Even the hardy desert tortoise needs help as the Earth warms and habitat disappears
EDWARDS AIR FORCE BASE, Calif. — Joshua trees and creosote brush poke up through the rocky earth of the Mojave Desert on an early spring afternoon. Mount Baldy towers in the distance, its snowcapped peak contrasting with the low, dry conditions on Edwards Air Force Base, where a field team of ecologists and wildlife specialists are bringing a rare species to their future home.
The wind whips through the brush as the team starts its work. Members of the San Diego Zoo Wildlife Alliance wear green ballcaps and a team from the Living Desert Zoo and Gardens sport matching windbreakers. Their hands wrapped in blue latex gloves, they carry gray plastic bins from the trunk of an SUV. Each container holds about six of the 69 desert tortoises that came from the zoo. They make their way to the outdoor “head start” pens on the base.
“All of us are very proud,” said Emily Thomas, conservation biologist at the Living Desert Zoo and Gardens, based in Palm Desert. “It’s exciting seeing them move on to the next stage in their journey before their release in these bigger habitats.”
There are two desert tortoise species in the Southwest: the Sonoran, which is most common east and south of the Colorado River, and the Mojave, found west and north of the river. The team at Edwards is focused on bringing the Mojave tortoise back to its natural landscape.
One of the desert's most iconic denizens, the tortoise, has been in peril for decades, its numbers declining as its range shrinks due to habitat loss and fragmentation. Infectious disease, vehicle strikes and rising temperatures have also affected the species' struggle for survival. The Mojave species is listed as threatened under the Endangered Species Act.
The goal of the tortoise program is to give the species a fighting chance by providing a safe environment during the most vulnerable stage of life. Team members from both the wildlife alliance and the zoo have been caring for this group of young desert tortoise hatchlings for the past six months and will continue to monitor the tortoises until they are ready for a full release into the wild.
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How heat affects hardy desert species
Once common throughout the deserts of the Southwest, Mojave desert tortoise populations have declined by an estimated 90% in the last 20 years. The U.S. Fish and Wildlife Service’s evaluation of population trends from 2018 indicate that the species is on a path to extinction under current conditions.
Last year, the wildlife alliance tracked a group of gravid adult females at Edwards Air Force Base, about 90 miles north of Los Angeles. The females were taken to outdoor rearing pens, where a field team monitored them and conducted behavioral trials. Once the females laid their eggs, they were returned to the wild.
A typical clutch for a female tortoise contains four to eight eggs. The clutches were meant to hatch at the outdoor pens and then brought to the Living Desert Zoo and Gardens, where they would be able to grow more than twice the size that they would in the wild.
But after nests began to emerge during a dangerous heat wave last fall, biologists excavated the eggs and partial hatchlings and moved them to the zoo early.
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“Hatchlings were emerging from their nest in the heat wave,” said Melissa Merrick, associate director of recovery ecology for the wildlife alliance. “And that caused us a lot of concern and that is one of the reasons we moved them to the Living Desert one month ahead of schedule.”
Although desert tortoises are well adapted to high temperatures, much is not understood about the effects of extended extreme heat on hatchlings in the wild. Rising temperatures over the last few decades is cause for concern by experts because young tortoises already have a high mortality rate. Around 95% of desert tortoises in the wild do not survive the first five years, according to the U.S. Geological Survey.
Tortoise eggs stop developing when the outdoor temperature reaches around 96 degrees Fahrenheit. Their upper critical maximum capacity is 109 degrees. At that point cognitive function is impaired and death could follow. It was around that temperature when hatchings began to emerge from their nest last fall.
“Obviously tortoises experience temperatures that are that hot on a regular basis and they have the capacity to use shelter sites and burrow to mitigate,” Merrick said. “But hatchlings are much smaller, and thermoregulation is harder for them because they don’t have any sort of thermal inertia.”
Adult tortoises are more equipped to handle high temperatures, as their larger mass allows for them to release heat faster. Smaller and young tortoises have difficulty adapting to the extreme temperatures found in the Mojave Desert, and longer and hotter summers are making recovery more difficult to manage.
Adopting tortoises in Arizona
On the other side of the Colorado River, Arizona Game and Fish Department and the Arizona Tortoise Rescue work to find homes for the vulnerable and threatened species.
Each year the state agency adopts out hundreds of captive Sonoran desert tortoises that are surrendered to the department. The tortoises cannot be released back into the wild because captive tortoises can transmit diseases that could decimate wild populations.
“We take in captive desert tortoises from all over the state and try to find new homes for them,” said Tegan Wolf, desert tortoise adoption program coordinator “We have close to 200 right now.”
Wolf says most of the tortoises come from illegal breeding activities or from people who can no longer care for their tortoises. In the last 10 years, the program has grown as the public becomes more aware of adoption. Last year the program received more than 400 tortoises.
Many of the tortoises the program has received in recent years are juveniles since they are coming from illegal breeding. Wolf says it has been a challenge to adopt out the young because of the long-term commitment. The tortoises can live to be up to 80 years old.
“We have a lot of little ones who need home,” “It’s the opposite with dogs and cats, the adults are more popular than the babies,” Wolf said.
The remote quiet of an Air Force base offers a haven
The San Diego Wildlife Alliance Zoo has been involved in desert tortoise research for the last 13 years. The program has focused on jump-starting populations in decline. The time the hatchlings spend under the care of the team helps get them through their most vulnerable stage of life.
Hatchlings are typically no more than two and half inches in length, which makes them an easy target for predators like ravens and coyotes. But part of the process of monitoring young hatchlings is to understand how different habitat characteristics influence their survival and growth once they are released.
The shelter and vegetation at Edwards Air Force Base make it an ideal home for the species. The area’s remote surroundings give the reptile a better chance for survival. With few exceptions, the area is largely uninhabited, which limits human-caused disturbances like traffic, new housing, and illegal killings.
Tortoises also prefer the rocky substrate found in the area over a prominently sandy environment, partly because the rocky cover provides camouflage for young. Juvenile tortoises use their size to blend in with small rocks found in the desert to deter predators.
Merrick said the area is also desirable for tortoises because of mounds that are formed when sand is blown and captured by shrubs. When these mounds form, the soil underneath becomes more friable for small mammals, like desert woodrats or cottontails, to dig into, which creates pre-made burrows suitable for young tortoises.
“It has this cool little microhabitat, where the shrub presence promotes a little community of small mammals that use that area to make burrows,” she said. “The little tortoises are about that size and fit really well into those burrows.”
Hatchlings from the Living Desert will be kept in managed care for another six months at the head start facility. The outdoor pens are covered by a net top to prevent predation. Before they are released into the wild, tortoises will be fitted with a radio transmitter to allow the team to track their movements.
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Team will monitor the tortoises in the wild
Monitored care is wildly important for a species that has been threatened in recent in decades. And even fewer young tortoises have been found in their range over the last 20 years, largely due to habitat fragmentation.
Urban development pushing into the desert has not only destroyed potential habitat for tortoises but has pushed predators into new territory as well. New structures give a heightened advantage to birds of prey that now have ready-made perches to look for their next meal.
“As communities in the desert continue to grow and expand, there’s other infrastructure with that like powerlines, billboards, and roadkill,” Merrick said. “And all of that provides additional resources to predators like ravens and coyotes to allow them to persist at densities that are unnatural to what they were in the past before humans became so prominent on the landscape.”
Desert tortoises take nearly 20 years to reach sexual maturity, which is why intervening at a young age is so important.
“One thing we could do to get them past that most vulnerable stage is to raise the young up from a couple years to make sure they are a bit bigger, and their shells are harder,” Merrick said. “By selecting careful release sites, it might provide more cover and just another little boost.”
The team will also investigate how nest placement and the resulting temperatures will affect sex ratios. Desert tortoises have temperature-dependent sex determination: Cooler temperatures result in male clutches and warmer temperatures result in female clutches.
“They can make decisions on where they put their nest, how deep they make their nest, and how far deep into the burrow they place their nest,” Merrick said. “And that may be in response to the current temperatures they are experiencing.”
The team has placed temperature sensors on juvenile tortoises, sub-adult, and adult female tortoises to look at how they are using burrows to thermoregulate throughout the day as a function of temperature. Merrick says this will offer a glimpse into how different thermoregulation strategies may vary dependent on age and size.
At the base site, the team carefully places the young tortoises one-by-one into the head start pens, where they begin to acclimate to their new home for the next few months. The tortoises nibble on small plants sprouting from the dirt, acclimating to their new home without hesitation.
It is a bittersweet moment for the team members who raised these tortoises from birth, a moment similar to parents dropping off their child to freshman year of college. It is a new beginning for the young, who are now more prepared to face the harsh elements of the Mojave Desert.
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Jake Frederico covers environment issues for The Arizona Republic and azcentral. Send tips or questions to firstname.lastname@example.org.
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This article originally appeared on Arizona Republic: Wildlife teams help desert tortoise species amid warming, growth