Threats to Desert Tortoises
Because it is so well adapted to and dependent on its habitat for survival, the desert tortoise is very sensitive to any changes in it. As a result, certain human activities have caused significant changes in the nature and condition of desert tortoise habitat in the Mojave Desert region. These changes have resulted in the elimination or steep declines of many desert tortoise populations, such as in the Western Mojave Desert in California where overall declines now approach 90 percent over just the past 40 years. Such losses are common near cities and urban developments, agricultural and industrial developments such as in the Las Vegas valley of Nevada, and the Antelope, Victor and Indian Wells Valleys in California. In more remote areas of the Mojave, development of highways, roads, railroads, canals, pipelines and electricity transmission systems have resulted in fragmentation of otherwise high-quality habitat, leading to isolated populations of desert tortoises and increased mortality from vehicle use and entrapment. Poaching, livestock grazing, excessive use of off-highway vehicles within tortoise habitat and crushing by automobiles continue to threaten tortoise populations.
Climate change projections indicate that difficult times lay ahead for the desert tortoise as increased temperatures and decreased precipitation and soil moisture expand over the region. Like all reptiles, tortoises have minimal ability to self-regulate their internal temperature, and thus must retreat to the shade of vegetation or their burrows once the air temperature climbs above about 90 degrees Fahrenheit, in order to keep their body temperatures out of the danger zone. Drought also decreases the amount of wildflowers and grasses available to desert tortoises, which is their primary food supply during the spring and fall seasons. During drought, tortoises spend more time in their underground burrows to minimize water loss and conserve energy.
Females tortoises also lay fewer eggs, or none at all, during severe drought years. Furthermore, the sex of tortoises is determined by the soil temperature where adult females lay their eggs. Temperatures above 88.7 degrees Fahrenheit during egg incubation favor development of female tortoises, so prolonged high temperatures could skew the sex distribution of future generations. Extremely high soil temperatures (above 95 degrees) are lethal to the developing young.
Raven and coyote predation causes significant losses of tortoises in many areas of the Mojave Desert where these naturally occurring predators have increased in numbers, especially near cities, towns, campgrounds and day-use recreational facilities. During times of drought, ravens and coyotes turn to desert tortoises as one of their primary sources of food. Ravens are especially adept in preying on hatchling and other small-sized desert tortoises.