In October 1983, ahead of an unusually harsh winter, groups of pronghorn in south-central Wyoming began what should have been a routine journey to their sage-freckled winter range on the Red Rim near Rawlins.

But a newly completed, five-foot-tall, 28-mile-long woven wire fence blocked the way. Rancher Taylor Lawrence said he'd erected it around the Red Rim to keep the pronghorn from competing with his cattle for forage, though some suspected his motive lay beneath the soil in unmined coal seams; wildlife can be an obstacle to development. Whatever his reason, it worked: Pronghorn have many athletic talents, but jumping isn't one of them. And when they can't scoot under a fence, they rarely try to go around it. Soon, some 1,500 of them were crowded behind the barrier.

By December, when Lawrence relented and opened the fence to let the animals through, an estimated 700 had died. Video footage of their carcasses bedded down against the fence made national news. A court case over the legality of the fence -- which enclosed both public and private land -- went all the way to the U.S. Supreme Court, which ordered Lawrence to take it down for good.

The incident is a dramatic example of a common problem: Private lands and their owners are not always hospitable to wide-ranging wildlife. And protected public swatches simply aren't enough to support animals on the move. The "Path of the Pronghorn" -- the route that a few hundred pronghorn travel between their fawning grounds in Grand Teton National Park and their winter range near Pinedale, Wyo. -- crosses only a few short stretches of private land. Still, fencing and subdivisions are among the key threats to the corridor's continuity. If the development squeeze gets too tight, pronghorn could stop making the trip altogether.

"In the greater Yellowstone area, almost every species spends a key piece of their life cycle on private land," says Luke Lynch, Wyoming state director of the Conservation Fund. "You've got the high, dramatic lands protected, but that's not what wildlife need for year-round survival." Bison stray beyond Yellowstone National Park in the spring to calve and graze on lower-elevation private land. Mule deer, elk and pronghorn often winter on valley floors and other privately held low-lying areas. Many pathways between core habitats for grizzly bears, wolves and mountain lions also lie on private land. And it's not just far-roaming species that rely on private property: The vast majority of the species protected by the Endangered Species Act are at least partly dependent on it for habitat.

In some places, environmentalists or public agencies have identified important corridors, and then worked to enlist landowners in protecting them. But from the Mexican border to the Crown of the Continent, where the U.S. and Canadian Rockies meet, a growing number of landowners are showing their own initiative, banding together to protect working land from development, and to link migration and dispersal routes across jurisdictional boundaries. Many conservationists say that the future of large-landscape conservation lies in these efforts.

"It's only going to come about through partnership, by stitching things together at the local level," says Joel Berger, a University of Montana professor and Wildlife Conservation Society scientist whose work was instrumental in protecting the Path of the Pronghorn. In the U.S., says Berger, the best opportunity to stitch together the pieces of a vast and intact ecosystem is in the Northern Rockies. "Because we have space, low population and a lot of public land, the complexities that go with private lands and higher human densities are lacking."

Among the landowners leading the charge are Brian and Kathleen Bean, a San Francisco-based couple who, in the late 1990s, set out to become "conservation buyers" -- landowners, usually well-heeled, who purchase property primarily to protect it. The Beans settled on 24,000 acres in the foothills of the Pioneer Mountains in central Idaho, a place they call the Lava Lake Ranch. Between their private acreage and grazing leases on public and private land for their sheep operation, the area they steward is just shy of 900,000 acres.