TRURO, Mass. – A Cape Cod idyll could be about to end for residents of rustic shacks who may be forced out by new federal rules.

A master plan being developed by the National Park Service could force out families who have lived for decades in so-called dune shacks along the Cape Cod National Seashore.

About nine of the 18 shacks are used by nonprofit groups and were taken by eminent domain by the Park Service more than 30 years ago. Residents who handed over their shacks were paid fair market value and others who opted for leases for 25 years or a lifetime received smaller payouts.

Residents are not charged rent, but must pay town property taxes.

William Burke, park historian for the National Seashore, says management of the shacks has been a case-by-case approach resulting in a mix of family residences and nonprofit users, long-term and short-term deals.

The park is now drawing up a plan that will set uniform rules to determine who uses the shacks, for how long and for what purpose.

After the current agreements expire, longtime families will probably be required to apply for leases. Officials at National Park Service regional headquarters in Philadelphia will approve new arrangements, holding in the balance decades of residency.

“The government cannot lease government-owned properties exclusively to a hand-picked family,” Burke said. “It has to be an open, fair and competitive process.”

The 25-year leases will begin to expire in 2014, he said.

The shacks along the tip of Cape Cod have been home to playwrights Tennessee Williams and Eugene O’Neill and poet Harry Kemp, helping foster a reputation of creativity in the community.