Texas researches have concluded that Yellowstone’s bison herds — which fluctuate between 4,000 and 6,000 animals — have merged into a single breeding population.
That doesn’t mean the Yellowstone bison are in imminent danger of becoming inbred, but the herd’s genetics could eventually suffer if current trends continue, an ecologist told Cowboy State Daily.
The Yellowstone herd’s prime genetics might be saved by transplanting bison to places like Montana’s Missouri Breaks country or Wyoming’s Red Desert, said George Wuerthner.
Wuerthner has worked in the Greater Yellowstone Ecosystem for decades and is president of the Montana Wild Bison Coalition.
“It (the Red Desert) is one of the largest areas of public land without fences,” he said. “Migration and movement are the way that bison have survived in the long term.”
The long-accepted narrative is that Yellowstone has two unique bison herds, the central herd and northern herd, with very little genetic exchange between them.
However, in a recently published study, researchers from the Texas A&M College of Medicine & Biomedical Sciences argue that for all practical purposes, the Yellowstone bison have merged into a single herd.
When contacted by Cowboy State Daily, the Yellowstone office of the National Park Service declined to comment.
Genetic Treasure
The researchers concluded that, at least for now, the Yellowstone bison are genetically healthy.
Wuerthner said he and other bison conservation advocates worry that current trends and management practices, which include hunting and/or rounding up and slaughtering bison as they leave the park, could eventually ruin the Yellowstone bison’s genetics.
The Yellowstone bison are a genetic treasure, the Texas A&M researches state in the abstract for their study.
“Yellowstone National Park is home to the only plains bison population that has continually existed as wildlife, on the same landscape, through the population bottleneck of the late 19th century,” according to the abstract.
“Nevertheless, by the early 1900s, only 23 wild bison were known to have survived poaching. Salvation efforts included the addition of 18 females from Montana and three bulls from Texas to augment this population,” the researchers added.
Natural Selection
Wuerthner agrees that the Yellowstone bison population is unique and incredibly valuable.
“The Yellowstone herd is the least-domesticated herd we have in the Lower 48,” he said. “I think they’re unique, genetically, for that reason.”
Other herds in the Lower 48 have been “domesticated” by being fended in, protected from predators and given supplementary food during harsh winters, he said.
The Yellowstone bison have lived under more natural conditions, including being picked off by predators. Or, by freezing or starving to death during harsh winters.
The epic winter of 2022-2023 cut a swath of death though wildlife populations in Wyoming. That included Yellowstone bison, many of which were pushed out of the park by brutal conditions, only to be killed by hunters.
The harsh tests of natural selection are reflected in the strength and purity of the Yellowstone bison genes, Wuerthner said.
Bison Leave The Park, Run Into Trouble
Bison leaving Yellowstone has long been a source of controversy, Wuerthner said.
Typically, the central park subpopulation has pushed out through West Yellowstone, Montana, while the northern Park Bison head out by the north gate near Gardiner, Montana.
Montana ranchers are concerned that bison could spread brucellosis to their cattle.
But Wuerthner argued that there doesn’t seem to be much concern about brucellosis coming from Yellowstone elk, which might actually be more prolificcarriers of the disease.
Brucellosis causes pregnant bison, elk and cattle to spontaneously abort their fetuses.
Ranchers worry about it, because maintaining a “brucellosis-free” status among their livestock is vital to the transport, sale and trade of cattle across state lines.
Wuerthner said he and other bison advocates think that the threat of brucellosis transmitting from bison to cattle has been overblown, and used as an excuse to cull Yellowstone bison — either through round-ups and slaughter, or hunting on lands adjacent to the park.
Such culling puts the Yellowstone bison at risk for more “genetic bottlenecks,”particularly if prime breeding bulls are taken out of the population, he said.
Bison are “tournament breeders,” he said. Meaning only a few of the strongest bison bulls that fend off competitors breed with most of the female bison.
Eliminating those dominant breeding bulls could weaken the Yellowstone herd’s strong, wild genetics, Wuerthner said.
Room To Roam
As far as how many bison are enough to keep the Yellowstone herds genetically healthy, Wuerthner said some biologists think “there is no minimum.”
If bison were better-tolerated on national forest land near Yellowstone’s borders, the landscape might be able to support 10,000 of them, he said.
Bison have a deep instinct to roam, he said. It’s their built-in survival mechanism.
Particularly before Native American hunters had firearms and horses, bison could escape over-hunting by moving around, he said.
The Red Desert and Missouri Breaks both offer vast expanses of public land. And, those lands at lower altitudes than Yellowstone, he said, which would give bison a chance to thrive.
“Free movement is really critical to them,” he said.
Mark Heinz can be reached at mark@cowboystatedaily.com.
