make them vulnerable to ravens, roadrunners, raptors, snakes, bobcats, foxes, and coyotes, until they reach five to eight years of age. That is when shell hardening and larger size can help protect them from most threats.
Since the early 1900?s, the desert tortoise has been threatened with a terrible array of impacts. One major threat is the loss of habitat. Between 1975 and 1988, an approximate increase in the human population of the Mojave Desert has taken over large areas of prime tortoise habitat and eliminated the desert tortoise from areas such as the Antelope Valley. Mining and energy related exploration and development within the desert have claimed additional habitat, and road building for these projects have carved the tortoise?s range into even smaller fragments. In addition to being barriers, roads have allowed once-remote habitat within reach of millions of city dwellers that can severely disrupt the habitat.
Other portions of habitat have been degraded by off-road vehicle use and livestock grazing, decreasing the availability of food plants for the tortoises. When food supplies drop, tortoises may not lay eggs that season, to conserve energy, therefore reducing their reproductive potential. Loss of vegetation cover due to livestock overgrazing can also leave young tortoises more exposed to predators, and grazing animals can crush tortoises and their burrows below them. Military activity in the desert, particularly training exercises that employ off-road maneuvers with tanks and artillery, are extremely destructive to the fragile desert landscape. Scars from training operations in 1942 and 1964 were still evident in aerial photographs taken in 1986 (Teller).
While encroaching on tortoise habitat, humans have also unintentionally encouraged the increase of a major tortoise predator. Garbage dumps and sewage ponds designed to serve the growing human populations attract large flocks of ravens, which, given the opportunity, prey upon young tortoises. The U.S. Fish and Wildlife Service (USFWS) estimates that raven populations in the Mojave Desert have increased over 1,500% between 1968 and 1990 (Teller). Biologists have observed ravens catching and carrying young tortoises and have noted the carcasses of young tortoises below raven nests.
Perhaps the most dangerous hazard to the desert tortoise is disease. Since 1988, the deadly upper respiratory disease syndrome ? marked by runny, mucous-filled nostrils and raspy breathing ? has struck tortoises throughout the western and central Mojave. Few, if any, infected tortoises survive this disease. The disease is highly contagious, in one area having spread to 40% of the population within a one-year period (Steinhart). This respiratory disease, of unknown origin and not yet fully understood, may lie dormant and appear in tortoises weakened by inadequate food or water by exposure to pollutants. Wild tortoises may have first caught the disease from sick, captive tortoises that were returned to the desert.
Due to evidence of steep declines in desert tortoise populations, and spurred by concern over the respiratory disease, the state of California designated the desert tortoise as a threatened species in 1989, followed by the federal government listing it as a threatened species in 1990. Efforts to protect the desert tortoise are underway on many fronts. In 1971, the U.S. Department of Interior Bureau of Land Management (BLM) declared 98 km? of tortoise habitat near California City ? the Desert Tortoise Natural Area (DTNA) ? off limits to mining, livestock, and vehicles. Since 1989, the BLM has also been developing a raven management plan to reduce raven predation upon tortoises. Under opposition, BLM Animal Damage Control Specialists and U.S. Department of Agricultures Animal Plant Health Inspection Service are currently shooting ravens known to prey on tortoises, or raven found within the DTNA (Steinhart). Since 1990, the agency has also suspended permits for three major off-road vehicle races in hopes that this will prevent racing vehicles from crushing more tortoises and their burrows, and further compacting the desert soil.
The desert tortoise is a hardy and unique creature of California?s Mojave Desert, surviving and adapting over millennia. The plight of the desert tortoise is hanging on the edge do mainly to human interference and what comes along with it. However, with the assistance of conservation groups and government, the desert tortoise may not see the brink of extinction anytime soon.