Desert Tortoise Recovery Office

Threats To Desert Tortoises

In 1989, information on high mortality rates and the presence of an upper respiratory tract disease in populations of the desert tortoise resulted in a temporary emergency listing as endangered. This means the desert tortoise was placed on the Federal Endangered Species Act List of Threatened & Endangered Species while scientists conducted further review to determine if the species needed long-term protection. In 1990, scientists determined that other threats existed such as losing habitat to development, the deteriorating quality of its habitat, and ravens killing young tortoises. This determination resulted in listing the desert tortoise as threatened.

Many of the threats attributed to the decline in tortoise populations continue today. Scientists are still learning about the precise relationship of the different threats to actual population decline. Based on more than 40 years of data, we know that tortoises are directly & indirectly impacted by natural as well as human-caused activities. These threats include disease, predation, expanding development, off-highway vehicles, invasion of non-native grasses and weeds, fire, collection, poachers, sheep & cattle grazing, mining, and drought. At this point, there is not one threat that seems to impact tortoises more than another. It is, rather, an accumulation of threats that are taking a toll. Drought, disease, predation, mining, grazing, and off-highway vehicles all impact tortoises. We'll look at several of these in detail. Additional information can be found in the Desert Tortoise Recovery Plan. A recent report by the U.S. Geological Survey also provides an overview and evaluation of the knowledge of the major threats to the persistence and recovery of desert tortoise populations.

Disease

Multiple diseases have been identified across the range of the desert tortoise, including bacterial respiratory disease, viral disease, and shell diseases. Scientists believe that the disease-related mortality may be a result of multiple factors including drought, poor nutrition, environmental toxicants, or habitat degradation including exotic plant invasion and fire. These factors, in combination, may cause disease and/or mortality.

There are several types of known diseases that affect tortoises, including the following:

Mycoplasmosis - an Upper Respiratory Tract Disease (URTD) that was recognized in captive tortoises before it was observed in wild tortoises. Wild tortoises with clinical signs of URTD were first seen on the Beaver Dam Slope, Utah, in the 1970's. It was first observed in California in 1988. At least two bacteria, Mycoplasma agassizii and Mycoplasma cheloniae are known to cause URTD. Both are transmitted through direct contact between tortoises.

Herpesvirus - has been reported in captive tortoises and in some wild tortoises in California. Little is known about the effects of this disease on wild tortoise populations.

Shell Disease - seen in most Arizona populations, it is common in tortoises in the eastern Mojave and Colorado deserts of California. It is found in the West Mojave populations, but to a less extent. It occurs in all sizes and ages of tortoise. The cause(s) have not been determined, and its direct effects on individual tortoises or populations are still unclear.

Disease research continues in many areas, including whether Mycoplasma cheloniae causes URTD in desert tortoises, how URTD spreads through the populations, the relationship between nutrition.

Roads & Off Road Access

Roads provide access to all regions of the desert and we know that desert tortoises have been killed and injured by vehicles using paved and dirt roads and trails. We do not know, however, how many have been killed or injured by road use because of the vast area of the desert.

Roads also facilitate and accelerate the spread of non-native plants. The Smithsonian Institution recently concluded that serious disruptions of the desert habitat are not only damaging the tortoise’s food source, but increasing the amount of non-native plants in the area. These plants are displacing native vegetation and have lower nutritional value, which may be a contributing factor in ill and malnourished tortoises.

These non-native plants also assist the spread of fire. Fire kills both desert tortoises and the shrubs they use as shelter, and it opens the environment to additional invasive plans. This sets the stage for this destructive cycle to repeat.

An additional unfortunate issue with regard to access is that some people continue to shoot desert tortoises, collect them as pets, and poach them for food.

The recovery plan specifically recommends that all competitive and organized off-road vehicle events be prohibited in critical habitat, or desert wildlife management areas. The Service and BLM have allowed events to proceed on roads through critical habitat during times of the year when desert tortoises are inactive.

Fencing is also helping protect tortoises in both the Sonoran and Mojave deserts from being killed on many roads. Annual road mortality of desert tortoises has decreased by 75 percent or more since Highway Departments began installing barrier fencing.

Predation

Free-ranging dogs have been reported to cause localized problems of killing or maiming tortoises, especially in parts of the Western Mojave Desert, but ravens have had more wide ranging impacts. Raven populations have increased in the Western US, largely as a result of abundant food supplies provided by the growing human population. Increased raven populations in the desert have resulted in artificially high predation rates on small tortoises. This problem is being addressed in Nevada through raven removal projects implemented by USDA Wildlife Services with assistance from the State of Nevada, Clark County, the Service, and and other land managers. In California, an impact assessment is being developed under the guidance of the Desert Managers Group.

Urbanization

	DTRO Staff			Land Management For Desert Tortoise
	Desert Tortoise Recovery Planning			How You Can Help The Desert Tortoise
	Science Advisory Committee			Announcements
	DesertTortoise.gov			Meeting Summaries
	Desert Tortoise Habitat & Life History			Reports
	Threats to the Desert Tortoise