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Mojave Desert Tortoise
Threats To Desert Tortoises |
| When evaluating a species for listing, the U. S. Fish and Wildlife Service (Service) considers five factors: 1) damage to, or destruction of, a species' habitat; 2) overutilization of the species for commercial, recreational, scientific, or educational purposes; 3) disease or predation; 4) inadequacy of existing protection; and 5) other natural or manmade factors that affect the continued existence of the species. When one or more of these factors imperils the survival of a species, the Service takes action to protect it. The Service is required to base its listing decisions on the best scientific information available. The following is an overview of the threats to the desert tortoise and its habitat by the five listing factors used to determine the desert tortoises' protection under the Endangered Species Act. |
Damage to, or Destruction of, the Desert Tortoise's Habitat |
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Since the 1800s, portions of the desert southwest occupied by desert tortoises have been subject to a variety of impacts that cause habitat loss, habitat fragmentation, and habitat degradation, threatening the long-term survival of the desert tortoise. 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 roads and highways, off-highway vehicle activity, poor grazing management, and habitat invasion by non-native invasive species. |
| Wildfire |
| A 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 the desert tortoise's habitat and available food. |
Landfills/Waste Disposal
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 such as ravens.
Development of Renewable Energy Sources
Applications for solar and wind energy facilities in California and Nevada total over 600,000 acres because of recent public emphasis on advancing alternative energy sources. Habitat loss through renewable energy development is a potential threat to the desert tortoise where many of these projects have been proposed. The Bureau of Land Management has committed to excluding these projects from designated critical habitat for the desert tortoise and Desert Wildlife Management Areas. However, potential long-term effects of large-scale energy development fragmenting or isolating desert tortoise conservation areas and cutting off gene flow between these areas have not been evaluated. |
| Overutilization of the desert tortoise for Commercial, Recreational, Scientific, or Educational Purposes |
Despite legal protection under federal and state laws, deliberate maiming or killing of tortoises and collection of desert tortoises by humans for food or as pets have been identified as potential threats to the species. |
| Disease or Predation |
To date, the available evidence indicates that upper respiratory tract disease, 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.
Contaminants
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. It has been suggested 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.
Predators
Desert tortoises, particularly hatchlings and juveniles, are preyed upon by several native species of mammals, reptiles, and birds. The common raven ( Corvus corax ) has been the most highly visible predator of small tortoises, while coyotes ( Canis latrans ) have been commonly implicated in deaths of adult tortoises. The population-level effects of these or other predators are unknown. Other avian predators of the desert tortoise include red-tailed hawks ( Buteo jamaicensis ), golden eagles ( Aquila chrysaetos ), loggerhead shrikes ( Lanius ludovicianus ), American kestrels ( Falco sparvarius ), burrowing owls ( Athene cunicularia ), and greater roadrunners ( Geococcyx californianus ). Coyotes, kit foxes ( Vulpes macrotis ), mountain lions ( Felis concolor ), ground squirrels ( Spermophilus spp.), and free-roaming dogs are some of the known mammalian predators. Invertebrate predators of eggs and hatchling tortoises include native fire ants. |
| Inadequacy of Existing Regulatory Mechanisms |
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The final listing rule acknowledged that 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. |
| Other Natural or Manmade Factors Affecting its Continued Existence of the Desert Tortoise |
| Climate Change
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.
Intergovernmental Panel on Climate Change models have predicted that precipitation will likely decrease by 5 to 15 percent annually in the region, 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 because of increasing temperatures and decreasing precipitation in winter.
Further predictions need to be developed specifically for the desert tortoise to help inform recovery efforts.
Non-motorized Recreation
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 maiming or killing of tortoises, increased raven predation, degradation of vegetation, and soil compaction.
Released Captive/Pet Tortoises
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 has been observed in tortoises removed from areas where previous releases of captive animals had occurred. |
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