Genetic Analysis Shows Just One Interbreeding Bison Population In Yellowstone National Park

A new round of genetic analysis has concluded that there is just one interbreeding bison population in Yellowstone National Park, not two genetically distinct populations.

The question of whether there were two genetically distinct herds first arose in 2012 after wildlife biologist Natalie Halbert theorized that the Yellowstone's Central Plateau and Northern Range bison herds were genetically separate. That theory became part of a legal battle over whether the park's bison needed to be listed as a threatened species under the Endangered Species Act.

Back in 2014, the Buffalo Field Campaign and Western Watersheds Project petitioned the U.S. Fish and Wildlife Service to extend "threatened" or "endangered" designation under the ESA to Yellowstone's bison. In a somewhat lengthy petition of more than 60 pages, the groups argued that Yellowstone's bison are "the largest remnant population of the Plains bison that ranged across much of United States until it was eliminated post-settlement."

The Fish and Wildlife Service opted in December 2015 not to extend such protection, holding that there was no "substantial scientific or commercial information" to justify such a designation and that bison numbers in the park were growing and so it wasn't going to do a "status review" on whether bison were doomed. But in January 2022 the Buffalo Field Campaign and Western Watersheds Project renewed their legal battle over that question.

The two groups had found Halbert's study and raised the question into whether, if there were two distinct herds, the National Park Service needed to be managing for 6,000 bison in Yellowstone, not 3,000? While the Fish and Wildlife Service again disagreed with the conservation groups, a federal judge in January 2022 once again said the federal agency didn't properly examine the issue and ordered the wildlife agency to re-examine, for a third time, the groups' petition.

U.S. District Judge Randolph D. Moss at the time did not rule one way or another on whether Yellowstone's bison merit ESA protection, but he did say the Fish and Wildlife Service needed to do a better job of explaining why it doesn't think they do.

While the Fish and Wildlife Service has yet to rule the ESA petition, the latest genetic analysis dismisses the theory that there are two distinct bison populations in Yellowstone.

"Even though there are multiple breeding herds and clear evidence of historical bison lineages, it appears substantial gene fow is occurring throughout the population," the study's conclusion states. 

The paper was prepared by Rick Wallen and P.J. White from the Yellowstone Center for Resources at the national park; and Sam Stroupe and James N. Derr from the Department of Veterinary Pathobiology at Texas A&M University.