Invasive grasses choking desert tortoise

The U.S. Fish and Wildlife Service and several partners, including U.S. Geological Survey, the Bureau of Land Management, and the National Park Service, are working to defend the tortoise and its Mojave Desert home from invasive grasses. Credit: USFWS

Desert tortoises are dying at alarming rates, primarily due to invasive species of grass, especially red brome and cheatgrass. Foreign to the Mojave Desert, these grasses are not as nutritious to tortoises as their normal forage.

Pacific Southwest Region External Affairs
July 27, 2017

The desert tortoise is fighting an aggressive invader. The invader’s name is Bromus, commonly known as red brome and cheatgrass, and they threaten to consume tortoise habitat in Southern California, Nevada and parts of Arizona and Utah, often at the expense of desert tortoise survival.

The U.S. Fish and Wildlife Service and several partners, including U.S. Geological Survey, the Bureau of Land Management, and the National Park Service, are working to defend the tortoise and its Mojave Desert home with a multi-pronged counter-attack.

The desert tortoise, the state reptile for California and Nevada, can live up to 70 years in the wild and spends most of that time burrowing in holes near mountain slopes. Credit: USFWS

The tortoise and the desert

The desert tortoise, the state reptile for California and Nevada, likes to spend its time burrowing in holes near mountain slopes. In the summer months, temperatures can easily reach a blistering 120°F in the Mojave Desert. Seeking shelter from the heat, tortoises burrow in the ground or seek refuge in the shade of shrubs. In the winter, temperatures can reach below freezing, and tortoises snuggle into burrows once again to stay warm and conserve energy when less food is available. Other species also rely upon the desert tortoise and its burrowing skills for shelter to escape the harsh climate.

The tortoise and desert habitat: In the summer months, temperatures in the Mojave Desert can easily reach a blistering 120°F. Seeking shelter from the heat, tortoises burrow in the ground or seek refuge in the shade of shrubs. Above, Service partners begin spraying cheatgrass. Credit: USFWS

In the spring and fall, desert tortoises can be found roaming the 25,000-square-mile Mojave Desert, looking for mates and the food and water needed for survival. Native forbs that only sprout during a narrow window in the spring provide most of their food. They rely on water stored in their bladders to prevent dehydration. This water storage allows them to go without a drink for a year. Finally, the tortoise’s exterior shell offers protection from predators. Adaptations like these allow desert tortoises to live up to 70 years in the wild. So, why has the population in the Mojave Desert dropped 90 percent in some areas since the 1980s?

Threats to the desert tortoise

Desert tortoises are dying at alarming rates, primarily due to invasive species of grass, especially red brome and cheatgrass (Bromus rubens and Bromus tectorum). Foreign to the Mojave Desert, these grasses are not as nutritious to tortoises as their normal forage.

Roy Averill-Murray, desert tortoise recovery coordinator for the U.S. Fish and Wildlife Service, warns that red brome is dangerous for the desert tortoise. Their mouths "could become infected, then they can have difficulty eating anything else and it could cause severe health issues,” he says.  Credit: USFWS

“The red brome in particular has these very spiky awns on the seedheads and those get lodged inside the tortoises’ mouths,” says Roy Averill-Murray, desert tortoise recovery coordinator for the U.S. Fish and Wildlife Service. “The tortoises will eat them especially if they don't have a lot of other choices. [Their mouths] could become infected and then they can have difficulty eating anything else and it could cause severe health issues.”

Research by U.S. Geological Survey scientists showed that juvenile tortoises that had to rely on red brome as forage grew less and declined in health compared to juveniles that had native plants to eat.

To make matters worse, the invasive grass dries quickly as spring temperatures warm and the grass forms dense undergrowth that burns easily by lightning strikes or cigarette embers. Many tortoises caught on the surface during a fire are killed, while those in burrows also may succumb to smoke inhalation. Wildfires are increasing in the Mojave Desert over the past two decades as a result of the invasion. More than 400,000 acres of tortoise habitat burned in 2005 alone.

Research by U.S. Geological Survey scientists showed that juvenile tortoises that had to consume red brome (shown here) as forage, grow slower and are less healthy compared to juveniles that had native plants to eat. Credit: USFWS 10:22 AM

After a fire, Bromus has an advantage by producing many seeds that sprout earlier than native Mojave Desert plants, outcompeting them before the natives even have a chance to recover.

Habitat restoration

In addition to supporting research to understand how invasive species affect desert tortoises, the Service is also working with other agencies to save the species by improving its habitat. The habitat restoration process is a two-pronged approach that includes herbicides and out-planting.

“We are looking to do some experimental restoration treatments that will help us learn how to restore desert tortoise habitats that are burned,” says Sara Scoles-Sciulla, plant ecologist at the U.S. Geological Survey.

Federal non-governmental agency partners begin treatment of desert tortoise habitat near Coyote Springs, Nevada. Credit: USFWS

The first prong involves knocking back the grasses. With two treatment types, the herbicides are designed to protect against the invasive species of grass. First, a pre-emergent herbicide targets the grass before it has an opportunity to germinate from a seedling. Second, a post-emergent herbicide targets the plant once it has sprouted. The specialized treatments are designed to cease the reproduction of additional foreign grass and allow the ecosystem a window for native plants to grow back without the competition.

Team members gather near Coyote Springs, Nevada,  for the first step in knocking back the invasive grasses, a process that requires treatment with herbicides designed to protect against the specific invasive species of grass. Credit: USFWS

“[The herbicide is] non-selective,” says Maura Schumacher, biological science technician at the National Park Service. “Wind and rain will cause this herbicide to get to the non-target areas and kill non-target native desirable species. We're very careful with where the herbicide goes, when we're spraying it, what time of day … so we can preserve the native vegetation.”

However, since so much of the Mojave Desert has already burned, there are not enough native seeds left to take advantage of this window. This requires the scientists to apply additional remedies in the second prong of the restoration strategy. Scientists administer treatments like seeding and out-planting greenhouse-grown native species. The scientists also rely on desert rodents to help the process along by fertilizing and dropping the seeds all over the Mojave.

Kelly Mathis, a biological technician with the National Park Service, carefully measures herbicide into a small measuring cup. A little amount of herbicide goes a long way in the application process. Credit: USFWS

“Once [native shrubs] are back on the landscape, we think they will provide the shade, and they’ll provide the structure for tortoise burrows that are necessary to keep this endangered species going out here on the landscape,” says Jonathan Smith, restoration project manager at the Bureau of Land Management.

Additional ways to help

There are ways the public can help the desert tortoise. First, be extremely careful with sparks or fire in the desert to avoid causing a wildfire that can quickly get out of hand. If you see a desert tortoise, the most important thing you can do is not touch, harass or pick it up unless it immediately needs to be moved out of harm’s way.

Brent Sparks, with Great Basin Institue, prepares to search for the signal of a nearby desert tortoise. Credit: USFWS


While they might look friendly, approaching a tortoise might scare it and cause the reptile to empty its bladder, losing the liquid it needs to stay hydrated in its harsh climate.

Another way to help the tortoise is to be cautious when traveling through its habitat: check under parked cars prior to ignition as tortoises might seek shade under cars; stay on marked paths while hiking to avoid disturbing its burrows, food or shelter; and drive slowly on designated routes to avoid crushing the tortoise.

In addition, desert tortoises have charismatic personalities which can entice people to pluck them from their natural habitat. Roy Averill-Murray advises people against taking tortoises they find home with them.

“It is really important that [people] do not collect tortoises from the wild; the population is too low and can’t sustain that,” he says.

Likewise, do not release pet tortoises because they can spread disease to wild tortoises.

The public can help the Service and its partners save the desert tortoise. Human activity caused the species population to decline, and human activity can bring it back.

Ways to help the tortoise include being cautious when traveling through its habitat: check under parked cars prior to ignition as tortoises might seek shade under cars; stay on marked paths while hiking to avoid disturbing its burrows, food or shelter; and drive slowly on designated routes to avoid crushing the tortoise. Credit: USFWS