The battered white pickup truck is bouncing across a pasture of sagebrush and alfalfa when Bronc Speak Thunder turns the steering wheel east and points to the far side of a creek bed. Scattered across a grassy slope are what appear to be a field of brown boulders — until they rise onto furry black legs and turn their shaggy heads in our direction. In another minute, the sun captures them in full relief, and we can make out the curving horns and distinctive humped backs of the American Plains bison. Bronc turns off the engine. The only sounds are the fluty singsong of meadowlarks and the whoosh of wind across miles of Montana prairie. The thirty cows stand silent, staring at us in unison, while a dozen cinnamon-colored calves frolic around their legs. For the calves, it's just another day of play in a spring ritual as old as the prairie itself. But for Bronc Speak Thunder and his tribe, these animals represent the future of an epic experiment. They are offspring of the historic bison of Yellowstone National Park, the largest wild bison herd on earth and the last direct descendants of the herds that once roamed the Great Plains and defined the American West.

They also mark one victory in an emerging struggle to see if two endangered populations — Plains Indians and bison — can recover in unison after two centuries of devastation at the hands of European settlers.

Speak Thunder's reservation, the Fort Belknap Indian Community in north-central Montana, is one of a handful of Plains tribes that are adopting wild bison herds, hoping to revive the fading culture of their ancestors while also helping rescue an animal whose history is so deeply intertwined with their own.

"Our lives ran parallel in American history,'' said Mark Azure, president of the Fort Belknap tribal council and a patron of the bison project. "We were both run almost to extinction.''

The tribal experiments, in turn, are part of a larger campaign by conservationists and biologists to save bison as a wildlife species and, by returning them to large swaths of the Great Plains, restore the region's valuable prairie ecology.

The Plains bison, or buffalo, have been described as the largest species extirpation in world history. Until the 19th century their numbers ran beyond counting — estimates range from 20 million to 75 million — and they traveled in herds so vast that pioneer witnesses describe stampedes that lasted more than 24 hours. A century of slaughter pushed them to the edge of extinction, to perhaps as few as 1,000 by 1900.

Now, their restoration as a wild animal would represent one of the most dramatic species rescues on record.

These efforts reflect a nascent but fundamental shift in the way Americans think about the natural world — a repudiation of the idea that the West was there to be won and that nature is something to be conquered.

"Over 150 years, we tamed a lot of wilderness and wiped out a lot of species,'' said Jonathan Proctor, Rockies and Plains program director for Defenders of Wildlife. "Then we realized it was a mistake, and now we're trying to bring them back.''

Protecting the herd

The sun is just rising over the Absaroka range near Yellowstone on a cool May morning, but already Stephany Seay is on her walkie talkie. Seay is a patrol coordinator for the Buffalo Field Campaign, a group of mostly volunteer conservationists who have established themselves as the protectors of Yellowstone National Park's great bison herd.

Early each spring scores of bison, hungry after a winter of foraging grass under Wyoming's deep snows, wander down from the park's high elevations to graze and give birth. Following migratory instincts laid down though centuries of evolution, they often roam beyond park boundaries. Every spring, rangers from the Montana Department of Livestock and the National Park Service saddle up to haze them back into the park. And each spring for 18 years, the Buffalo Field Campaign has set out to document the roundup — and its sometimes cruel results — while building a case that bison should be left free to roam.

This annual rite is a source of long-standing friction between the nation's conservationists and Montana's ranching community. But it signifies a remarkable success in wildlife preservation. The Yellowstone herd, started with a few animals by Teddy Roosevelt and William Hornaday early in the 20th century, has since grown to a herd that numbers between 3,000 and 4,000. By 1990, the herd was so big that its spring migration beyond park boundaries triggered a backlash.

In Montana, where cattle are king and gun ownership is endemic, the solution was simple: Hunters and ranchers would line up along Highway 191 near West Yellowstone and shoot any bison that stepped outside the park.

After a decade, this annual slaughter had triggered a national uproar, and the shooting came to a halt.

Meanwhile, however, the gory showdown had captured the attention of conservationists and Native American leaders. Saving the Yellowstone bison became one focus in a much larger set of restoration efforts across the Great Plains — and a larger movement to save the prairie ecosystem itself.

Bison are what biologists call a "keystone'' species: Wherever they flourish, a large and complicated ecosystem grows up around them. Wild plovers and buffalo birds follow them, eating insects from the dust beneath their hooves and grubs from their droppings; prairie dogs build cities where the grazing bison have cropped the grass short. Soon predators follow — black-footed ferrets, coyotes, wolves and hawks. Where bison carve "wallows'' into the tough prairie turf — shallow depressions where they thrash about in the dust — rain collects and creates temporary watering holes for other animals and habitat for prairie amphibians.

They are also an extraordinary study in environmental adaptation: A bison's fur is 10 times more dense than that of a cow, and snow will collect on its outer coat and sit for hours without melting from body heat. Naturalists in Canada were once taking a winter count of moose and wolves by flying overhead with an infrared camera; an entire bison herd didn't trigger a signal for the lens because their insulation is so perfect.

While North America's bison population has recovered dramatically, to roughly 500,000, all but a few thousand live as livestock, raised by private ranchers and destined for the slaughterhouse at a young agee. Most contain some cattle genes, a result of old cross-breeding experiments, and many are bred for qualities such as meat production and docility.

If bison are to survive as a wild species, the nation needs an entirely different effort, said James Bailey, a retired biology professor from Colorado State University and author of "American Plains Bison: Rewilding an Icon."

"It is the whole animal we try to perceive and preserve for others to ponder,'' Bailey writes. "We cannot preserve wild bison without wildness.''

Renewed focus on bison

That's exactly the sentiment behind projects unfolding across several Plains states. The Nature Conservancy has established about a dozen grassland preserves populated by bison; a nonprofit called the American Prairie Reserve is raising bison as part of an effort to re-create a massive wildlife preserve in central Montana, and the Wildlife Conservation Society is leading the charge to re-establish bison as protected wildlife at several sites. In June, the Obama administration threw its support behind the movement, identifying 20 federal sites suitable for reintroducing bison as a wild species.

But to cattlemen such as Kerry White, a third-generation rancher and Montana legislator, these efforts sound a death knell for a life that he and his family have built over three generations. Alarmed by efforts to expand the range of wild bison, White helped found an organization called Citizens for Balanced Use, which has filed a series of lawsuits to block the Yellowstone bison.

To White and his allies, a bison is 2,000 pounds of trouble — an unruly neighbor that will wreck their fences, trample their fields and upset their cattle. Their core argument is that Yellowstone bison carry brucellosis, a bacterial disease that can cause spontaneous abortions in mammals. Even a few cases in cattle could endanger Montana's certification as a brucellosis-free state and pummel a hugely valuable beef industry.

"We've got nothing against the buffalo,'' White said. "It's a noble animal, a beautiful animal. We just want it managed properly.''

Biologists point out that no case of bison-cattle brucellosis transmission has ever been documented in the field, and argue that the ranchers' real motive is to protect valuable grazing rights and pastures.

Nevertheless, their fierce opposition worked, causing years of gridlock over any efforts to expand the Yellowstone herd. Then, about a decade ago, a biologist named Keith Aune had an idea. Aune, who was a top wildlife official for the state of Montana, had helped establish a small quarantine program at Yellowstone where bison could be studied and tested until proven free of brucellosis. Why not take some of these healthy animals and use them to establish satellite herds outside the park?

Aune's plan won the support of Montana Gov. Brian Schweitzer — a rancher himself but also a friend of the state's tribes and environmentalists and a formidable politician. The first cohort to emerge from quarantine went to CNN magnate Ted Turner, a large landowner in the West and an ardent environmentalist.

But Aune had also worked closely with Montana's tribes: Their reservations offered huge tracts of perfect habitat and many already raised bison herds for cultural and educational purposes. In 2011, two reservations with long experience raising buffalo — the Assiniboine and Sioux tribes at Fort Peck and the Assiniboine and Gros Vent tribes at Fort Belknap — stepped forward with proposals.

"We had the land. We had experience with a bison herd,'' said Azure. "And we have a historical connection to the buffalo.''

In March 2012, the first truckload of quarantine bison arrived at Fort Peck; in 2013, after another bitter year of litigation, a caravan of bison arrived at Fort Belknap from Fort Peck.

"There were crowds of people lined up along the highway and in the meadow,'' Azure recalled. "I wasn't expecting anything — I was just exhausted by the long battle and relieved that we made it. But we came over the hill and into the pasture — they burst into cheers and applause. My heart soared.''

At nearby Fort Peck, wildlife manager Robert Magnan was equally excited. With a supportive tribal council, Magnan had designed an elaborate suite of new pastures and was beginning to imagine an eco-tourism business built around the famed Yellowstone bison.

This year, under an agreement with the state of Montana, Ted Turner returned 145 of the Yellowstone bison, and suddenly the state found itself besieged by people who wanted them. Applicants ranged from the well-funded American Prairie Reserve to the Cherokee Nation to the parent organization of the Bronx Zoo in New York.

On Oct. 16, in a quiet announcement from the Department of Fish, Wildlife and Parks, the state awarded all 145 bison to the Fort Peck tribe.

Magnan was bowled over by the announcement, calling it "a dream come true,'' but quickly set to work building quarantine and testing facilities that will allow Fort Peck, in its turn, to become a breeding source for others who want to raise herds of the storied animals.

"For hundreds of years, the bison kept our people alive,'' Magnan said last week. "Now it's our turn to save them.''