On a cold, sunny day on the treeless plains of north-central Montana, hunters close in on their prey. Several bison and their calves watch nervously as a pickup truck slowly circles them, a rifle pointed out of the passenger window. A shot rings out, and a few minutes later, a young bison calf plops down on the ground, grunting and squirming. The hunting party — a team of biologists — moves in, warily eyeing the larger bison, eager to get a blood sample and move away from the agitated creatures. Once they fill a giant plastic syringe, they give the calf a shot, and it stands up on wobbly legs and staggers back to the herd.

Welcome to the American Prairie Foundation preserve, the front lines of the efforts to save America’s bison and restore a large swath of the North American Great Plains. The bison is often heralded as the nation’s first and greatest conservation victory — in the last century the population grew from fewer than 1,000 to half a million — but the story is not that simple.

In the late 1990s, James Derr, a geneticist at Texas A&M, discovered that most of the roughly 500,000 bison in North America have a tiny amount of cattle DNA mixed into their genome — the consequence of ranchers crossbreeding bison and cattle a century ago. The revelation that all but 10,000 bison are hybrids shook the conservation community. In 2004, the American Prairie Foundation learned that their bison, which today number 45, are among the few that are pure. Others haven’t been as lucky. When Derr’s discovery came to light, scientists realized that conservation efforts then underway wouldn’t ensure the survival of genetically pure bison. Extinction is still a threat. Now conservationists like those in Montana are hunting bison in a benign but urgent sense because saving them goes hand in hand with saving the prairie, the world’s least protected terrestrial ecosystem. So while the species was almost decimated by the Great Hunt, their survival depends on this Next Great Hunt. 

Curt Freese, a researcher who has studied bison genetics, is head of the World Wildlife Fund’s Northern Great Plains Program. Freese was instrumental in setting up the foundation’s preserve. Though the long-term consequences of hybridization are unknown, he says, “if you have a chance to maintain them as pure, it makes sense. Once the genes are in there, there’s no getting them out.”

Scientists say that only the purest bison — those most similar to the ones that made up the massive, rambling herds of yesterday — should be used to repopulate the Plains. (Though biologists are reluctant to use the word pure because genetic markers might be present at levels current technology can’t detect, today’s tests find some bison to have no cattle genes.) Maintaining the bison’s genetic purity will help ensure the survival of the species and pave the way for the animals to reassume their historical ecological role. These herds are critical to rehabilitating large, landscape-scale prairie habitat because other species rely on their grazing for survival. Cattle genes, which make up less than one percent of the genome of most affected bison herds, could influence a multitude of traits, including the bison’s ability to resist disease, withstand cold weather, and migrate. The truth is no one knows what the effects of hybridization will be. But when it comes to the future of the bison, no one wants to be surprised.

The Great Plains is now considered one of the most degraded ecosystems on the continent. Cattle grazing under fence, irrigation systems converting grasslands into croplands—a century of these and other agricultural practices has taken a serious toll, depleting soil and aquifers. The wildlife hasn’t fared much better. Species such as wolves and grizzlies were completely pushed off the Plains and into the Rocky Mountains, and many of those that remain are dwindling: Approximately 128 Plains species are listed as endangered.

Restoring the Great Plains

The idea of restoring the Great Plains surfaced two decades ago. In 1987, academics Frank and Deborah Popper argued that the best use of the region was turning it into a vast Buffalo Commons, bringing back the great bison herds that once swarmed the prairie. This, they said, would reestablish biodiversity and create a sustainable economy, attracting tourists and hunters.

The American Prairie Foundation is just one of the organizations working to reclaim the Plains. The Nature Conservancy, the World Wildlife Fund, Ted Turner, and the Popper’s Great Plains Restoration Council, as well as state and federal agencies, have set aside vast acreages of grasslands in North America, especially in the northern Plains, and many are trying to bring back bison. “Of all the grasslands, the northern Great Plains stood out globally,” Freese says. “It’s one of the few places where most of the grassland hasn’t been plowed, and the native prairie is fundamentally intact.”

In 2004, the American Prairie Foundation, which is affiliated with the World Wildlife Fund, bought its first ranch near Malta, Montana; it now owns five. So far the group has blocked up more than 70,000 acres of deeded and leased land. The Foundation hopes to connect the acreage they control to protected federal land, creating a refuge of several million acres (larger than Yellowstone National Park). This contiguous stretch of land would allow bison to roam freely as they did in the days before the Great Hunt.

When settlers came to America’s Great Plains, the grass was crawling with wildlife: bison, birds, antelope, deer, elk, wolves, grizzlies, prairie dogs. Bison, the most emblematic of prairie species, were present in numbers so great, they looked like a vast, brown inland ocean.

Bison are more than symbols of the Plains: They’re a keystone species, critical to biodiversity. Herds of the wild, free-roaming animals mow the grass low in some places and leave it longer in others, benefiting a range of birds with different habitat needs. Prairie dogs follow the herds and graze the grass further to build elaborate, socially complex towns of thousands of individuals. By digging and aerating the soil—and creating habitat for burrowing owls, snakes, and insects—prairie dogs extend the ecosystem underground. The creatures also fill most of the shelves in the Great Plains grocery—eagles, coyote, and swift fox hunt them. The highly endangered black-footed ferret depends on prairie dogs for shelter and food: the ferrets live in prairie dog burrows and attack them while they sleep.  

“We need to let the bison graze on the landscape to create these complexities, this mosaic of grazing intensity,” says Freese.

Bison also helped shape the landscape and cycle nutrients throughout the ecosystem. For instance, the animals created wallows — saucer-like depressions — by rolling on the ground. “These became mini-wetlands and had a diversity of species within them,” Freese says. When wild bison died, they became food for scavengers that then scattered a nitrogen-rich deposit, fertilizing the land and nurturing a diverse habitat.

But the U.S. government viewed the enormous herbivores, a staple for many Native American tribes, as a barrier to western colonization. So it launched the Great Hunt. Bison were hunted out of existence all across the southern Plains by the 1870s.

By 1880, railroads had pushed into the Northwest, and the hunters set to work wiping out bison in Montana, Wyoming, and the western Dakotas. In 1882, Montana and the Dakotas shipped 200,000 hides to tanneries in the East. By 1884, just 300 hides were collected.

Tens of millions of bison had been reduced to fewer than 1,000, and the species teetered on the precipice of extinction. About two dozen were hanging on in Yellowstone National Park, and another 250 survived in Wood Buffalo National Park in Canada.

Some of the cattle ranchers who owned the rest hypothesized that a cross between native bison and Asian cattle would make a more disease-resistant animal. They began breeding the two species to produce a cross they called cattalo, which has proven over time to be a less robust animal than either of its ancestors.

Descendents of the cattalo

Today, many U.S. bison are descendents of the original cattalo, which agencies and individuals used to restore herds across the country. As a result, the vast majority of bison have a smattering of cattle in the family tree. The few remnant populations of pure bison are scattered, from Ted Turner’s Vermejo ranch in New Mexico to Wind Cave National Park, where the American Prairie Foundation’s bison originated. But the extent of the phenomenon was unknown until efforts got underway to restore the Great Plains.

Bison testing, like that done on the American Prairie Foundation preserve, continues to yield surprising results. In September, DNA analysis conducted by Derr confirmed that the bison of California’s Santa Catalina Island, long thought to be purer than those on the mainland because they have lived in isolation since the 1920s, are hybrids. A microscopic pinch of cattle genes might seem unimportant, but it could result in the extinction of the North American bison. Even if thousands of bison remain, if they all have a sprinkling of cattle genes, there would be no pure bison genome—they would become what biologists call “genomically extinct.”

While some might argue that it’s a question of semantics, the missing bison genes in the hybrids could have a real impact on the species. “You can’t see any difference visually” between bison that have the cattle genes and those that don’t, says Kyran Kunkel, a World Wildlife Fund biologist who works with the American Prairie Foundation. “But we don’t know what the long-term ecological or biological impacts would be.” Scientists do have a few guesses, though. They know that crossbreeding has led to less fit animals, rather than the desired  “hybrid vigor.” The missing genes could affect the ability to digest certain plants or resist a disease, says Freese. “If cow genes became too common in the mitochondria of the cell, which affects metabolism, they could affect the adaptation to cold weather,” said Freese.

Bison are threatened in other ways. Biologists say bison are in effect “ecologically extinct” because they no longer act in their former ecological role—migrating long distances, for example, or gathering in huge herds. That means that even if large numbers of the purest bison are restored to giant swaths of prairie, old behaviors might be absent, lost through generations. If restored bison don’t roam in massive herds, nourishing the ecosystem, it could make the restoration of the Plains incomplete.

The widespread domestication of bison could also be driving these types of behavior changes, says Freese. Ranchers select for certain traits all the time, which is akin to man-made evolution. “Ranchers might get rid of a cantankerous bull, for example,” says Freese. “Breeding bison to be docile and meaty are the kinds of things that affect the wildness of the species.”

Dave Carter, executive director the National Bison Association, the trade group for people who raise bison, says that isn’t so. “The ranchers I know don’t want their animals to be docile. [The bison] are equipped to survive, and they want to keep them that way. They don’t want them just to be meat wagons.”

The Next Great Hunt—this time to save the bison species—is well underway. Across the country, biologists are testing bison and gaining a clearer understanding of which herds have polluted genomes. The challenges now involve separating hybridized bison from those with no detectable levels of cattle genes—and making sure they remain separate.

Isolation is the best way to assure purity, but it doesn’t necessarily guarantee the bison genome will remain cattle-free. Yellowstone National Park has the largest of the near-pure herds, with more than 3,000 animals, an invaluable genetic reservoir. Officials there had a scare when a lone bull from a nearby bison ranch wandered into the park, mixing with the herd. They removed him from the population before he could pass on his heritage.

Now the American Prairie Foundation is working to ensure the bison and their offspring remain pure, as they are seed stock for what they hope will become a free-ranging herd tens of thousands strong. That massive herd may well one day roam the hundreds of thousands—or even millions—of acres the Foundation is buying and rehabilitating. It might be a pipe dream to think that the far-flung refuges currently being developed will connect, allowing the great herds to return. But scientists know the best bet for the future of the Plains lies in the tiniest of things—the genome of the purest bison.

Story by Jim Robbins. This article originally appeared in "Plenty" in January 2008.

Copyright Environ Press 2008