Crops, dams, fences, roads and other human footprints can block animal movements, but efforts are afoot to open wildlife corridors for many of North America’s iconic species.
Pronghorn ford the Green River near Pinedale, Wyoming, along the famed Path of the Pronghorn, a route fragmented by roadways and fences. Wildlife crossing structures now are helping pronghorn and other species make the journey more safely.
A FRIGID WIND whips across the northeastern Montana prairie as a herd of pronghorn forage on a hillside. Pronghorn are America’s fastest land mammal, able to sprint at 60 miles per hour, but this group moves methodically, conserving energy. “They are just trying to make it through the winter,” says National Wildlife Federation biologist Andrew Jakes, an expert on pronghorn migration.
As winter turns from spring to summer, pronghorn in this vast expanse of prairie will begin their move northward, seeking green waves of nutritious new grass. Jakes has discovered that it’s a surprisingly epic journey. After tracking GPS-collared pronghorn in the region for several years, in 2018 he published a study documenting the longest pronghorn migration ever recorded—more than 550 miles round-tip between winter and summer ranges within the transboundary grassland and sagebrush landscapes of Alberta, Saskatchewan and northern Montana.
Just the year before, U.S. Fish and Wildlife Service biologist Jason Tack coauthored a study documenting the longest greater sage-grouse migration on record, stretching more than 150 miles round-trip from northeastern Montana to breeding leks in Saskatchewan. Both Jakes and Tack were surprised to see how closely the sage-grouse and pronghorn routes—and behaviors—overlapped.
Both species migrate at roughly the same time in the spring and fall to skirt areas with roads, croplands and other human impacts while stopping to refuel on undisturbed habitat, frequently on private ranchlands. About half the land in this region is private property, making landowners—especially ranchers—key allies in the effort to protect migration corridors. “The take-home message,” says Jakes, “is that wildlife don’t know borders or property lines, and they need space to move.”
That need is true across the globe, where myriad species of birds, mammals, amphibians and other wildlife need movement corridors to roam in search of food, water, shelter and mates. The world’s longest-distance migrants, Arctic terns may fly up to 44,000 miles annually between Greenland and Antarctica, while spotted salamanders and wood frogs may travel just 1,000 feet between hibernation sites and vernal pools for breeding. No matter the length, North America’s wildlife corridors sustain keystone predators such as wolves, mountain lions and grizzly bears as well as some of the world’s longest-distance migrating ungulates, including pronghorn, mule deer, moose, elk, bison and caribou.
The ability to move is key to sustaining species’ genetic diversity as well as healthy ecosystems, but the growing field of “movement ecology” is showing how imperiled free movement has become. In 2018, Science published a major global study of animal movements that analyzed GPS data from more than 800 individual animals of 57 species ranging from pocket mice to elephants. Researchers found that the range of movement for mammals in areas with a heavy human footprint was cut by one-half to two-thirds. The study concludes that such curtailed movement impacts “population persistence” as well as “predator-prey interactions, nutrient cycling and disease transmission.”
The main culprit in the movement crisis, says NWF Chief Scientist Bruce Stein, is “human-caused fragmentation of natural habitats, which has profoundly affected the ability of most wild species to move freely across the landscape.” This loss of habitat and connectivity, he says—paired with factors such as climate change, invasive species, pollution and disease—has left one-fifth of North American species vulnerable to extinction and one-third at risk.
Fragmentation comes in many forms. Among them:
• Crops. “Cropping is the big habitat killer,” especially across the Great Plains, says Brian Martin, Montana grasslands conservation director for The Nature Conservancy. From 2008 to 2012, the United States created more than 7 million new acres of croplands, with production of corn and soybeans for biofuels driving much of the habitat loss. Grassland birds—such as Sprague’s pipits, Baird’s sparrows and Western meadowlarks—are suffering. According to the North American Bird Conservation Initiative, roughly a third of all grassland bird species now are in steep decline.
• Dams. Salmon and steelhead spawning runs in the Columbia River Basin at one time constituted the largest anadromous fish migration on the planet. Some traveled nearly 900 miles inland to spawn in Idaho’s Salmon and Snake rivers. Now there are four hydroelectric dams on the Snake, and “the fish runs are just a husk of what they once were,” says Brian Brooks, executive director of the Idaho Wildlife Federation (IWF). According to the American Fisheries Society, 106 runs of Pacific Northwest salmon in the Columbia River Basin have gone extinct, mainly because of dams.
• Fences. Millions of miles of barbed wire crisscross the West, mainly to contain livestock. Migratory ungulates—including deer, elk and pronghorn—can be blocked, injured or killed while attempting to get around barbed wire. Jakes says some pronghorn in his study had to navigate past 700 to 1,100 fences annually. “That’s a lot of fences,” he says, “and it only takes one woven-wire fence to stop them.”
• Roads. No one knows the exact number of animals killed by vehicles, but a study by the Federal Highway Administration estimates as many as 2 million collisions between cars and large animals occur every year in the United States. For 21 species federally listed as threatened or endangered—including the Hawaiian goose, desert tortoise, San Joaquin kit fox and California tiger salamander—the study cites road mortality as “one of the major threats to their survival.”
• Climate. Shifts in temperature can alter food availability and contribute to droughts, floods, fires and the spread of invasive species and disease, making climate change the greatest future threat to wildlife. “Given that species’ ranges already are shifting to follow suitable climatic conditions, maintaining or restoring connectivity among habitats is becoming even more important,” says Stein.
Recognizing that survival depends on freedom of movement for a host of species, the National Wildlife Federation and its affiliates “have made protecting, restoring and connecting wildlife habitat and migration corridors a cornerstone of our work,” says NWF President and CEO Collin O’Mara.
That’s certainly true in the Northeast, where NWF and affiliates are part of the Staying Connected Initiative (SCI), a coalition of some 50 partners working to restore corridors for moose, bears, coyotes and other species across the northeastern United States and Canada. Since 2009, SCI has made significant headway helping wildlife survive by conserving more than 288,000 forested and wetland acres that include wildlife corridors and road crossings vital to healthy wildlife populations. “We use remote cameras to track animal movements,” says Chris Hilke, NWF’s senior manager of climate adaptation. “It’s a thrill to see everything from bobcats to beaver moving through these landscapes.”
The Federation has also taken a lead in promoting habitat health and connectivity across the Farm Belt. NWF fought for language in the 2018 Farm Bill to increase wildlife habitat by expanding the Conservation Reserve Program, which takes marginal croplands out of production and plants them with native grasses. The bill also included language to prioritize lands that improve or create wildlife corridors. “This new language could help direct significantly more acres towards grasslands that improve habitat connectivity,” says Aviva Glaser, NWF’s agricultural policy director.
Encouraging ranchers to maintain grasslands rather than convert to crops, NWF works with these landowners to help make fencing more wildlife friendly. This may involve moving some fences away from migration pathways or simply adding clips to raise the lowest strand of wire, allowing ungulates to crawl underneath. To save low-flying sage-grouse, which sometimes strike fences with deadly force, NWF worked with the Sage Grouse Initiative and Montana Conservation Corps to place white plastic tags on barbed-wire strands, making them more visible. Research shows that such fence tagging can reduce sage-grouse collisions by 70 to 80 percent in high-strike zones.
Farther west, the Federation has been working for 25 years to encourage wild salmon restoration in the Columbia, Snake and Salmon river basins. “Science is telling us removing the four lower Snake River dams is our best option for saving our fish,” says IWF’s Brooks, “but any resolution to this complex issue has to be a grassroots effort.”
Dam removal elsewhere has proven to restore fish populations. In 2014, after the removal of two dams on Washington’s Elwha River in Olympic National Park, the river began to flow freely to the Pacific for the first time in a century. Populations of bull trout and salmon—which had plunged from about 400,000 to 3,000 after dam construction—have begun to rebound, and nutrients from the fish are now nurturing species from insects and birds to bears.
For land-bound species, wildlife crossings can significantly reduce road mortality from vehicle strikes. The Banff region in Canada has one of the largest densities of these structures in the world, with 44 wildlife overpasses and underpasses crossing the Trans-Canada Highway. Collisions have fallen by 80 percent during the past 20 years, sparing the lives of moose, deer, elk, bears, lynx and a host of other species.
One of the best-known wildlife crossings is on Wyoming’s Path of the Pronghorn, the first designated wildlife migration corridor on federal lands. Every spring and fall, pronghorn migrate about 170 miles between the Green River Valley and Grand Teton National Park, a route that crosses U.S. Highway 191. After the completion of a $9.7-million project to build two overpasses and six underpasses, the number of collisions fell from roughly 85 per year to 16, according to the Wyoming Department of Transportation.
The world’s largest wildlife crossing project is taking shape in Southern California: an estimated 200-foot-long, 165-foot-wide concrete bridge spanning 10 lanes of pavement over the 101 Freeway in Agoura Hills. Slated to open by 2023, the overpass would connect habitat for mountain lions in the Santa Monica Mountains. Without the bridge, the small population on the south side of 101 will likely go extinct. “This has never been done before in an urban setting on this scale,” says Beth Pratt, NWF’s regional executive director for California. “Saving these wild cats is a story that has captivated the world.”
An equally charismatic species, grizzly bears are also gaining freedom to move. NWF and its affiliates have helped create movement corridors for grizzlies in the Northern Rockies that allow populations to disperse from the Greater Yellowstone Ecosystem, where the bears have increased from fewer than 200 a generation ago to more than 700 today. “It’s a big success story,” says Tom France, director of NWF’s Northern Rockies, Prairies and Pacific Regional Center. The goal now is to help those Yellowstone populations expand outward and repopulate Idaho’s Salmon-Selway-Bitterroot Ecosystem, the largest unoccupied grizzly habitat in the Lower 48.
The ultimate vision for connectivity is the Yellowstone to Yukon Conservation Initiative, a plan to link 502,000 square miles from Wyoming to the Northwest Territories in Canada. “Having north-south corridors and lower-elevation to higher-elevation connectivity is increasingly important as climate change becomes more pronounced,” says Stein.
“Support for restoring wildlife corridors is uniting people across this country,” adds O’Mara. With such broad support, the nation’s wildlife heritage may remain as sure and strong as a pronghorn sprinting across the plains.
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