Last Nov. 1, about 400 spectators watched in delight as 10 huge, shaggy bison rumbled out of a holding corral onto 1,000 acres of windy shortgrass prairie, 30 miles north of Fort Collins, Colorado. The fenced grassland here is part of some 32,000 acres of city and county natural areas stretching from the foothills of the Rocky Mountains to the Great Plains. Local managers plan to gradually expand the herd’s range to 2,500 acres as it grows through a combination of natural reproduction and more reintroductions. The herd already has its own Facebook page and, of course, a limited-release commemorative microbrew, Prairie Thunder Imperial Brown Ale.

The release restores the bison to the merest sliver of the species’ vast historic range, and yet it represents a major conservation success. These animals are descended from the bison in Yellowstone National Park, the only population to survive wholesale slaughter by settlers during the late 19th century, and the last major reservoir of bison genes that have not been polluted by cattle DNA from cross-breeding. Yet using them in restoration efforts outside the park has been difficult because many Yellowstone bison carry brucellosis, a disease that can cause cattle to abort or prematurely give birth. The Laramie Foothills herd, however, is brucellosis-free, thanks to novel assisted-reproduction technologies. That makes these bison an early test case for efforts to expand the species’ gene pool outside Yellowstone.

Up to 60 million bison once wandered the plains. The largest land mammal in North America, the bison is now recognized as a keystone species that helps maintain the ecology of grasslands. Their grazing habits influence the diversity of forbs and grasses, and their hooves help aerate the soil. Even their dirt wallows create seasonal habitat for birds and affect how fire moves through grasslands.

Today, there are an estimated 500,000 scattered across the plains but nearly all are managed as livestock, destined to become buffalo burger. Fewer than 21,000 are part of 62 “conservation herds” that are managed for environmental purposes with limited human intervention, and many of those have cattle genes. Even fewer genetically pure animals are considered truly free-roaming and “wild.” Many scientists consider the species to be ecologically extinct, meaning that its functional role in the landscape has been eliminated.

So while the reintroduction of 10 bison in the Laramie Foothills may not sound like that big a deal, genetically pure conservation herds like this one are a crucial step toward restoring wild bison to the Western landscape. They could help calm ranchers’ longstanding worries about disease, and over time new herds have the potential to become self-sustaining populations that more closely resemble historic herds — if, that is, state and local managers are willing to give them room to grow.

Today, Yellowstone is home to 4,900 bison. It’s the largest of four wild populations in North America, and contains 75 percent of the species’ genetic diversity. Every winter, state and federal officials round up most bison that wander outside the park’s borders. Up to 900 are removed annually through hunting or slaughter, largely to prevent the possible spread of brucellosis. They are either hazed back into the park, or quarantined and tested for brucellosis; any infected animals are killed.

These heavy-handed tactics have come under increasing attack in recent years. For one thing, there have been no documented cases of brucellosis transmission from bison to cattle, and even though elk also have been known to spread the disease, they aren’t as aggressively managed as bison. “The (disease) myth has been pretty much debunked now, but the stigma has spread far and wide,” says Steve Forrest, an ecologist for Defenders of Wildlife.

Assisted reproduction may help solve the problem. Jennifer Barfield of Colorado State University adapted techniques traditionally used for livestock, which involve cleaning semen from bison from the Yellowstone bloodline in the lab, and then using it to impregnate disease-free females. She has also collected eggs from brucellosis-infected Yellowstone bison destined for slaughter, cleaned them and fertilized them with clean sperm in the lab. The embryos are then implanted in surrogate bison cows.

This is one way conservationists can draw from the Yellowstone gene pool, while ensuring that newborn calves don’t carry the disease, Barfield says. Already, biologists plan to use male calves from the Laramie Foothills herd to build up other conservation populations, while minimizing inbreeding in the local population.

These steps are important because it’s risky to have so much of the species’ genetic material banked in one wild population. An unexpected disease outbreak or other catastrophic event in Yellowstone could be a significant setback for the entire species. But for conservation herds to become viable populations on their own, scientists estimate they must grow to at least 1,000 animals. And because a herd of that size probably needs at least 100,000 acres to roam, getting there will likely entail letting them mingle with cattle and roam across jurisdictional boundaries.

For now, the only genetically pure and brucellosis-free conservation herds allowed to roam freely on federal public lands in the U.S. are in South Dakota’s Wind Cave National Park and Utah’s Henry Mountains. Both herds are descended from Yellowstone bison, but managers have limited the populations to 400 or fewer animals.

“Every little herd is important in its own way,” says Montana State University wildlife ecologist Dustin Ranglack, who has studied the Henry Mountains herd. Ranglack has found that, due to eating and grazing habits, the bison rarely compete with cattle for forage, another common misperception. “But for an ecological future for bison, we need relatively large herds on large landscapes,” he says.

The most promising space for large-scale bison recovery is in eastern Montana. Along the Missouri River, the Charles M. Russell National Wildlife Refuge encompasses 915,000 acres. The refuge is slightly smaller than Delaware, and even today, Lewis and Clark would probably recognize its windswept grasslands.

The National Wildlife Federation has already bought out grazing allotments on 54,000 acres of the refuge, with the idea that bison will replace cattle when — and if — state managers who have jurisdiction over native wildlife approve. The neighboring Fort Belknap and Fort Peck Indian reservations are also using Yellowstone bison to build their own herds and help recover the species.

Perhaps most significantly, the American Prairie Reserve has already leased or purchased 307,000 acres of public and private lands adjacent to the refuge and plans to acquire a total of 500,000 acres. The reserve’s lands now support 620 bison, with spring calves on the way, and spokeswoman Hilary Parker says the group hopes to eventually run 10,000 animals.

But the state of Montana has been reluctant to openly support the reintroduction or expansion of bison herds, either in the refuge or around Yellowstone. The idea of a new, publicly managed wild herd is being floated in the state’s draft bison conservation plan, released last year. But the draft failed to identify any possible sites, or explicitly endorse a large and free-ranging bison population. To support just “another small ‘display’ herd of bison on a confined pasture,” Defenders of Wildlife’s Forrest says, “will be a colossal failure of planning.”

In the Laramie Foothills, the small herd is currently fenced in to avoid close encounters with hikers, mountain bikers and wildlife watchers. That won’t change even when managers expand the range in a few years. Still, a long-term, regional conservation plan for the foothills encompasses the natural areas, as well as surrounding federal and state lands, conservation properties and additional open space. It could eventually follow the American Prairie Reserve’s example and open a much greater section of public and private lands to a much larger bison herd.

Other Western conservation herds are also growing and gaining attention. Sixty miles south of Fort Collins, federal managers at the Rocky Mountain Arsenal, an Army weapons complex turned wildlife refuge outside of Denver, plan to double its herd size to 180 or more animals on up to 12,000 acres in the next four years. That herd has also proven that bison are a tourist attraction: 330,000 wildlife watchers flocked to the arsenal in 2015.

To further boost support for the species, some scientists and conservation groups are pushing for hunting seasons in states with growing conservation herds, even as managers are trying to increase their numbers. Ranglack believes that could encourage rural communities and wary state managers to regard free-ranging bison as an asset instead of a threat.

“It’s been so long since we managed these species as wildlife,” says Steve Woodruff, a senior policy manager for the National Wildlife Federation. “We’re never going to see millions of bison again across the West, but there are still some places where we can have wild bison along with a healthy cattle industry. There’s plenty of land to do both.”