Long-haul trucks roar along Interstate 80, a transportation backbone that stretches from San Francisco to just outside New York City. Traffic is so heavy here that the state’s transportation department recently counted a passing vehicle every 10 seconds, on average.
This vital, four-lane corridor of commerce also threatens wildlife. It blocks the ancient north-south paths of mule deer, elk and pronghorn, creatures that embody the American West. Animals that try to complete annual migrations, which their species have made for thousands of years, risk deadly encounters with passenger cars or 18-wheelers.
And as the climate changes, altering where animals graze and find suitable habitats, migration corridors are more important than ever to their survival.
“We can’t predict exactly what the impacts of climate change are going to be, or what species are going to be impacted,” said Hall Sawyer, a research biologist at Western Ecosystems Technology. “We do know one fundamental truth: That if we can keep this landscape connected, improve that permeability, they’ll be better off.”
Wyoming officials and scientists have a plan: Build wildlife crossings to preserve migrations. Bridges, tunnels and other structures — imagine protected bike lanes, but for animals — can protect animals from hazards like highways and help them navigate a warming planet.
For the female pronghorn known to researchers as 700031A, the valley in western Wyoming is a paradise in summer.
The valley surrounds a horseshoe lake that curves through the Wind River Range mountains. Sagebrush and mountain flowers, mixed with aspen trees, grow on the valley slopes. The vegetation she eats here will help fuel her 150-mile migration south, a journey longer than the distance between Washington, D.C., and Philadelphia.
She leaves in September, before winter snow buries edible plants and forms dangerous drifts.
700031A follows a route walked by generations of pronghorn. Bones discovered in Wyoming suggest that pronghorn used nearby migration corridors for at least 8,000 years, said Greg Nickerson, a writer and filmmaker for the University of Wyoming’s Wyoming Migration Initiative.
The pronghorn lineage in America stretches back 20 million years. Their ancestors endured nearly cataclysmic disasters, such as a Yellowstone supervolcano eruption that blanketed the West in ash approximately 640,000 years ago.
Researchers placed a GPS collar on the 700031A in February 2018 — giving her a number instead of a name, to preserve their scientific detachment. Since then, Sawyer, who studies migration patterns, has monitored her movements on his computer.
As the pronghorn heads south to a milder winter habitat, she nears Highway 191, where tall fences corral her to a crossing site. She walks across an overpass named Trappers Point, a gentle hump of bridge designed for animals and completed in 2012.
“We know that pronghorn and mule deer have been using that overpass,” said University of Wyoming graduate student Benjamin Robb, who watched 700031A’s movements on a computer screen in his Laramie office. “But seeing a collared animal using it is really cool.”
700031A skirts to the west of two gas fields. “She’s actually avoiding all the infrastructure,” said Sawyer, who estimates the pronghorn is an adult between 6 and 8 years old. She moves swiftly, traveling several miles a day, until she reaches the Green River. The water level drops in the fall, so she probably wades across, he said.
The coffee-colored pronghorn, following a narrow band of sagebrush, crosses two more highways. She navigates these roads without using wildlife bridges. She walks along the edge of the fenced-off solar farm “smack dab in the middle” of the corridor, Robb said.
By early November, 700031A arrived near Wyoming’s southern lip. Here, this descendant of supervolcano survivors encountered a new kind of obstacle: Interstate 80. The superhighway stopped her in her tracks.
An impassable road
Interstate 80 began killing wildlife even before it opened in 1970. As archivist John R. Waggener writes in “Snow Chi Minh Trail,” his history of I-80 named after a particularly perilous section of the road, construction crews collided with wildlife as early as 1967.
“They keep running into wildlife, and so the engineers and the road planners realize that they are building this highway smack-dab in the middle of a fairly major migration corridor,” Waggener said.
He can attest to this firsthand. Waggener was a passenger in a vehicle that struck a pronghorn on I-80 during a whiteout blizzard in the late 1980s. The humans were fine; the animal was not.
In Wyoming, 15 percent of all vehicle collisions involve deer, antelope or other big game animals, said Corinna Riginos, director of science at the Nature Conservancy’s Wyoming field office. These 6,000 annual collisions cause $50 million in damage to cars and loss of wildlife. In 2017, a semi-trailer truck killed 25 pronghorn on I-80 when the truck plowed into a herd.
As the highway neared completion, engineers dug underpasses below I-80 to allow animals to cross.
Their motivations were sound. “If you were to install highway underpasses or overpasses and fencing on a segment of road,” Riginos said, “we know from many, many studies and data sources that that is going to reduce the number of collisions.” A study of the Trappers Point overpass and nearby underpasses found that collisions with mule deer dropped by 79 percent and those with pronghorn stopped completely.
But without accurate migration data, the first culverts were constructed in the wrong places. Few animals used them.
Mule deer and other grazers that migrate in Wyoming follow spring vegetation. Migration isn’t a journey between two endpoints; migration is the point — the animals eat their way along the path, scientists say.
In what University of Wyoming biologist Matthew Kauffman calls “surfing the green wave,” migrating animals spend the spring eating plants that sprout first at low elevations. The deer then follow the foliage as plants appear along the slope to higher elevations.
Kauffman and his colleagues are only beginning to understand how warming temperatures can disrupt the green wave. In drought years, the wave is much harder to surf, he said.
But a migration corridor that is expansive enough and free of obstacles offers animals a chance to survive a warming planet, Kauffman said.
“That 150-mile migration? That animal has a 150-mile foraging template to … mitigate the effects of climate change,” he said.
I-80 runs for 400 miles through lower Wyoming. A railroad, another obstacle to wildlife trying to move north and south, runs parallel to the interstate for miles.
Animals traveling south for the winter are forced to stop when they reach I-80, which runs east-west through Wyoming’s Red Desert. In the summer, mule deer will travel more than 100 miles to the Hoback Basin, following a migration corridor that Sawyer discovered. Thousands of deer eat their way along this path.
Across Wyoming, fences protect motorists from wildlife attempting to cross roads. Although fences deter animals, some make it through by vaulting over the barriers. Others end up tangled in the fencing; those who cannot pull free die.
But I-80 is such a strong barrier that pronghorn are “over a thousand times more likely to successfully cross a fence than this interstate,” Robb said.
One afternoon last month, Nickerson counted seven dead mule deer on a 100-mile stretch of the interstate between Rawlins and Laramie. At the point where it bends around Elk Mountain, I-80 becomes a roadkill hot spot. There, at Halleck Ridge, a proposed crossing site for deer and elk, the transportation department tallies about 25 roadkill carcasses per year.
Wildlife will use some underpasses on I-80. In one of these boxy concrete tunnels, birds made mud nests stuck to the walls. In late January, Nickerson’s cameras recorded a small herd of deer as they marched below the highway.
Given the varying lengths of the country’s corridors, scientists are trying to determine whether they provide a way for wild species to outrun the climate crisis.
At Georgia Tech, Jenny L. McGuire, an expert in spatial ecology, worked with colleagues to develop a model that divvied up the American landscape into hot and cool habitats.
“We said: ‘Okay, what’s the coolest place that something can get to, by moving through connected, adjacent natural land areas?’” McGuire said. They predicted the most sensible paths animals would take to navigate fragmented habitats, avoiding urban areas in favor of land with little or no human development.
The researchers reported in a 2016 paper that only 41 percent of the wild habitats in the United States would allow species to maintain their preferred temperatures over the next 100 years of warming, assuming a global increase of 2.7 degrees Celsius over that time period. But, if those habitats were connected by wildlife corridors, 41 percent jumps to 65 percent, McGuire and her colleagues found.
North-south movement is critical, but so is elevation. One estimate projects that wildlife would have to travel nearly 250 miles toward the poles to escape an overall 5-degree Fahrenheit temperature increase. Or they could run 500 yards up a mountain.
The Nature Conservancy is mapping habitats that, its conservationists predict, will allow Wyoming’s wildlife to move into new ranges as the climate changes. Researchers already know that deer “migrate from low to higher elevation areas every year,” Riginos said. “And, in the future, other species may need to shift their ranges to the same higher elevation areas.”
Ecological connections serve two purposes: They allow individuals to survive by providing more space to roam, access to food or a bigger mating pool. And they enable animals to relocate in the long term. “If you establish a permanent corridor, it’s important for both needs,” McGuire said.
After linking football-field-size sections of longleaf pine savanna, a rare habitat in South Carolina, ecologists have seen the diversity of plant species balloon since 2000.
Last year, scientist Jodi Hilty and her co-authors updated their textbook on corridor ecology. In the 13 years since the first edition, “we found 183 new papers on connectivity and climate change,” she said. “The field literally exploded.”
There are some concerns that corridors could help spread invasive species or wildfires. “But the preponderance of evidence — and, certainly, the recommendations resounding from the climate change science community — show that connecting these remaining wild areas is a very sound approach,” Hilty said.
One of the best-regarded experimental studies of climate and connections took place a decade ago in a forest in northern Quebec. There, researchers tweaked the moisture and temperature in the habitats of tiny mites that live on moss patches. Some pieces of moss were connected, creating a corridor.
Biodiversity plummeted in the dried-out habitats, as McGill University biologist Andrew Gonzalez and his co-authors showed in a 2012 paper.
But linking the “carpets of mosses,” Gonzalez as described them, helped preserve some of the richness of the mite species. “Extinction rates are slowed and colonization rates increased,” he said.
The study of corridors and migration has evolved, thanks to high quality GPS data. Scientists can track the movements of animals in real time and use that information to help identify where to establish the best road crossings.
That revolution also allowed University of Wyoming biologists to track record-setting animals, such as mule deer 255. The animal’s one-way, 242-mile trek from Idaho to Wyoming is the farthest ever recorded for a deer walking in the Lower 48 states.
Yellowstone to Yukon, also known as Y2Y, is one of the most ambitious corridor projects on the continent. The project, begun in 1993, aims to protect a 2,000 mile stretch of habitat in North America from Wyoming to the Arctic Circle. Yellowstone to Yukon’s conservationists have supported highway overpasses, helped track wolverines and purchased more than 500,000 acres of land to preserve wildlife routes.
The project is “really incredibly useful for the northwestern portion of the United States,” said McGuire, the Georgia Tech ecologist. Yellowstone’s grizzlies, for instance, aren’t especially genetically diverse, Hilty said. Breeding with Canadian bears could help their cubs resist disease or gain other beneficial traits.
Corridor and connectivity projects have sprung up across the country: in Los Angeles as a bridge for mountain lions; below Florida highways as structures for panthers and alligators; and, in the Capitol, as legislation. Last year, Rep. Don Beyer (D-Va.) reintroduced the Wildlife Corridors Conservation Act with bipartisan support.
The bill would authorize $50 million in grants for the national corridor system as well as several million dollars each to the Interior, Agriculture and other departments. Rights of way or easements on tribal and private land would link up with corridors on federally owned habitats. The bill passed out of the House Natural Resources Committee in January.
Beyer said he was influenced by the naturalist E.O. Wilson, who proposed in his book “Half-Earth” that humans should retreat from half of the planet’s surface. Because humans fractured wildlife corridors, it “becomes our responsibility as best we can to reconnect them,” Beyer said.
Also last year, Sen. John Barrasso (R-Wyo.) introduced a $287-billion highway infrastructure bill with bipartisan support that would provide $250 million in funding for wildlife crossings.
In spring of 2017, the state of Wyoming, with backing from environmentalists and hunters, launched a multimillion-dollar corridor plan. Officials identified hundreds of possible locations for crossing structures and narrowed the list to 10, prioritized by human safety and ecological factors. Some of these, like Halleck Ridge, would cross I-80.
Expense remains the biggest obstacle, said Scott Gamo, environmental services manager at the Wyoming transportation department. The Trappers Point overpass cost $9 million. Each of the top 10 sites will cost about $10 million to $20 million, he said. His department recently secured a $14.5 million federal highway grant to build a wildlife crossing near Dry Piney Creek in Wyoming. Wyoming drivers have purchased 1,100 specialty license plates featuring mule deer to help fund the overpasses.
Underpasses, though cheaper, may not always work. For instance, pronghorn are reluctant to lose sight of the horizon and avoid tunnels. But because collisions are reduced so dramatically, the benefits of overpasses exceed the costs over time, Riginos said.
In February, as Nickerson drove back toward Laramie, he spotted a mule deer on the bank of a river, which crossed below the highway at a right angle. This wasn’t good, he said. “It could just jump over one fence, and then it’s in a rough spot.”
He made a U-turn, mashed the accelerator and took an exit road that descended from the highway to a gravel lot on the riverbank.
Signs of mule deer were everywhere. Scat and tracks covered the lot.
Nearby, a line of hoof prints marched over the ice. Here, the animals had found a wide natural crossing under the highway. At least, until the river thaws.
About this story
Video editing by Jayne Orenstein. Copy editing by Anne Kenderdine. Development and design by Andrew Braford.
GPS routes of pronghorn, mule deer and elk sourced from collaborative efforts by the University of Wyoming (Jeffrey Beck, Adele Reinking, Katie Taylor, Kurt Smith), Wyoming Game and Fish Department (Embere Hall, Lee Knox, Jeff Short), Western EcoSystems Technology Inc. (Hall Sawyer), Wyoming Migration Initiative, Wyoming Cooperative Fish and Wildlife Research Unit (Matthew Kauffman), and the Monteith Shop of the University of Wyoming. Existing wildlife underpasses along I-80 sourced from Wyoming Department of Transportation. Viable underpasses created from surveys by Benjamin Robb, William Rudd and Gregory Nickerson. Fences sourced from Earl DeGroot, Wyoming Cooperative Fish and Wildlife Research Unit. Ungulate/vehicle collision maps sourced from Corinna Riginos, the Nature Conservancy of Wyoming, with data from the Wyoming Department of Transportation. Cartographic reference and source material from “Wild Migrations: Atlas of Wyoming’s Ungulates” (OSU Press). Yellowstone to Yukon boundary provided by the Yellowstone to Yukon Conservation Initiative.