Desert Tortoise
© Dan Fillipi

Desert Tortoise

Threats

Human Disturbance

Portions of the desert southwest occupied by tortoises have been subject to a variety of human disturbance impacts dating back as far as the 1800s. Certain activities have resulted in significant tortoise habitat loss and degradation, thereby threatening the long-term survival of the species. 

These include the illegal collection of the species as pets, the spread of invasive plants and increase risk of wildfires, as well as tortoise injury and mortality associated with vandalism, livestock grazing, poaching, off-road vehicle use and incidents with automobiles.

Expanding development from urban centers, agriculture, the energy industry and military installations, have destroyed, degraded and fragment tortoise habitat and resulted in steep population declines. These population crashes are most notable near cities and suburban developments, agricultural and industrial developments such as in the Las Vegas Valley of Nevada, and the Antelope, El Mirage, Indian Wells, Stoddard and Victor Valleys in California. 

Even in remote areas, the development of highways, roads, railroads, canals, pipelines and electrical transmission systems have led to fragmentation of otherwise high-quality habitat. 

Sadly, even when attempts are made to relocate tortoises from planned areas of disturbance, the results have been discouraging. In fact, nearly half of the 158 tortoises relocated for the expansion of Fort Irwin in 2008, died within three years. 

Renewable Energy Development

An extensive number of solar and wind energy generation facilities, and supporting transmission infrastructure, have been developed throughout the range of Agassiz’s desert tortoise. Many of these developments have been located in important desert tortoise habitat that has displaced and necessitated the relocation of desert tortoises. Plans for future projects are only estimated to compound the potential for loss of life and the amount of habitat loss for these tortoises, with some estimates putting the overall loss at more than 37,000 acres.   

Climate Change

Climate change projections indicate that difficult times lie ahead for the desert tortoise. Increased temperatures, changes in seasonal weather patterns, decreased precipitation and more frequent droughts are likely to occur in much of this species’ range. 

An increased likelihood of drought, would diminish the availability of wildflowers and grasses, which are their primary food supply during the spring and fall seasons. Less vegetation and water combined with higher temperatures, would cause tortoises to spend more time in their underground burrows. Females may lay fewer eggs or none at all in severe drought years – as is common when resources are scarce – and since soil temperatures determine the hatchlings’ sex, prolonged high temperatures could skew the sex distribution for future generations. Temperatures above 88.7 degrees Fahrenheit during egg incubation favor development of female tortoises and soil temperatures above 95 degrees Fahrenheit, are lethal to the developing young.

The effects of climate change on the plant community and remaining habitat and population linkages remains to be seen. However, working now to protect Agassiz’s habitat and improve conservation land resiliency, are critical components of addressing the projected impacts of climate change.    

Invasive Plants and Wildfire

Surface disturbance facilitates the establishment, growth and spread of invasive plant species. These invasive plants can abnormally influence the frequency, extent and severity of wildfire in the Mojave Desert by providing continuous fuel that often ignites and burns easier than native species. Many of the invasive species are able to recover faster than native species after a fire and if wildfire occurs too frequently, native plants – especially those tortoises depend on for shade – may not be able to re-establish themselves. Mojave Desert wildfires have been increasing in frequency and extent in the last decade and climate change impacts may only worsen their threat to tortoises. 

Predation

Raven and coyote predation cause 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. Landfills, agricultural areas, roadways, fast-food restaurants and transmission lines have benefited these scavenger species by providing simple food, increased nesting areas and travel opportunities.

During times of drought, ravens and coyotes turn to desert tortoises as one of their primary sources of food. Ravens are especially adept at preying on tortoise hatchlings and small adults.

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