Avoiding death's highway

Danger on the highway: Desert tortoise researchers want to help the animals use culverts to get from one side of the highway to the other, especially where the tunnels connect their habitat. A joint 2015  Bureau of Land Management and U.S. Fish and Wildlife Service research effort  found that Mojave Desert tortoises use culverts for shelter and safe passage underneath the roadways that stand between them and additional habitat, mates and food. Credit: Brad Sutton/National Park Service

How does a desert tortoise cross the highway? It can use a 'wildlife underpass'

By Dan Balduini
May 22, 2018

Motorists traveling along highways in southern Nevada might not even see them. Those that do notice them probably don’t give them a second thought—unless they are driving through a rare downpour and can see water rushing through them.

However, for Mojave Desert tortoises and other wildlife, they are life savers.

“Road mortality is one of the highest-priority threats to address across the range of the desert tortoise. Therefore, the Service is working diligently with our partners to address the issue,” said Roy Averill-Murray, manager of the Service’s Desert Tortoise Recovery Office. Credit: USFWS

It is remarkable how something as simple as a drainage culvert can serve purposes beyond channeling water under a road or highway. Many animals use culverts to get from one side of the highway to the other, especially where the tunnels connect their habitat.

A joint Bureau of Land Management and U.S. Fish and Wildlife Service research effort, which began in 2015 along U.S. Highway 93, found that Mojave Desert tortoises use culverts for shelter and safe passage underneath the roadways that stand between them and additional habitat, mates and food. The research also determined that the desert tortoise is just one of many species of wildlife that benefit from the presence of these “wildlife underpasses.”

The Mojave Desert tortoise faces numerous manmade obstacles when seeking suitable habitat in the wild. Roadways pose one of the greatest dangers in that regard, fragmenting habitat and accounting for the deaths of at least 270 tortoises per year within their critical habitat.

Originally intended as storm water conveyance, this 36-inch diameter culvert along U.S. Highway 93 in Hidden Valley, south of Coyote Springs, Nevada, serves as an underpass for a range of wildlife.  Intended as storm water conveyance, culverts in this area are a specific benefit for the desert tortoise. Credit: Kerry Holcomb/USFWS

“Road mortality is one of the highest-priority threats to address across the range of the desert tortoise. Therefore, the Service is working diligently with our partners to address the issue,” said Roy Averill-Murray, manager of the Service’s Desert Tortoise Recovery Office.

Highways create impacts well beyond the actual asphalt. For example, habitat becomes depopulated as tortoises living up to several hundred yards from highways eventually try, unsuccessfully, to cross to the other side.

In 1995, Clark County and the Nevada Department of Transportation began installing tortoise barriers along southern Nevada highways.

This concrete culvert along U.S. Highway 95 near Cactus Springs, Nevada, north of Las Vegas, includes tortoise-walks designed by the Nevada Department of Transportation. The tortoise-walks are intended to usher them around the large plunge-pools on the downstream side of the underpass. Credit: Kerry Holcomb/USFWS

The barriers are designed to keep the reptiles, listed as a threatened species under the Endangered Species Act in 1990, off of highways and guide them toward culverts through which they can reach the opposite side. That strategy became part of Clark County’s Multiple Species Habitat Conservation Plan, including highways in Clark, Esmeralda, Lincoln, Mineral and Nye counties. As a result, Nevada has the most complete network of tortoise barriers within the species’ range.

According to Service biologist Kerry Holcomb, studies conducted by the U.S. Geological Survey in California and the Service in Nevada documented that the barriers reduce tortoise road mortality by as much as 93 percent—enabling some repopulation of habitat adjacent to roadways.

“Nevertheless,” Holcomb said, “the goal of adequately protecting the Mojave Desert tortoise from road mortality faces many hurdles. One such hurdle involves tortoise behavior when encountering a new barrier.

“Research shows that tortoises meeting a barrier for the first time often pace back and forth trying to find their way through, especially when a culvert is not nearby. This can cause them to overheat during hot weather in their desperation to get to the other side,” Holcomb added.

Mojave Desert tortoises survive in the most extreme conditions, but they need a little help crossing the roadways. Credit: USFWS

Fortunately, this problem has an easy solution. Simple shade structures placed along the barrier provide opportunities for the tortoises to get out of the sun until they cool down and can resume their activity.

A lot goes into the design and construction of culverts for their original purpose of flood control. In southern Nevada, that work is a bit more challenging due to the presence of Mojave Desert tortoises.

Along with barriers, Department engineers must also consider the species when determining which materials to use for erosion control in plunge pools created by water rushing into and out of the culverts. Unmanaged erosion will ultimately make it difficult, if not impossible, for tortoises to access the culverts to get to the other side. At the same time, engineering designs to prevent such conditions must consider the ability of tortoises to safely negotiate the erosion-control material, as well.

Loss of habitat, due mostly to human impacts, is the main reason this iconic desert species is protected. Barriers designed to keep the reptiles off of highways do help, but the tortoises must be able to safely access sections of their habitat fragmented by those highways.

Mojave Desert tortoises survive the extreme conditions of the Mojave Desert, but they need a little help crossing the roadways. To that end, the Service is working closely with the Nevada Department of Transportation and the Federal Highway Administration to design the most tortoise-friendly highways and drainage culverts possible.

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Dan Balduini

About the writer...

Dan Balduini serves as the public affairs officer for Southern Nevada Fish and Wildlife Office in Las Vegas, Nevada.

A former radio and television reporter with more than four decades in communications, Dan spends his free time enjoying his family, while pursuing his other loves—playing golf, fly fishing, and watching college and pro football.

More stories by Dan:

Unconventional help for the wild desert tortoise
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