Fences built to protect threatened desert tortoises may be killing some

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In a twist of tragic irony, roadside fences designed to protect desert tortoises may be leading some to their death, according to new research from UC Davis. 

The fences are designed to keep the threatened tortoises from getting hit by cars. But in a new study, Davis researchers found that when tortoises in the Mojave Desert encountered the fences, they often paced back and forth alongside the barriers for hours in the blazing sun.

This led to their body temperatures rising to dangerously high and even lethal levels. 

“When they’re confronted with a new barrier, they’re actually very stubborn and persistent animals,” said Brian Todd, a wildlife biologist who co-authored the new paper published in Biological Conservation. “And they will pace back and forth along that barrier trying to get back to a burrow or another part of their home that they know has been there for the last 20 years.”

Red dots show movements of a female desert tortoise, outfitted with a GPS tracker and temperature sensor, along a fence beside a California agricultural inspection station on I-15 over a 15-month period. The tortoise overheated and died. Courtesy J. Mark Peaden, UC Davis.

The study has implications for how to best protect the desert tortoise, whose population has declined by 90 percent since mid-century. It is now listed as "threatened" on the federal Endangered Species List due to car strikes, predation by ravens and coyotes and loss of habitat from residential and solar development.

The fences are required components of new road construction in tortoise habitat, but they don't appear to be the best way to help, Todd said. At least not wire or mesh fences.

“When the animals can see through the fence, they don’t realize why they can’t get to their favorite habitat or the burrow they used to use,” he said.

 

A desert tortoise paces along a fence in the Mojave Desert. Courtesy J. Mark Peaden, UC Davis.

This is not a new finding. In 1982, the proceedings of the Desert Tortoise Council included a UC Santa Cruz study showing that tortoises spend more time walking along open-mesh fences than along solid fences. 

“The solid metal fence presented a complete visual barrier,” the study found. “Tortoises could not see through it and so could not have recognized it as something to go through. The chicken-wire fence, at the other extreme, presented a clear view to open desert. That probably served as a stimulus for the tortoises to go directly toward the fence in an attempt to reach that desert habitat. Then the fence presented a physical barrier to passage."

But solid fences aren’t as commonly used because they are more expensive than chain link fences, said wildlife biologist Jerry Roe. He founded the fencing company Animex, which builds fences that reduce pacing and shade animals while they walk along them.

“[Chain link] is quick and easy, and [companies] are familiar with the product, but when you look at it, it just doesn’t work,” he said.

Still, there may be ways to make fencing safer for tortoises, even if it is of the see-through variety. Todd found that tortoises are most likely to try to cross roads in spring and late summer (much of the rest of the year, they hunker down in burrows). During these times, he said fencing could be removed and speed limits could be lowered to encourage drivers to take it slow to avoid hitting tortoises on the road.

Fences could also be used to funnel tortoises towards arroyos and dry washes, where culverts could be built to help tortoises cross safely beneath roads. Or temporary burrows or shade structures could be built alongside fences.

Tortoises, which can live to be 50 years old, spend most of their time underground in burrows and can carry 40 percent of their weight in water in their bladders -- adaptations that allow them to survive  extreme desert conditions. 

Improving fences is important, Todd said, because losing even one tortoise can have an impact on the dwindling population. 

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