Inholders, as the property owners inside the parks are known, "live in fear that a change in administration, a new Congress — all of a sudden they're going to have land acquisition agents at their door, trying to force them out," Cushman said.

He said many of the landowners are the third and fourth generations to live there, adding that his own cabin in Yosemite National Park is "the one constant my kids have always had" even though the family moved often.

Park Service Director Mary Bomar has said the agency buys land only from willing sellers.

At Zion, Hank and Mariangela Landau bought the 10 acres coveted by the Park Service in 2005. The land is a 2 1/2-hour drive from Las Vegas, near a site where some of the 1972 Robert Redford movie "Jeremiah Johnson" was filmed.

Landau said he and his wife tried to be environmentally sensitive in remodeling the dilapidated tavern constructed some 40 years ago — a one-story building with light-gray siding — and establishing their retreat, The Center for the True North. Its features include solar power and low-impact lighting.

"We feel good about it because of the way in which we remodeled it," Landau said. "We're not Frank Lloyd Wright, but we did the best we could to try to blend it."

Landau would not disclose what he paid for the property. Whitworth, the park superintendent, said the land was on the market for about $340,000.

Missing an opportunity to buy will not make it any easier the next time. Parks are such desirable areas that the price will probably be higher if a property hits the market again.

In the past few years, the Park Service, on its own or with help from conservationists, bought and demolished the Home Sweet Home Motel at Gettysburg, and purchased an old AT&T communications center at Point Reyes National Seashore in California.

At Glacier National Park in Montana, where deer, elk, moose and wolves roam, Warren Heylman of Spokane, Wash., built a cabin this year on land his grandparents homesteaded before President Taft signed the 1910 bill making Glacier the nation's 10th park. The 130-acre tract had been at the top of Glacier's acquisition list for years.

In July, a helicopter delivered materials for construction of a one-room cabin on the land. Heylman said that the family is exercising its rights and that the land is "not for sale, period."

"We think development of that land is inappropriate," said Brace Hayden, a Park Service official at Glacier. "You can draw your own conclusions about what's best for the American public versus what's best for this family."