Decline in Desert Tortoise Spells Bad News for the Mojave

The desert tortoise, a key link in the life cycle of desert critters throughout the Southwest, is being decimated by human activities, and its population has declined by as much as 50% during the last six years in the western Mojave Desert, a study by the U.S. Bureau of Land Management has found.

Steep tortoise population declines are also being observed in Arizona, Nevada and Utah, where malnutrition brought on by a loss of native plants to cattle grazing and other human incursions--including the theft and wanton shooting of the tortoises--are posing the greatest threat faced by the reptile in its millions of years of evolutionary history.

“We’re losing the species in the wild. In the Southwest, the tortoise is in trouble,” said Glenn Stewart, a zoology professor at Cal Poly Pomona who has studied the reptiles extensively and is chairman of the Desert Tortoise Council.

The declining number of tortoises has a ripple effect throughout the desert habitat. Tortoises provide food for the desert kit fox, coyote, bobcat, golden eagle and raven. The tortoise’s burrows become shelter for snakes, lizards and small rodents, which tunnel passageways off the tortoise burrows. Tortoise holes are a major nesting site for the burrowing owl.

But if the spiraling decline in the tortoise population continues, wildlife biologists fear that within 20 years the amicable tan-and-dark-gray-shelled herbivore will disappear from vast regions of the southwestern deserts.

“We know they’ve dropped over 90% in the last 50 years,” Bureau of Land Management wildlife biologist Kristin H. Berry said in an interview. “In the last seven or eight years, we’ve probably had a 50% drop in numbers and a retraction of their (western Mojave) habitat.”

In the 1920s, she said, there were 1,000 desert tortoises per square mile in a vast triangular region bounded by Ridgecrest, Palmdale and Barstow. Now there are sometimes as few as 20 to 50 per square mile. Even in protected areas like the Desert Tortoise Natural Area, about 20 miles north of Edwards Air Force Base, the numbers are no greater than 200 per square mile.

The precipitous decline in tortoises comes at a time when it is accorded little protection under the law. California designated the desert tortoise as a “protected species” in 1973. The classification makes it illegal for people to kill or own one. But, as Berry’s study indicated, it has not stopped the theft and carnage of tortoises. Nor does it require developers, for example, to take the tortoise’s habitat into consideration when planning desert construction.

Meanwhile, the state Fish and Game Department and the U.S. Fish and Wildlife Service have yet to act on recommendations to declare the tortoise a threatened or endangered species.

Used for Target Practice

Nowhere is the crisis more apparent than in the western Mojave, where the desert tortoise--the official state reptile--has been used for target practice, grabbed by poachers for food and pets, run over by off-road motor vehicles and attacked by growing numbers of natural predators like ravens. The ravens prey on baby tortoises whose fingernail-thin shells make them easy targets. The tortoises’ habitat has also been cultivated for crops, bombed by the military, made the site of a solar energy power plant and crisscrossed by roads that invite human access.

Berry told of a shocking discovery last spring when she returned to the Fremont Valley, located just north of the Desert Tortoise Natural Area, where a tortoise plot was studied in 1976. Until 1981, she reported 209 tortoises in the 39-square-mile area.

“We went back in last spring and found only 77 live ones. A large portion of the dead ones found were shot. . . . Some had their backs pounded in,” she said. She and other researchers filled three large boxes with dead tortoises. “Most of the ones found dead were adults, which is very serious.” It takes 15 years for a tortoise to reach reproductive maturity.

Berry said her findings are based on a survey of 14 of 27 tortoise study sites. At eight plots in the western Mojave, she said, population drops ranged from lows of 20% to 30% to highs of 70% over the last six to eight years.

“We have set up our study sites in relatively undisturbed areas. . . . When we look at these figures, we have to recognize that things are likely to be worse, and are probably worse, where there are concentrations of humans,” she said.

Sharp Population Decline

Indeed, sharp population declines have been recorded at two study sites within the Desert Tortoise Natural Area that an estimated 6,000 people visited last year. The preserve was designated as a natural area in 1980, although the BLM took initial steps in 1973 to protect at least part of the habitat from sheep grazing and later from off-road vehicles.

Berry said that inside a fenced area of the preserve near an interpretive center for visitors, the population declined by 34.1%, with young tortoises hardest hit. Of the deaths recorded there, one out of five were blamed on shootings, mutilation or vehicle kills.

“We were surprised to see so much decline inside the fence,” Berry said. District wildlife biologist Larry Foreman added, “The interpretive center brings a large number of visitors into that study area. Some take tortoises home.”

In the second study area, part of which lies just outside the preserve’s fence, the number of tortoises plummeted by 44.7%.

By contrast, in areas that are not readily accessible to humans, including the eastern Mojave and the interior reaches of the Desert Tortoise Natural Area, the population is doing far better. It increased to 158 tortoises from 136 over a six-year interval in one remote area of the preserve.

In Nevada, off-road vehicle races, expansion of subdivisions from Las Vegas, industrial projects and grazing are degrading and shrinking the habitat.

In Utah, a new BLM study by Tucson veterinarian James L. Jarchow found a “disturbing incidence” of malnutrition and starvation among desert tortoises because of the long-term decline of native perennial grasses such as bush muhly caused by cattle grazing over the last several decades.

Easy Prey

The grasses are a critical source of nutrition during midsummer, allowing female tortoises in particular to develop fat, protein and calcium necessary for producing eggs. The lack of nutrition led to thinning of the tortoise shells. “They became easy prey for predators,” Jarchow said. He noted that coyotes and badgers turned to eating the more vulnerable tortoises when the numbers of desert cottontail and black-tailed jack rabbits diminished.

He said that over the last 40 years, tortoises have declined in their size, especially females. Jarchow reported similar findings in northwestern Arizona, next to Beaver Dam.

“If we take care of desert lands to keep the tortoise alive, we’ll be taking care of most other species. We call it an indicator species. It can be an indicator of the well-being for many, many other animals,” she said.

That is precisely what is planned by the Ridgecrest-based Desert Tortoise Preserve Committee, which plans to announce today a $2.5-million fund-raising campaign to acquire 9.5 square miles of privately owned property within the existing Desert Tortoise Natural Area.

Funds raised by the committee will complement another $500,000 in the federal budget for BLM land acquisition. The California Department of Fish and Game has also helped acquire land.

The committee also plans to fund an education campaign to familiarize visitors to the desert with the importance of the tortoise and its vulnerability to human activity.

Said Berry, “The tortoise’s prospects in the Southwest with the growth of the human population is poor without management action and a commitment on the part of the public to do something for this animal. We are seeing some very strong management actions and efforts. But it will take more than the activities of one or two agencies. The public must be involved. And several segments of the public must change its behavior,” she said.