Editor’s Note: This story accompanies the May 2016 issue of National Geographic magazine, devoted entirely to America’s first national park. Find more at natgeo.com/yellowstone.

Just a few miles south of Bozeman, Montana, in the rolling foothills of the Gallatin Range, spacious dream homes pepper the landscape. Forty years ago, wapiti, the Shawnee name for elk, poured out of the mountains in December and spent winters grazing in farmers’ alfalfa fields.

Today an ever expanding human footprint weighs on these hills, as it does on many corners of the Greater Yellowstone Ecosystem, where public and private lands intersect. The Greater Yellowstone’s 22.6 million acres include both Yellowstone and Grand Teton National Parks, plus national forests, wildlife refuges, and surrounding chunks of 21st-century America: highways, towns, parking lots, malls, and suburbs.

Dennis Glick, founder of conservation group Future West, assesses the scene.

For the second half of the 20th century, he, like many U.S. conservationists, believed that if anything would destroy the integrity of Yellowstone and neighboring lands, it would be the noose of natural resource extraction tightening around the national park’s borders.

Hard-rock mining, conservationists believed, would foul the rivers. Oil and natural gas wells would fragment wildlife habitat; industrial-strength logging would lay waste to national forests; and livestock grazing on public lands would cause conflicts with grizzly bears, wolves, and other predators.

But Greater Yellowstone survived the era of natural resource extraction that swept across the West for a century. Ask Glick now to identify the most ominous threats facing the ecosystem, and he doesn’t hesitate. “Number one would be the effects of climate change: droughts, big wildfires, and diseases that were never here before.”

But close on the heels of climate change is people.

“It isn’t that we are behaving callously,” he says. “Humankind may very well be loving this place to death.”

View Images This band of 1,400 sheep spends the summer grazing season in the Gravelly Range of Montana. They’re tended by three ranchers along with a sheepherder and two Akbash guard dogs. Constant vigilance replaces bullets as a way of deterring predators that live in and around Yellowstone. Photograph by David Guttenfelder

Visitors from around the world are swarming to the region in record numbers. Last year both Yellowstone and Grand Teton set visitation records, and they are expected to shatter those marks again in 2016. Meanwhile waves of others summoned by an instinct to live closer to nature—“lifestyle pilgrims,” as Glick calls them—are leaving cities and relocating to Greater Yellowstone.

“Greater Yellowstone management is on the map for doing things right,” Glick says, “protecting big pieces of public land so it now supports every major mammal species that was here before Columbus arrived on the continent.” But how people develop a few million acres of private land has huge implications for the ecological integrity of public land and the wildlife that depends on it.

As long as adjacent private lands remain open and undeveloped, species can survive. For example, nine major elk herds pass through Greater Yellowstone on epic migrations. Although they spend a majority of their time on public land, they also spend crucial winter months on private ranches.

View Images A state wildlife manager in Cody, Wyoming, checks on a problem grizzly that’s been tranquilized so it can be relocated away from people. Wyoming and other states around Yellowstone argue that grizzlies, still protected on the “threatened” list under the Endangered Species Act, have recovered enough for trophy hunting to be allowed. Photograph by David Guttenfelder

Back in the day, Glick says, most population growth in this region occurred in existing communities or, if in rural areas, remained tucked into draws out of the wind or above the floodplain. “Nowadays,” he says, “rural sprawl has been the dominant development pattern, and it has ripple effects for public lands.”

Andy Hansen, a conservation biologist and professor at Montana State University, points out—in a study due to be published this summer—that the number of private land tracts with no homes or few homes is declining. And the number of parcels with one home per 40 acres increased 328 percent from 1970 to 2010, he says.

By 2013, 30 percent of Greater Yellowstone was considered “developed,” and some wildlife migration pathways were believed to be imperiled. By 2020, between 5 and 40 percent of the ecosystem’s most biologically rich habitats will undergo conversion from ranch and farmland to exurban development.

Nic Patrick survived an attack by a grizzly bear at his own ranch. He recounts his story, holding no grudges.

Today Yellowstone and Grand Teton, plus Glacier National Park, located along the U.S. border with Canada, are the only national parks in the lower 48 states that support populations of three major North American predators: grizzlies, wolves, and mountain lions. In Yellowstone and Grand Teton this success is due to the parks’ good neighbors: surrounding private lands that support habitat for the prey species—elk, deer, and pronghorn—that the predators eat.

But over time the severing of corridors will likely contribute to a decline in migratory elk, mule deer, and pronghorn populations inside the national parks.

People are also pushing deeper into grizzly habitat, causing conflicts that typically result in bears being removed or killed. And climate change, a looming wild card, is expected to reduce the carrying capacity of public lands, making private lands on the periphery of Greater Yellowstone all the more important.

View Images Kids having fun with guns, just north of Yellowstone National Park, or a sign of the times? The Yellowstone region, like much of the West, is uneasily divided over a fundamental question: Who should manage the land and its wildlife, and how, and to what end? Photograph by David Guttenfelder

In recent decades local land trusts have protected important parcels of private land through conservation easements. But the process isn’t keeping up with the rate at which land is being steadily fragmented.

Foresighted federal environmental laws preserved Yellowstone and the national forests. But protecting private lands requires a different kind of thinking. And at a time when anti-regulation sentiments serve as an obstacle to community planning in the West, the coming decades will be critical.

In the long term what will save Greater Yellowstone from experiencing the same fate as most other regions in the lower 48? “I’ll say it in one word: ‘restraint,’” Glick says. “In 1872 the creation of Yellowstone National Park was an exhibition of restraint against the prevailing forces of Manifest Destiny, and we are reaping huge benefits. What’s going to save Greater Yellowstone comes down to the same ethic. But I’m sorry to say I’m not seeing much of that these days. If we are going to act, the time is now.”

View Images Four-year-old Elle Anderson chases a ball and a future near her family’s house on J Bar L Ranch in Montana. “A hundred years from now,” says Hilary Anderson, “I hope this place is a thriving ecosystem full of everything that should be here—wolves, bears, humans, livestock.” Photograph by Louise Johns