In the western Pacific Ocean, on the largest island in the Marianas Archipelago, invasive species are adversely impacting Guam National Wildlife Refuge. And the refuge has a response.

To protect and restore critical habitat, the refuge is installing a multi–species barrier fence around 125 acres of native forest. The goal is to create habitat as free as possible of invasive snakes, pigs, deer and cats for the benefit of Guam’s native bird species.

Guam Refuge includes a pristine beach, native limestone forest and a rich cultural heritage. It provides habitat for the island’s last remaining populations of the endangered Mariana fruit bat and Serianthes nelsonii tree as well as extirpated endangered bird species, including the Mariana crow and Guam rail.

“Guam’s ecosystem has been so devastated by invasive species that, for restoration of native habitat to occur, a pest–free area needs to be established,” says refuge manager Joseph Schwagerl. The weather–resistant fence is scheduled to be completed this fall. Once it is finished, Schwagerl says, invasive pest species will be removed from (or controlled within) the enclosed area to enable forest regeneration and native bird repatriation.

“Our long–term goal is to release the flightless Guam rail—which has been extinct in the wild for more than three decades—within the enclosed area,” Schwagerl says.

Invasive species first arrived on Guam with the Spanish, who colonized the island in the 1600s. They brought animals that have had severe impact on native forest. “Pigs and deer eat and trample young trees and plants before they reach maturity, thereby slowing propagation and dramatically decreasing the understory as well as the amount of mature trees and vegetation,” Schwagerl says.

Most damaging, though, has been the accidental introduction of the brown tree snake, which is thought to have stowed away in military cargo after World War II. With no natural predators and abundant prey, the snake population exploded and native forest bird populations plummeted.

The snakes “have had the most devastating effect because they consume both the bird and eggs, which has led to the extinction or extirpation of nine of 11 forest bird species on Guam,” says Schwagerl. “The only native mammals found on Guam were three species of bats, of which only the Mariana fruit bat remains in small numbers [less than 50] on the island and is an occasional visitor to the refuge.”

After the barrier is complete, the plan is to remove adult brown tree snakes from the enclosed area using known trapping, baiting and hand–removal methods. Removing juvenile snakes is more difficult because there is no known method of baiting and trapping them. But the refuge is working on that with the U.S. Geological Survey Guam Brown Tree Snake Project.

“We are still developing methods to remove small snakes that have yet to make the transition to warm–bodied prey,” says U.S. Geological Survey project research manager M.J. Mazurek. Nonetheless, trapping adult snakes will allow for the safe release of several native bird species within the enclosure, including the endangered Guam rail—eventually.

The rail, or ko’ko in the native Chamorro language, was extirpated from Guam in the 1980s. It is now bred in captivity. If all goes well, the refuge anticipates a soft release of Guam rails into the enclosure by 2015.

Jennifer Cruce is a wildlife refuge specialist at Guam National Wildlife Refuge.