Now, a small, genetically pure, disease-free, free-ranging herd in the Henry Mountains in Utah has scientists and conservationists excited about the future. The development could be instrumental in bringing back some of the species' splendor.
"A pure bison is genetically a genuine descendent of the original Plains bison that used to roam North America," said Utah State University Professor of Ecology, Dr. Johan du Toit.
"Most of the bison alive today in North America are essentially hybrids. They're a mix in some way of bison and cattle genes," du Toit said.
Over the 19th and part of the 20th centuries, ranchers confined and crossbred bison with cattle in the hopes of creating livestock with the bison's drought-resistant traits and cattle's docile nature, according to du Toit. Some thought bison might therefore crossbreed with cattle in the wild if given the opportunity.
Utah State and Texas A&M University researchers collected genetic samples over several years.
Their efforts were supported by staff from the Utah Division of Wildlife Resources and the Bureau of Land Management.
Despite the fact that the Henry Mountains herd has been grazing freely and side-by-side with cattle for decades, the genetic research indicates the animals have not crossbred with cattle. This means bison may be able to be managed in a mixed grazing system in other parts of the country, giving the Henry Mountains herd both the genetic pedigree and the "source herd" potential that most bison don't have.
"This hybridization issue is purely a function of humans forcing it. Under natural conditions it just does not happen," du Toit said.
The Henry Mountains herd is also disease-free, showing no signs of brucellosis, a crucial factor in determining the long-term conservation potential value of a herd.
Brucellosis is a fast-moving bacterial infection that affects bison, cattle, other animals and humans. It can cause both bison and cattle to abort their calves, said Bill Bates, wildlife section chief for the Utah Division of Wildlife Resources.
"It's important in the state of Utah that we have a brucellosis-free status for our livestock industry," Bates said.
Unlike the vast majority of the estimated 500,000 bison around the country, the herd in the Henry Mountains is free-ranging. There are no fences. The animals go where they want to, when they want to.
"All of the movements that these animals make through the seasonal cycle are in ecological response to the landscape that they're moving through and not any other human pressures," du Toit said.
"There's no supplementation, they are not provided with anything by management."
This has opened the door to studying their responses to seasonal vegetation variations.
Controlled through hunting
The population of the herd is managed through hunting. According to Bates, the Henry Mountains population of about 350 has a high survival rate and produces many calves each year.
"We need to keep bison numbers in balance with the available habitat," Bates said.
"We're most concerned about a healthy ecosystem, a healthy range. We have livestock, we have bison, we have other wildlife species, and we don't want to hurt the range's resources."
A small number of tags are issued every year to sport hunters. They then get the opportunity to hunt a bison, and in so doing help control the population size. The money raised from hunting also helps offset some costs associated with managing the herd and the land, Bates said.
A unique herd
Transplanted from Yellowstone National Park in the 1940s, the original herd of about 20 animals made its way to the picturesque Henry Mountains and has lived there since. The land is filled with vegetation, woodland and other wildlife, and is surrounded by baking-hot desert.
Du Toit said the herd has all the genetic potential and hardiness to be a strong source herd well into the future.
"This species was given a very hard deal by the European settlers that moved across the Great Plains, and here we have a remnant, almost a refuge population," du Toit said.
"This bison population provides a source of inspiration to people who are interested in ... restoring some of the splendor of the wildlife that used to occur on this continent. Particularly for a species that has played such an important part in the history of the indigenous people of North America."
A few other genetically pure herds exist, notably at Wind Cave National Park and at Yellowstone National Park.
After a mulityear effort by senators, members of Congress and dozens of advocacy and conservation groups, U.S. President Barack Obama named the bison the American "national mammal" in April.
Though the species once grazed the plains of most of the United States in the millions, its population dwindled to a little over a thousand at the turn of the 20th century, according to a highly cited study published by the late William Hornaday, a taxidermist, hunter, zoologist and first director of the New York Zoological Park, known today as the Bronx Zoo.
Hornaday's study noted that this decline coincided in large part with the mass slaughter of the animal during the years of the United States' westward expansion.
Through preservation efforts led by environmentalist and former U.S. President Teddy Roosevelt, the American Bison Society, legislators, the Bronx Zoo and hundreds of people across the country, the bison was slowly but consistently reintroduced into the wild, according to a Wildlife Conservation Society historical analysis.