JACKSON (AP) - The National Park Service needs to investigate whether a subspecies of bison might still roam Yellowstone National Park, a former park ranger said.

Bob Jackson, a critic of the Park Service and former Yellowstone ranger, says mountain bison lived on the high-elevation Mirror Plateau south of Lamar Valley for thousands of years before the plains bison were reintroduced to the park.

Though the bison have likely interbred, he said, the Mirror Plateau bison likely retained much of their genetic heritage and their "culture," which has enabled them to survive for thousands of years without leaving the park.

"This is an animal that doesn't go outside of Yellowstone because they are so wary," Jackson said of the mountain bison.

The mountain herd numbers roughly 300 animals and spends winters in Yellowstone, he said.

But outfitter camps set up in and around the area where the bison roam could be disturbing the herd, he said.

Jackson, who is a private bison rancher, said Yellowstone biologists need to step up their research on the Mirror Plateau to ensure the mountain bison's long-term viability. Further, he said wildlife managers need to give the herd access to more backcountry areas.

Yellowstone spokesman Al Nash said recent reviews of scientific literature suggest that there is no difference between the Mirror Plateau bison and reintroduced bison.

"There was a time when scientists thought there might be two subspecies, the mountain bison and the plains bison," Nash said. "But most scientists really don't see that distinction any more."

Still, Nash said researchers are conducting genetic sampling of the bison on Mirror Plateau.

"And we do have some bison from the different herds or smaller groups that are radio collared," he said. "We are working to gather data on their movement to see if this small group does indeed interact with other groups, especially during the rut, and to see if these animals move outside the park boundaries."

"We don't think there's a separate mountain bison, but there are ongoing questions about how these groups interact," Nash said.