When the new West is won, will there be cowboys? In light of what her neighbors are up to, Double O Ranch owner Vicki Olson isn’t so sure. “I guess the point that I keep hammering at is that if they succeed, that means all of us third- and fourth-generation ranchers are gone,” Olson said. She is the average Montana rancher, 56 going on 70, working a spread gouged from the pebbly soil by her grandparents 100 years ago. Her neighbor, the nonprofit American Prairie Foundation, is methodically acquiring ranches and crafting a 3.5-million-acre wildlife reserve out of private property...