American bison symbolize resilience, and today, their thriving population represents humanity’s greatest conservation story. In the 16th century, around 30 million bison roamed the grasslands across the US, but widespread poaching wiped out over 99 percent of their population, and by the late 19th century, only 325 were left. Just 23 bison roamed Yellowstone in 1884.

Thanks to consistent and dedicated conservation efforts, their numbers have rebounded to around 500,000 today nationwide — but this isn’t the end of the great bison story. The next challenge for conservationists is maintaining a genetically healthy population. 

While their numbers have increased, most bison today live in managed or commercial herds, with only a small fraction roaming freely in the wild. Maintaining genetic diversity is crucial for protecting them against diseases and ensuring their long-term adaptability to environmental changes. 

A new study reveals interesting insights into the genetic health of the bison at Yellowstone National Park — the birthplace of bison conservation efforts and the only place in the US where bison have continuously existed.

“There has long been a debate among conservationists about how to best manage genetic diversity in Yellowstone bison,” Sam Stroupe, lead study author and a postdoctoral researcher in the Department of Veterinary Pathobiology (VTPB) at Texas A&M University, said.

Two herds became one

Studies conducted in the last 20 years suggest that the bison population at Yellowstone National Park includes two separate herds, which together comprise between 4,000 and 6,000 individuals. As a result, there has been less exchange of genes between them, which could have led to genetic differences over time. 

Conservationists have been worried that if the two herds remain isolated for a long time, this could lead to inbreeding and harm the genetic diversity. But there’s good news! 

The researchers studied Yellowstone bison from two main groups that breed in the summer and two areas where bison spend the winter. They collected 282 tissue samples and examined changes in bison DNA.  Surprisingly, their analysis revealed that the two herds have merged into a single interbreeding population. 

“The bison in Yellowstone would be better described as initially two subpopulations of genetically distinct lineages that have become a single interbreeding population,” the study authors note.

This means that after 120 years of protection and management, Yellowstone’s bison now mate and mix as one large group. This change would further encourage genetic mixing and lead to improvement in overall population health and resilience.

“This finding certainly has a direct impact on the long-term conservation and management of this iconic bison population,” James Derr, one of the study’s authors and a professor at Texas A&M University, added.

Not one but many factors triggered this change

Yellowstone bison are unique because they are the only continuously wild and genetically pure herd in the US with minimal cattle gene introgression. Most other herds have some degree of cattle hybridization or are managed more intensively.

According to the National Park Service, “a number of Native American tribes especially revere Yellowstone’s bison as pure descendants of the vast herds that once roamed the grasslands of the United States.” 

There are currently 4,000 to 6,000 bison in Yellowstone. Breeding among such a large population is a positive sign, but what made the two herds merge into one? The study suggests that there is not one but many factors that drove this change.

“They become a single interbreeding population through gene flow, population growth, range expansion, response to environmental pressures, and migration instead of a historical divergence then convergence of a single source population,” the study authors note.

Future research focusing on these factors could reveal valuable insights into how the Yellowstone bison population may evolve in the future.

The study was published in the Journal of Heredity.