In the late winter sun, Yellowstone National Park‘s blanket of snow is blindingly white. The great shaggy beasts—​bison, in their thick, burnished winter coats—thrive in the glare. A few yards from where I stand on the road, a small group is gathered, sweeping their upturned horns from side to side, grunting softly as they plow away the snow to graze. I’m transfixed, so it takes a minute to notice the hundred or so more scattered in the valley ahead of me. Across the steep rise of the Gallatin Range, bushes dotting the ridge come into focus; each, I realize, is a buffalo, lumbering out of the mountains toward lower elevation.

Each winter, Yellowstone’s bison move from the high country in groups of a few dozen, seeking better feeding grounds. The evidence of wild bison migration is etched into our continent, where the movement of vast herds shaped the land. The countless pounding hooves formed wide passages called buffalo traces, as the beasts followed watersheds and ridgelines to new territory. The early pioneers followed these paths—through the Cumberland Gap in the Blue Ridge Mountains, across the Ohio River near Louisville, and the Vincennes trace through Indiana and ­Illinois—​to settle the West.

Of course, these places have long been purged of wild buffalo. By the 20th century, ­westward-​­moving colonizers, a thriving hide-​­hunting trade, and efforts to wipe out native populations by diminishing their primary food source had reduced a teeming mass of tens of millions of bison to just 23 fugitives, holed up in a valley a few miles south of where I glimpsed my first Yellowstone grazer.

Today, the American buffalo is one of the environmentalist movements’ great success stories. Thanks to more than a century of conservation efforts, there are roughly half a million bison in the United States, though they live strikingly different lives than their ancestors. More than 96 percent are livestock, raised for meat. The remainder, about 19,000, are mostly behind fences, in managed herds across the country.

Yellowstone’s bison are different. The animals that occupy the park are the last of their kind. They exist as their forebears did, in this tiny sliver of the U.S. where they’ve never been extirpated. They fight, graze, and repopu­late without the help of humans. They fall through thin ice and drown in lakes and rivers. They lose their old and sick to grizzly bears and wolves. And in winter, when the grass dwindles, they get going, roaming across the reserve’s 2.2 million acres, an area far less than 1 percent of their historic range. They are America’s last truly wild bison.

That wildness is what makes them iconic. In 2016, President Obama named the bison our first national mammal. They appear on the seals and flags of five states, and bring in hundreds of millions in tourism dollars every year. But scientists and conservationists warn that the Yellowstone bison are approaching a precipice—a moment that may make it nearly impossible to preserve their untamed nature.

For the ranchers and civilians who live on the park’s outskirts, migrating bison have never been convenient, but an infectious disease has heightened tensions. Scientists estimate that more than half of Yellowstone’s female bison herd carry brucellosis, a bacterial infection that causes domestic cows to abort their fetal calves. Though there’s never been a documented case of transfer from bison to cattle, theoretically the disease could spread to the herds that graze on public lands outside the park. Since 2000, the Department of Livestock for the state of Montana, which controls the land north of the park, has had an agreement with Yellowstone officials, the U.S. Department of Agriculture, and tribal entities: Inside the park, the bison are free to move as they please; once they go beyond the boundary, they’re fair game. Some are killed by hunters and local tribes. More are captured by wildlife managers and sent to slaughter. Almost none of the bison that attempt to migrate beyond Yellowstone’s borders survive.

Humans have played a role in natural selection since the first hunter threw a spear. But wildlife biologist and retired Colorado State professor James Bailey warns that by culling the migrating bison, and therefore selecting for the trait of staying put, we might be “disorganizing and diminishing the wild, adaptive genome.” He acknowledges that the proximity of people to places like Yellowstone makes some intervention necessary, though he calls the field he taught for 20 years—wildlife management—an oxymoron. But already, there are serious consequences.

“People are always looking for a number,” he says. “How many do we need? How much genetic diversity? How much natural ­selection? All these lines are largely arbitrary, so there’s no value in waiting for an absolute answer. Meanwhile, it’s business as usual, which is degrading the wildness of our herd.”

The animals in conservation herds will keep the species safe from extinction, but if we domesticate the mobility out of this last wild group, the ecology of the landscape will suffer, perhaps irretrievably, and we will have lost our wild buffalo after all.

In the American imagination, the West has often been a cattleman’s paradise. In reality, that land has always belonged to the bison. Within their habitat—a range that once stretched across at least 40 states—the bison is a keystone species, on which nearly everything else in the ecosystem depends.

staff send straying bison to slaughter

End of he Roam

At Stephens Creek Capture Facility, staff send straying bison to slaughter.

As they graze, their hooves and horns turn the soil, planting seeds and creating pockets of moisture that encourage growth. When they shed hair, small mammals and grassland birds use it to insulate their nests. Wallows, the depressions bison form by rolling in the dirt, fill with water and create miniature pond habitats for insects and frogs. Over millennia this mutually beneficial co­evolution has built an ecosystem in which the buffalo—and their ability to roam—are vital.

Then, we nearly wiped them out, and ­replaced them with cows.

In the late 19th century, after tanneries developed a process for making hides into leather, bison slaughter peaked. Hunters killed an estimated 2 million in 1870. For the next three years, hide hunters took down roughly 5,000 buffalo every day. By mid-1883, almost every single bison in the U.S. was dead.

Once the buffalo were gone from the plains, ranchers took advantage of all that grazing land to drive in their cattle. In truth, cattle aren’t built for the ecosystem. They are indiscriminate eaters who will chew a summer pasture down to its roots, decimating native grasses and promoting fast-growing invasive species, some with little nutritional value. They struggle to survive harsh winters; they can’t dig through heavy snow to reach surviving foliage. If they wander more than a half-day’s walk from a permanent water source, they risk death from dehydration.

In contrast, bison are built to survive. They “will eat snow, search out tiny springs, even paw the earth for water,” writes Dan O’Brien, a rancher in South Dakota, in his book, Buffalo for the Broken Heart, a memoir about converting his cattle operation to a free-​range bison ranch. The diversity of their diet “allows them to roam many miles from major sources of water. That difference between cattle and buffalo is obvious enough for anyone to see.”

We’ll never glimpse our nation’s landscape as it might have looked had the bison survived, but thanks to Yellowstone, we can make a pretty good guess. Chris Geremia, the park’s soft-spoken, bespectacled lead bison biologist, says the herd shapes the local ecology in a way other large ungulates—pronghorn, deer, sheep, and elk—do not. As temperatures begin to climb in the spring, all the animals “surf the green wave,” moving from low to high elevations as grasses and plants emerge. Unlike the other species, though, buffalo stop surfing in mid-spring, gathering in huge numbers in central areas they turn into grazing lawns.

“They look just like a lawn in the suburbs,” Geremia says. At the end of the season, pastures where domestic cattle graze are nearly barren. The places where bison wander and feed, on the other hand, stay lush and abundant with life. Prevent buffalo from migrating, Geremia tells me, and you remove the landscape’s ability to renew itself.

Preventing bison from migrating is part of his job, though. In 1995, the state of Montana sued Yellowstone National Park, arguing that the wandering buffalo were a business handicap. Livestock interest groups and local ranchers asserted that in addition to competing with cattle for grass and causing property damage, the bison could transmit the brucellosis pathogen to their herds.

bison following migration path
We still have a shot at preserving this animal’s wild nature. To really save them it’s people who will have to do the evolving: learning to live in concert, not conflict, with the bison. Chris Douglas

Five years later, a drawn-out court battle resulted in the Interagency Bison Management Plan, a collective that meets each November to set a culling target for the coming winter. The group includes the Montana Department of Livestock; Montana Fish, Wildlife & Parks; the National Park Service; the USDA Animal and Plant Inspection Service; the U.S. Forest Service; and three tribal entities. They manage an annual slaughter aimed at keeping the Yellowstone buffalo population between 3,000 and 4,200, just barely enough to maintain genetic diversity.

A young bison thrashes against the cold steel of a squeeze chute, bucking and tossing its head in a panic. It takes two National Park Service employees, wielding electrified cattle prods, several attempts to close the hydraulic-powered sides of the stall around the animal. The immobilized yearling’s eyes open wide as Geremia and his colleagues examine its teeth, draw a syringe full of blood to test for brucellosis, and stick a numbered tag to its woolly flank. After a few minutes, they release it to a holding pen, where it joins several other unlucky buffalo.

This is Stephens Creek Capture Facility, a complex of corrals nestled in a valley along Yellowstone’s northern border. The site handles wild bison like livestock: Most of them, including some that test negative for brucellosis, board trailers and head to slaughter. The IBMP considers this the best way to mitigate risk.

It was cattle that first infected bison and elk with brucellosis, almost a century ago, when they were free to graze within the park. By the mid-20th century, brucellosis was the most common zoonotic disease in the world, affecting an estimated 124,000 U.S. cattle herds.

The illness, which could sweep through large swaths of livestock, delivered a financial hit to ranchers. But the risk to humans dealt a larger blow: Transferred through unpasteurized milk, the illness didn’t cause spontaneous abortion in ­people—just joint pain, night sweats, and headaches. But it did spread widely: In 1947, there were 6,400 reported cases of brucellosis in humans. These spurred a nationwide effort to eradicate the disease, which meant culling entire herds if a single animal showed evidence of infection. The strategy eventually evolved to include a vaccine for cattle that, while only 65 percent effective, at least eliminated the need to kill whole stocks.

By all accounts these measures succeeded. Today, the number of affected cattle herds is in the single digits, and fewer than 100 human cases get diagnosed annually. But because culling every bison and elk in a national park was never an option, and because there’s no effective method for vaccinating wild animals, 50 percent of the bison herd still carries brucellosis. The greater Yellowstone area is the last reservoir of the disease in the country.

The danger of brucellosis is how effectively it spreads. “When an infected cow aborts a fetus, the other members of the herd get curious,” Marty Zaluski, Montana’s state veterinarian, tells me. “They come to investigate, and they sniff, lick, and nibble the tissue.” For ranchers, ­allowing ­Brucella-carrying buffalo anywhere near domestic cows is a risk they are just not willing to take.

The handful of documented ­brucellosis transmissions to cattle in recent decades have been tied to elk. Yet there are no constraints on the movement of elk outside the park. Some argue that’s due to breeding habits. Bison congregate during calving, when the risk of transmission is highest, while elk tend to separate their newborns from other animals. Others, like wildlife biologist Bailey, argue that the issue of brucellosis is just a straw man for a cultural clash. “This politically successful tactic,” he writes in his book American Plains Bison: Rewilding an Icon, “has been used to avoid the embarrassing argument that public bison will compete with private livestock for public forage on public land.”

Zaluski doesn’t embrace the slaughter, but he’s also against relaxing the rules. “It’s like saying, ‘My dog never gets out of my yard, so I don’t need to keep this fence.’ In that regard, we’re victims of our own success.”

The consequences, Bailey explains, are a slow but steady domestication of the genome through disease management, and demonstrable changes to the way bison move throughout the park. Hunting and capture operations on Yellowstone’s western boundary over several generations might explain why fewer bison now head that way each year. Buffalo learn migration paths from the previous generation. “Mobility, which gave bison the opportunity to utilize a large diversity of habitats in response to seasons and weather,” is part of what defines their wildness, Bailey says. But when nearly all the animals that try to take a certain route die, that aspect of wild behavior dies with them.

At mile marker 10 on highway 89, near ­Gardiner, there’s a bend in the Yellowstone River. For 50 years, Hank Rate, an Iowa-born rancher with a Harvard degree and a U.S. Forest Service background, has lived here with his family and a few dozen head of cattle. He came to this place, which he calls “the Serengeti of the temperate zone,” to live among wild things. I have to interrupt our conversation to stand in amazement on his porch, watching two bald eagles fight over a fish.

Rate doesn’t mind being pressed up against the bison, and he’s never worried much about the threat of brucellosis. As he sees it, this is the ­buffalo’s land; he’s just living on it.

And in recent years, Rate’s actually watched tolerance for bison start to grow in Montana. In late 2015, Governor Steve Bullock designated a year-round buffalo buffer zone outside Yellowstone’s northern and western entrances, where the law says they can’t be picked off or hazed back into the park. The territory includes tracts of public land surrounding Rate’s home, and one by one his neighbors responded by moving their cattle herds away.

Rate recognizes that his inherent comfort with wildness doesn’t work for everyone. Two-thousand-pound animals capable of charging at nearly 40 miles per hour aren’t all that compatible with the highways and housing developments that accompany humanity’s relentless sprawl. But if allowing the Yellowstone bison to roam just a little farther means they hang on to the natural behaviors that make them so special, well, Rate figures that’s an OK compromise.

Alternatively, Geremia, the park biologist, believes the best way to really preserve this wild herd is to use them to start new ones in other places. “Then maybe my son will always be able to see them,” he tells me. He and his colleagues have been trying for years to establish a quarantine program at Stephens Creek. Ideally, an agreement with Montana’s Department of Livestock would allow wild bison that repeatedly test negative for brucellosis to be relocated in the state and around the West, on enormous acreages far from cattle operations, where they can thrive and migrate freely. But progress on all fronts is slow.

Millions pilgrimage to Yellowstone every year to see the bison, their restoration touted as one of the great conservation success stories of the last century. But evolution marches on, and the next generation of visitors may find a herd that’s been intrinsically, irrevocably altered. The people who have dedicated their lives to the buffalo—studying them, conserving them, and, yes, helping to ship them to slaughter—are hopeful we won’t let it get that far.

Because we have Yellowstone, we still have a shot at preserving this animal’s wild nature. To really save them, it’s people who will have to do the evolving: learning to live in concert, not conflict, with the bison. “I hope we can do it,” Geremia says. “I think there are ways, where there’s enough land. There are other places where bison can be bison.” For now, at least, safe in the national park, the last ­living symbol of America’s Wild West abides.

This article was originally published in the Summer 2019 Make It Last issue of Popular Science.