A new federal wildlife refuge is being proposed in Massachusetts that could include acquisition of 2,000 acres of unprotected land in Plymouth and Mashpee.

The ambitious proposal from the US Fish and Wildlife Service involves six states — a first in the Northeast — and would encompass about 300,000 acres in 10 areas in Massachusetts, Connecticut, Maine, New Hampshire, New York, and Rhode Island.

The Fish and Wildlife Service wants to acquire 15,000 acres of unprotected land for the proposed Great Thicket National Wildlife Refuge, including 500 acres in Plymouth and 1,500 in Mashpee. The Massachusetts portion of the refuge would spread over 43,035 acres in Plymouth, Carver, and Wareham surrounding the Myles Standish State Forest, and 28,633 acres in Mashpee.

Although the proposal, which was announced in January, is new, it is connected to conservation efforts that have been ongoing for many years, said Meagan Racey, Fish and Wildlife Service spokeswoman for the Northeast.

“The Fish and Wildlife Service took a look at what was happening with the young forest conservation and the New England cottontail [rabbit] and determined that we need more protected shrublands and young forests in order to address this decline we’ve seen,’’ Racey said. “Nationally speaking, these larger-scale conservation effort approaches are absolutely necessary in order for us to do a good job protecting wildlife and healthy forests and other wildlife that support our quality of life.’’

The goal of the proposed refuge is to increase the scope of federally protected areas to keep shrublands and young forests from disappearing due to development or overmaturation. The 10 areas were chosen based on declining populations of certain wildlife dependent on that type of habitat, including the New England cottontail rabbit and young songbirds, including the blue-winged warbler, the chestnut-sided warbler, and the field sparrow.

Sharl Heller, president and cofounder of the Plymouth nonprofit Southeastern Massachusetts Pine Barrens Alliance, said the group is thrilled at the prospect of having federal investment and habitat protection in the area.

“Development is happening at an alarming rate and it’s not just industrial and houses, it’s these solar farms and sand and gravel operations,’’ Heller said. “The pine barrens aren’t thought of enough as an endangered habitat, and we’re trying to change that in people’s minds. It’s called pine barrens because it was thought not to have a lot of value, but it hosts about 182 species. It’s very significant.’’

A regional inventory of the New England cottontail by the Fish and Wildlife Service found that its range had declined by 86 percent since 1960. The percentage of New England young forestland, classified as forest up to 20 years old, decreased to 3.55 percent in 2014 from 4.53 percent in 2009. In Massachusetts alone, land categorized as young forest went from 2.01 percent in 2009 to 1.77 percent in 2014.

Parts of Plymouth and Mashpee were chosen to be included in the proposed refuge because they had already been identified by the state Division of Fisheries and Wildlife as high-priority areas for certain species, including the New England cottontail and the northern red-bellied cooter turtle, said Tom Eagle, who oversees the state’s eight refuges for the Fish and Wildlife Service.

“Some refuges have a very single focus or purpose; some refuges are established for migratory birds, for instance,’’ Eagle said. [But] “the Great Thicket is a habitat that benefits a slew of species in trouble. We’ve lost habitat due to development, especially in the Massachusetts area.’’

At this early stage, Fish and Wildlife officials have not delineated the exact property lines of the parcels they would like to acquire, but they have estimated the cost in fee titles or easements of obtaining all 15,000 acres to be between $84 million and $129 million. And because they would only deal with willing sellers, the process could take 30 years or more to complete, according to the proposal.

The 500 acres identified for acquisition in Plymouth are in the south part of town, abutting the Massasoit National Wildlife Refuge. The proposal estimates the 500 acres could cost $4.5 million in fee titles or $2.9 million in easements. Mashpee’s 1,500 acres proposed for acquisition could cost $13.5 million in fee titles or $8.8 million in easements.

The proposal won’t be finalized until after the public comment period, which runs through March 4. After that, Fish and Wildlife Service officials will adjust the plan if needed, Racey said.

“The plan gives us permission to look for landowners and funding to acquire the land,’’ she added.

If approved, Great Thicket would be the first new national wildlife refuge in Massachusetts since the Assabet River National Wildlife Refuge was established in 2000, Eagle said.

The draft plan and instructions on how to submit comments are available online at www.fws.gov/northeast/refuges/planning/lpp/greatthicketlpp.html.

Katheleen Conti can be reached at kconti@globe.com. Follow her on Twitter @GlobeKConti.