A migration route north of Yellowstone is being pinched by human development, and the park's pronghorn antelope are feeling the squeeze.
By Tom Arrandale
With their slender legs, oversized lungs, and big protruding eyes that can spot movement up to four miles away, pronghorn antelope are matched to the bold sweep of the American West like few other animals. They can sprint 70 miles an hour, cruising over treeless deserts and plains mile after unbroken mile. True nomads, pronghorn are always on the move, looking for fresh forage in the spring and shelter from snowstorms in the winter.
But this icon of the West’s wide-open spaces might soon vanish from America’s first national park. Most of Yellowstone National Park’s pronghorn still complete the trek from mountain valleys to snow-free plains and back, tracking the changing seasons. But even within the Greater Yellowstone Ecosystem’s protected 40,000 square miles, these ancient seasonal currents have been cut short beyond the park’s northern boundary by fences, agricultural fields, and most recently, the rapid rise of housing subdivisions.
“If that path stays severed and a disease were to spread through the herd or a harsh winter took its toll, there might not be a native pronghorn herd in northern Yellowstone 20 years from now,” says Tim Stevens, NPCA’s Yellowstone field representative. “That would be a tragic loss to the West’s natural wealth.”
Cornered by obstacles and confined to a depleted winter range, Yellowstone’s pronghorn have begun losing the nomadic instinct that suits them so perfectly to Big Sky country. Their plight demonstrates the challenges that human barriers can present to wildlife.
Just like wolf packs howling at rendezvous sites, elk bugling in fall, bison bulls sparring over mates, and grizzlies rambling for calories before their winter slumber, the pronghorn’s spring and fall migrations mark Yellowstone’s dramatic seasonal shifts. Such innate animal behaviors remain intrinsic parts of the natural world—just the sort of thing that national parks are meant to preserve in their fullest glory.
“Big migratory pronghorn herds are down to a handful, and those that are left are just barely hanging on,” says John Varley, former director of Yellowstone’s natural resources division. With the reintroduction of wolves more than a decade ago, the park’s biodiversity is nearly the same as when Columbus set sail. “We’re trying to hang on to those conditions,” Varley says, “but Yellowstone is increasingly becoming an island.”
Around the globe, more and more man-made barriers stand in the way of migrating elephants and wildebeest on African savannahs, chiru on Asian steppes, and caribou on Arctic tundra. Just south of Yellowstone, another herd of pronghorn makes a 350-mile round trip between Grand Teton National Park and Wyoming’s Red Desert, the longest terrestrial migration on the continent. Although the migration of the Yellowstone herd is on a much smaller scale—what was once a 70-mile journey has been whittled down to merely 30 miles—the instinct to move is just as strong. And it unfolds each spring, within sight of the century-old stone arch that marks Yellowstone’s northern gateway. In early April, pronghorn gather on the brushy flats, ready to start trekking to higher elevations. “Just like birds [sensing that the time has come to fly north each spring], a few pronghorn sort of get itchy, and then one or two will take off, and then they all start moving,” says Troy Davis, a Yellowstone ungulate biologist.
But human development now clogs the valleys where rivers race down from Yellowstone, obstructing 75 percent of the migratory routes that pronghorn, bison, and elk once traveled. Just to complete a shortened migration, Yellowstone’s pronghorn must cover formidable terrain, swimming rivers and venturing across forested 7,500-foot-high mountains in the spring before spreading out in the park’s mountain valleys. In summer, on the Blacktail Plateau, a solitary buck often browses sagebrush right along the paved road, displaying the black 12- to 20-inch lyre-shaped horns with forward-jutting prongs that give Antilocapra americana its common name. (Although females sport horns, only the male’s are pronged.) Summertime visitors spot tan-and-white–marked does, many nursing twin fawns prancing on long spindly legs. Yellowstone’s pronghorn occupy a crucial ecological niche. Vulnerable newborn fawns provide prey for coyotes and bobcats, which seldom tackle healthy bison or elk calves.
European explorers described them as “antelope” or “goats” because of their resemblance to more familiar animals, but pronghorn are actually unique to North America and have no close relatives anywhere else in the world. The sole survivors of species that evolved fleeing from giant wolves and fast cheetahs that once hunted North American prairies, they’re thought to have occupied the Great Plains for more than a million years.
To this day, pronghorn rely overwhelmingly on their vision, seeking out open ground where their speed gives them an edge if danger approaches. Between 30 million and 60 million pronghorn once roamed alongside bison, across unbroken short-grass desert and prairies from Canada to Texas. Around the time of the nation’s centennial, Plains residents described a massive migrating herd of pronghorns—an estimated 1 million animals—stretching across 70 miles. But overhunting and settlement of the West dramatically reduced pronghorn numbers to a mere 15,000 nationwide. Yellowstone, established in 1872, provided one of the last refuges where a handful hung on alongside the remnants of once-endless bison herds.
State game and fish departments brought the pronghorn back, and today roughly a million of the animals roam throughout the West, from Montana and Wyoming as far south as Arizona. Inside Yellowstone, pronghorn numbers peaked as high as a thousand in the era when the National Park Service actively maintained big ungulate herds for tourists to enjoy watching. In the 1940s and 1950s, with the population burgeoning, wildlife managers transplanted nearly 1,200 of the park’s pronghorn to the Great Plains rangelands around the West. But Yellowstone’s herd fell on harder times when the Park Service began to let natural forces regulate wildlife populations. A few years ago, pronghorn numbers crashed to slightly more than 200 animals as drought hit their winter range. Yellowstone’s coyotes also preyed heavily on fawns, striking in the few perilous days right after birth, when fawns must cower in sagebrush cover until they’re strong enough to flee danger.
The herd bounced back to roughly 300 last winter, but pronghorn no longer return to Yellowstone’s vast Hayden Valley and other remote summer ranges where transplant operations removed significant numbers from the population 50 years ago. Biologists speculate that the herd may be losing the memory of the migratory routes that generation after generation had followed. What’s more, roughly a third of the herd no longer migrates at all, sticking year-round to sparse winter range.
Now, isolation puts Yellowstone’s pronghorn in peril. When the pronghorn return in October, the entire herd is trapped on a barren, severely degraded landscape on the park’s northern boundary. That confinement leaves Yellowstone’s pronghorn at risk if disease breaks out, inbreeding weakens their genetic stock, or the occasional blizzard buries winter forage. Bison and elk can plow or step through deep drifts, but pronghorn make it through winter only by gathering where snow seldom piles up more than a couple of inches.
Although the Park Service had acquired more than 7,500 acres of land decades ago, extending Yellowstone’s northern boundary and increasing winter range for ungulates, rainfall there averages less than ten inches a year, and the land remains beaten down by the residual effects of irrigated farming and grazing. This spring, both the Park Service and adjacent Gallatin National Forest will again try to clear out noxious weeds and other invasive plants and replant native sagebrush and grasses, but it will take decades to bring natural conditions back to the landscape. By then, park biologists say, Yellowstone’s pronghorn could vanish unless they can reestablish a longer migration route down the Yellowstone River to reach Montana’s Paradise Valley.
There was a time when much of the Yellowstone ecosystem’s bison, elk, pronghorn, and deer spent harsh winters in that spectacular 40-mile-long valley. But today, the journey to the valley would take pronghorn across private lands, where numerous fences block the migration route.
Starting in the 1860s, ranchers transformed the corridor into lushly irrigated fields and fenced them off for livestock pastures. Pronghorn, so finely tuned to open ground, have never developed any instinct to jump over barriers. Most are too befuddled to try to get through even the flimsiest four-foot-high barbed wire fencing; a few try lunging across, and some die entangled in wire. Sturdier steel mesh fences built to hold sheep or calves have taken even deadlier tolls around Western rangelands. Pronghorn have been known to pile up behind impenetrable barriers and die trying to flee winter storms or move to greener forage. As the landscape was carved up, most pronghorn vanished altogether from the Yellowstone’s valley—leaving the park’s herd disconnected from Montana’s recovering population.
But most pronghorn learn to scoot beneath fencing if they can find spots where the bottom strand is high enough off the ground. A few years ago, at least two Yellowstone pronghorn managed to navigate the steep-walled gorge north from the park to found a new herd, now 50 animals strong, that’s thrived on alfalfa fields at the south end of the valley. Their journey gives park biologists hope that Yellowstone’s pronghorn could rediscover a migration corridor that would reconnect them with healthy antelope herds where the Yellowstone River emerges from the mountains and starts meandering across Montana’s high plains. To that end, NPCA is exploring opportunities to work with local ranchers and community leaders to clear the way across private lands so pronghorn can reach wintering grounds farther northward.
Danielle Blank, NPCA’s Yellowstone outreach coordinator, is hoping to work with local ranching families to replace old barbed-wire barriers with wildlife-friendly fencing. New designs string a smooth bottom wire at least 18 inches off the ground, leaving pronghorn enough of a gap to maneuver beneath it.
Some Paradise Valley ranches have already installed “let-down” sections built so cowboys can easily drop the wires to the ground when wildlife need to move through. Although Yellowstone’s wolves aren’t popular among ranchers for the threat they pose to sheep and calves, and the park’s elk sometimes consume hay and destroy fencing, most ranchers say they have no qualms about sharing the range with pronghorn. “I haven’t heard anybody complain about antelope,” says Marty Malone, the Park County, Montana, agricultural extension agent. “Ranchers just admire them.”
So do other Westerners who still can glimpse a band of pronghorn in full flight, stretching out single file with smooth flowing strides, like a streamliner gliding along the boundless skyline. “Along with the bison, pronghorn have always been a dominant feature of this landscape,” Blank says. “It would be a shame if future visitors lost the chance to watch these iconic animals moving across the plains en masse—just like the first settlers in this region were able to see, so many years ago.”