Officials at the American Prairie Reserve say they have done everything possible to be good neighbors and have not foisted their vision on anyone. They have installed electric fences to ensure that their 275 bison do not roam onto other people’s property. They allow hunting on the land. They lease back some of their land to allow ranchers to graze their cows.

They say they take an understated approach to buying land. They approach families after they have decided to sell, and sometimes negotiate arrangements that let ranchers live or work on their land for years after a sale goes through. Because the reserve project is nonprofit, officials say they can bid only fair-market value and do not artificially drive up property prices.

“It’s a misnomer that we’re paying top dollar,” said Sean Gerrity, the president of the American Prairie Reserve. “There are some properties we’re interested in, but they’re currently priced at above market value and we can’t go there.”

Still, the financial profiles of the reserve’s supporters have created a divide in a county where the average job pays about $25,400, according to Montana State University. The group has several current and retired fund managers and retail billionaires on its board, and counts heirs to the Mars candy fortune as supporters. It has raised a total of more than $63 million in donations and pledges.

Mr. Gerrity estimated it would take 15 to 20 more years to quilt together the patchwork of public and private lands that represent the group’s vision of three million acres of preserved prairie. Right now, the group owns about 58,000 acres outright and has grazing leases on an additional 215,000 acres of federal land.

The reserve’s goal is to revive a landscape that existed when Meriwether Lewis and William Clark passed through in the early 1800s. They have taken down 37 miles of fence. They have replanted some tilled ground with native grasses. They have pulled down barns and sheds and cleared away heaps of trash. Their bison saunter across dirt roads.

“The idea is to open this place back up,” said Dick Dolan, who oversees acquisitions and finances for the reserve. “The vision is to have an ecosystem functioning as naturally as possible.”