Where the Bison Could Roam

Between 30 million and 60 million bison once roamed parts of the United States, primarily in the Great Plains. They were a “keystone” species in a complex ecological web, creating a cascade of environmental conditions that benefited countless other species.

Intact grasslands are very productive for biodiversity. In part because of the loss of bison and other megafauna, intact grassland biomes are now among the most endangered in the world, and the numbers of many species that depend on them have collapsed.

Agriculture has also taken a huge toll on the prairies. “It’s being plowed up fast and mismanaged for cattle,” said Curt Freese, a wildlife biologist and one of the founders of American Prairie. Nearly a million acres in the eight-county region around the reserve have been converted to cropland in recent years, Dr. Freese said.

The primary task here now, researchers and managers say, is to increase the number of bison and acres. In 2008, more than two dozen ecologists and experts, in a paper known as the Vermejo Statement, estimated that to foster a functioning prairie ecosystem at least 5,000 bison would need to be able to migrate freely on some 450,000 contiguous, fenceless acres.

The first 16 bison were brought to American Prairie in 2005, and their numbers have grown to 774. The reserve has set a goal to settle 6,000 bison on 500,000 contiguous acres, Mr. Heidebrink said. The hardest part of the task, though, has been building up enough land.

Someday, the bison on reservations, American Prairie and nearby wildlife refuges in the United States and Canada, may become one vast herd, roaming across about 3 million acres. Native American communities in Montana, including the Fort Belknap Reservation and Blackfeet Nation, already have herds of their own.