When the brown tree snake was transported accidentally to the Pacific island of Guam sixty years ago it slithered into paradise: a banquet of birds that had no fear of snakes–and no predators to keep snakes in check.

Today Guam is the text book example of what invasive species can inflict on an ecosystem. The brown tree snake has wiped out most of the island’s indigenous birds and is making serious inroads into Guam’s other small animals.

Photo courtesy Isaac Chellman/University of Washington

Food is so plentiful that the snake can grow to ten feet on Guam, well over the more typical six feet it reaches in its native Australia.

Scientists estimate there are 3,000 brown tree snakes per square mile on Guam, an island 30 miles long and 5 to 15 miles wide about 3,800 miles west of Hawaii. The snakes have taken up every nook and cranny and frequently slip into people’s homes, where they are said to be aggressive when confronted.

Having munched its way through so much of Guam’s animal diversity, you’d think the snake’s population would be on the edge of collapse because of a dwindling food supply. But not so, according to the U.S. Geological Survey, which maintains a comprehensive Web site about the reptile. The snake has adapted to feeding on a ubiquitous lizard that apparently breeds as fast as it is eaten, sustaining the island’s enormous snake population.

A new study published today suggests that the brown tree snake could mean Guam will lose more than its birds. Native tree populations could be declining as well.

Photo courtesy Isaac Chellman/University of Washington

“It has been 25 years since the birds disappeared,” said Haldre Rogers, a University of Washington doctoral student in biology, in a statement announcing the research. “It seems to me the consequences are going to keep reverberating throughout the community if birds are fundamental components of the forest.”

Birds are important for pollination, spread seeds around the forest and control insects that feed on plants, Rogers notes. But in Guam 10 of the 12 species of native forest birds have become extinct and the other two species have fewer than 200 individuals. None of the non-native bird populations that have moved to Guam live in the forest.

Rogers compared tree seed dispersal on Guam with that on nearby islands, which do not have brown tree snakes. On Guam seeds and seedlings were found in close proximity to the nearest adult tree. On the other islands seeds were more widely dispersed and the nearest adult trees were found two to three times farther away from seedlings.

These traps are among 119 placed under or near trees on Guam and the nearby island of Saipan to determine the distribution of tree seeds.

Image courtesy Haldre Rogers/University of Washington

The biggest indirect impact, Rogers says, could be altered seed scattering that in turn might, in the near future, transform the remaining forest from a diverse mixture of tree species to clumps of trees of the same species, separated by open space. “That could have serious consequences, including extinction, for plant and animal species that still live in the forests.”

Rogers suspects there could be other indirect impacts the brown tree snake has had on Guam. There is anecdotal evidence that there is a substantially higher spider population on Guam than on other nearby islands, she says, and that could largely be because of the decline of the native bird population.

Rogers is concerned that what is happening to the island’s ecosystem could indicate something of the future for the entire planet. Birds are in decline everywhere, she notes.