On fields where cattle graze and wheat grows, a group of conservationists and millionaire donors are stitching together their dreams of an American Serengeti. Acre by acre, they are trying to build a new kind of national park, buying up old ranches to create a grassland reserve where 10,000 bison roam and fences are few.

The privately financed project — now a decade in the making — has ambitions as big as the Montana sky, tapping private fortunes to preserve the country’s open landscapes. Supporters see it as the last, best way to create wide-open public spaces in an era of budget cuts, government shutdowns and bitter battles between land developers and conservationists.