The desert tortoise is widely distributed throughout major portions of the Mojave and Sonora deserts of California, Nevada, Utah, Arizona, and of Sonora and Sinaloa, Mexico.

Different genetic, body plan and behavior features within the desert tortoise species have resulted in the naming of two major populations; the "Sonora population" and Mojave population". It is the Mojave population that is listed as "threatened" by state and federal agencies. California listed the tortoise as threatened in 1989, and the US included it in 1990. Historically, the desert tortoise served as a source of food, medicine and useful material for Native Americans. Later, tortoises were eaten by settlers and traders. Before the early 1950's, many populations reached densities of several hundred tortoises per square mile.(Tortoise population is figured out by density measurements: the number of animals found divided by their land range.)

The Mojave population lives mostly in 6 regions, and the numbers of tortoises in each area (density) vary within the regions. For example, in 1984 the density of tortoises in one area ranged from 0 to 250 animals for every square mile. At one Mojave population site in California, there was a 76% decline between 1979 and 1992. Today, most populations contain no more than five to fifty tortoises per square mile. It is difficult to track tortoise populations, because the tortoises borrow underground in cold and hot seasons, and can be difficult to find.

Exact numbers on total population are difficult to calculate, but these population densities have been generally decreasing.

The populations have been decreasing due to several reasons:

Direct tortoise threats:
Collisions with vehicles
Illegal collection
Disease (mostly from pets released into the wild)
Droughts
Fires
Predators

Indirect Threats:
Habitat loss from construction and farming
Invasion of non-native plants into tortoise habitats Pollution of the atmosphere
Landfills and illegal waste dumping
Global warming

Actions have been taken to provide tortoise protection areas within their natural habitats, but it is difficult to quickly measure the effects of the protection partly because tortoises have long lifespans (80-100 years!) and take 10 years or more to reach sexual maturity. Actions include building fencing along roads and highways to keep tortoises from being hit by vehicles, and restricting human access to protected lands. It is a conservation goal that population densities do not fall below 10 tortoises per square mile, or 50,000 adults total, so that the species will not go extinct.

Interesting fact: Tortoise dens can be used by many generations, and some dens in southern Utah are estimated to be 5000 years old!