Nevada Fish & Wildlife Office
Pacific Southwest Region

Mojave Desert Tortoise

Threats To Desert Tortoises

In determining whether to list, delist, or reclassify (change from endangered to threatened status, or vice versa) a species under the Endangered Species Act, the U.S. Fish and Wildlife Service evaluate the role of five factors potentially affecting the species. These factors are: 1) destruction or modification of its habitat; 2) overutilization for commercial, recreational, scientific, or educational purposes; 3) disease or predation; 4) inadequacy of existing regulatory mechanisms; and 5) other natural or manmade factors. The following is an overview of the threats to the Mojave desert tortoise and its habitat by the five listing factors used to determine the desert tortoises' protection under the Endangered Species Act.
 

1. The Present or Threatened Destruction, Modification, or Curtailment of Its Habitat or Range

Since the 1800s, portions of the desert southwest occupied by desert tortoises have been subject to a variety of impacts that cause habitat loss, fragmentation, and degradation, thereby threatening the long-term survival of the species. Some of the most apparent threats are those that result in mortality and permanent habitat loss across large areas, such as urbanization, and those that fragment and degrade habitats, such as proliferation of roads and highways, off-highway vehicle activity, poor grazing management, and habitat invasion by non-native invasive species. Indirect impacts to desert tortoise populations and habitat are also known to occur in areas that interface with intense human activity. Photo: Highway through desert tortoise habitat in Mojave National Preserve

Another threat that has come to the forefront is the increased frequency of wildfire due to the invasion of desert habitats by non-native plant species. Changes in plant communities caused by non-native plants and recurrent fire can negatively affect the desert tortoise by altering habitat structure and species available as food plants.

Fire threatsOff-highway vehicle activity, roads, livestock grazing, agricultural uses, and other activities contribute to the spread of non-native species (or the displacement of native species) and the direct loss and degradation of habitats. For example, unmanaged livestock grazing, especially where plants are not adapted to large herbivorous mammals or where the non-native species are less palatable than the natives, can preferentially remove native vegetation, leaving non-native plants to grow under reduced competition. Photo: 2005 wildfire in desert tortoise habitat in southern Nevada

Landfills and other waste disposal facilities potentially affect desert tortoises and their habitat through fragmentation and permanent loss of habitat, spread of garbage, introduction of toxic chemicals, increased road kill of tortoises on access roads, and increased predator populations. Military operations (e.g., construction and operation of bases, field maneuvers) have taken place in the Mojave Desert since 1859 and can affect tortoises and their habitats similarly to other large human settlements (i.e., illegal collection of tortoises, trash dumping, increased raven populations, domestic predators, off-highway vehicle use, increased exposure to disease, and increased mortality).

Solar PanelsConflicts between energy development projects, which can range size in the thousands of acres, and the desert tortoise have been recognized at least since 1986. Desert tortoises may be killed during exploration, construction and ongoing operations, and maintenance activities associated with energy facilities and transmission corridors. Project sites are typically contoured and fenced, resulting in direct mortality and habitat loss. Ground-disturbing activities that may cause negative impacts to the desert tortoise can increase soil erosion, increase establishment of invasive plant species, reduce cryptobiotic soil crusts (biological crusts composed of living bacteria, algae, fungi, lichens, and/or mosses that stabilize the soil), and alter drainage patterns that impact plant communities downstream from the project footprint. Photo: Photovoltaic solar array in desert tortoise habitat in southern Nevada

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2. Overutilization for Commercial, Recreational, Scientific, or Educational Purposes

Data and anecdotal observations indicate that collection for personal or commercial purposes was significant in the past. Even though Sonoran desert tortoises are protected in Arizona in addition to Mojave desert tortoises, the probability of collection of Sonoran desert tortoises by motorists was greatest on maintained gravel roads compared to non-maintained gravel and paved roads. While illegal collection of desert tortoises still occurs and collection could possibly impact local populations, there is no quantitative estimate of the magnitude of this threat.

Research projects may result in injury or loss of individuals. These activities may be permitted under section 10 of the Endangered Species Act, but terms and conditions to minimize injury and mortality of individuals are mandatory.

 
3. Disease or Predation

To date, the available evidence indicates that upper respiratory tract disease, as caused by the bacteria Mycoplasma agassizii and M. testudineum, is probably the most important infectious disease affecting desert tortoises. Less is known about other diseases that have been identified in the desert tortoise (e.g., herpesvirus, cutaneous dyskeratosis, shell necrosis, bacterial and fungal infections, and urolithiasis or bladder stones). There is evidence that any one disease may predispose an animal to other diseases. However, it is not known whether this is a cause or effect. Additional research is needed to clarify the role of disease in desert tortoise population dynamics relative to other threats.

The role of environmental contaminants in directly inducing toxicosis-related diseases (i.e., liver diseases) and increasing susceptibility to infectious diseases has recently been suggested as a significant source of mortality. Elevated mercury and arsenic levels have been associated with diseased tortoises in the wild. Necropsy and analyses of kidney, liver, and scute tissues suggested that tortoises from California with a variety of diseases (upper respiratory tract infection, urolithiasis, metabolic disease, and shell diseases) had statistically significantly higher levels of several potentially toxic elements compared to healthy tortoises. Illegal dumping of hazardous wastes in the desert may expose tortoises to increased levels and possible consumption of toxic substances and affect populations on a localized level where these activities are concentrated. It has been postulated that elemental toxicity may compromise the immune system of desert tortoises or otherwise detrimentally affect physiological function, rendering them more susceptible to disease, but further investigation is needed.Ravens

Desert tortoises, particularly hatchlings and juveniles, are preyed upon by several native species of mammals, reptiles, and birds. The common raven has been the most highly visible predator of small tortoises, while coyotes have been commonly implicated in deaths of adult tortoises. Except for extreme predation events brought on by unusual circumstances, predation by native predators alone would not be expected to cause dramatic population declines. This highlights the importance of combined and synergistic effects of threats. For example, predation pressure by ravens is increased through elevated raven populations as a result of resource subsidies associated with human activities. Ravens obtain food in the form of organic garbage from landfills and trash containers, water from sewage ponds and municipal areas, and nesting substrates on billboards, utility towers, bridges, and buildings. Photo: Common ravens picking through trash at a highway rest area

Other avian predators of the desert tortoise include red-tailed hawks, golden eagles, loggerhead shrikes, American kestrels, burrowing owls, and greater roadrunners. Coyotes, kit foxes, mountain lions, badgers, ground squirrels, and free-roaming dogs are some of the known mammalian predators. Invertebrate predators of eggs and hatchling tortoises include native fire ants.

 
4. Inadequacy of Existing Regulatory Mechanisms

All four states within the range of the Mojave population of the desert tortoise have laws in place to protect the species. In addition, a great deal of effort has been dedicated to planning by the various land management agencies whose jurisdictions include desert tortoise habitat. Many of the existing plans include language specific to protection of the species, such as limiting off-highway vehicle use and competitive/organized events, grazing, vegetation harvest, and collection of desert tortoises. However, the multiple-use mandates under which the agencies function require a complex balance between conservation and use of public lands, and management agencies frequently do not have sufficient funding to enforce their regulations. Also, state law in Arizona, Nevada, and Utah does not regulate habitat degradation, making mitigation of impacts to potentially unoccupied but suitable habitat difficult.

 
5. Other Natural or Manmade Factors Affecting its Continued Existence

Global climate change and drought are potentially important long-term considerations with respect to recovery of the desert tortoise. There is now sufficient evidence that recent climatic changes have affected a broad range of organisms with diverse geographical distributions. While little is known regarding specific direct effects of climate change on the desert tortoise or its habitat, predictions can be made about how global and regional precipitation regimes may be altered and about the consequences of these changes. Generally, climate change predictions for the geographic range of the Mojave desert tortoise suggest a 3.5 to 4.0 degree Celsius (6.3 to 7.2 degree Fahrenheit) increase in annual mean temperature, with the greatest increases occurring in summer (June-July-August mean up to 5 degrees Celsius [9 degrees Fahrenheit] increase). Precipitation will likely decrease by 5 to 15 percent annually within the range of the desert tortoise with winter precipitation decreasing up to 20 percent. Because germination of the tortoise’s food plants is highly dependent on cool season rains, the forage base could be reduced due to increasing temperatures and decreasing precipitation in winter. Elevated carbon dioxide and altered precipitation regimes may facilitate invasive plant species, thereby increasing fire frequency. Effects of altered nitrogen dynamics on the Mojave Desert are less clear; for example, increased nitrogen deposition from dust in the vicinity of metropolitan areas could result in higher plant production, exacerbating the effects from carbon dioxide noted above. Alternatively, increased temperatures may release nitrogen gases from Mojave Desert soils, reducing fertility of those soils and the ability to support plant life. Further predictions need to be developed specifically for the desert tortoise to help inform recovery efforts.

Other activities that may impact the species include non-motorized recreation such as camping, hunting, target shooting, rock collecting, hiking, horseback riding, biking, and sight-seeing. These activities bring with them threats associated with increased human presence, such as loss of habitat from development of recreational facilities, handling and disturbance of tortoises, increased road kill and deliberate killing of tortoises, increased raven predation, degradation of vegetation, and soil compaction. Desert habitats are also disturbed by construction and maintenance of linear utility corridors and ancillary facilities and to some degree by vandalism and harvest of vegetation for personal or economic purposes.

Another potential threat facing the desert tortoise is the unauthorized release or escape of pet tortoises to the wild. Captive releases have the potential to introduce disease into wild populations of desert tortoises. The highest prevalence of clinical signs of upper respiratory tract disease in one study was observed in tortoises removed from areas where previous releases of captive animals had occurred.

 
Last updated: April 16, 2014