A wildlife expert tries to shoot a female water buffalo with a contraceptive-filled hypodermic needle

From International Correspondent Gary Strieker

(CNN) -- The forests on Guam are disappearing -- partly because non-native animals have upset nature's equilibrium.

The island's water buffalo, for example, are prized by the people of Guam, but controlling their numbers has become a priority.

Descended from domestic animals brought to Guam from Asia centuries ago, hundreds of feral water buffalo now live in the shrinking forests, grazing far too much for the land to support their numbers.

Wildlife experts are shooting the female buffalo -- with hypodermic darts loaded with a contraceptive that prevents them from giving birth for at least a year.

"It's not permanent ... so you don't lose the genetic material. You alter which females are breeding," said Leslie Morton, natural resources manager for the U.S. Navy.

Pigs, deer damage trees

On Guam -- a U.S. territory in the Pacific Ocean that is the largest of the Mariana Islands -- other creatures are threatening the forests as well.

An unhealthy Guam forest

Wild pigs and Asian deer, whose ancestors were brought to the island by the Spanish, also are leaving their mark. The foraging animals cause extensive damage to trees.

Also contributing to the problem is the disappearance of birds and bats -- most of which were wiped out by another alien species, the brown tree snakes.

Without birds and bats pollinating and dispersing seeds, the forest is not regenerating.

"And now, when trees set seed and the seeds fall to the ground, pigs and deer are there to eat the seeds and fruit," said Mike Ritter of the U.S. Fish and Wildlife Service.

'An entirely clear forest floor'

In addition, a typhoon last year caused severe damage to Guam's forests. Much of that damage will not be restored by new growth.

Guam bats have been wiped out by brown tree snakes

"If you're walking through the forest, you'll see no seedlings in the forest. If you look down at the ground, you just have an entirely clear forest floor," said Gary Wiles of the Guam Department of Agriculture.

Humans also are contributing to the destruction.

Poachers use fire to flush out game, burning large areas of forest every year. And, spreading development is pushing back forest boundaries.

In a few decades, most of the island's unique forests could disappear -- unless efforts to save them can make a difference.