Changing Planet

Snake Plague on Guam Impacts Trees

Brown tree snake.jpg

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.

brown tree snake 2.jpg

 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.

seed trap.jpg


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.

Forty years in U.S., UK, and South African media gives David Braun global perspective and experience across multiple storytelling platforms. His coverage of science, nature, politics, and technology has been published/broadcast by the BBC, CNN, NPR, AP, UPI, National Geographic, TechWeb, De Telegraaf, Travel World, and Argus South African Newspapers. He has published two books and won several journalism awards. He has 120,000 followers on social media. David Braun edits the National Geographic Society blog, hosting a global discussion on issues resonating with the Society's mission and initiatives. He also directs the Society side of the Fulbright-National Geographic Digital Storytelling Fellowship, awarded to Americans seeking the opportunity to spend nine months abroad, engaging local communities and sharing stories from the field with a global audience. Follow David on Facebook  Twitter  LinkedIn
  • Francisco

    I’M FROM GUAM TOOOOOOOOOO!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

  • roberta marcin

    i lived in guam many years ago and i would like to get in touch with a donald garetto

  • Keith Stone

    I lived on Guam in the early 70’s to mid 70’s. We had lots of birds, lizards, mice and rats. Especially mice and rats, a big problem where I was. Most of my off time was spent in the jungle, from the south to the north and east portions of the island. Loved to search the boonies after fires and found lots of cool stuff but never any dead or live snakes. When I went there I was told no snakes on Guam. I find it interesting a couple years later I buddy on Guam said they had a snake problem. I’d bet they were introduced to control rats and something went hay wire!

  • Samantha

    The snakes are often found in dumpsters or around people’s outdoor trash piles. On my property alone, we’ve caught and killed at least 6 snakes in the last two years. We don’t even keep any trash outside and we have dogs (our dogs will chase and kill snakes). We don’t have a lot of trees on our property. But, we still find them….The snakes are plentiful!

  • donald lindstrom

    i lived on guam in 1978 and found a snake in the jungle and took a picture of it . my island friend said guam had no snakes’

  • Donald Garetto

    FINALLY!!! roberta i have found you. come to guam we may start a new life together and have many ninos

  • Melissa Ward

    I lived on Guam during 77-78, and was told by my Chamorro friends that there were no snakes on Guam. There were plenty of critters: birds, rats, lizards, mice, all sorts of insects. I heard from a friend who remained in Guam in later years that yellow snakes had appeared on island and one had crawled into the crib of my former neighbors and bit their baby. I lived on Tumón Bay right next to the “Guam Zoo” a place where any unacceptable animals that showed up on island in boats or planes were taken for safe keeping and to prevent them infiltrating the island. I don’t remember them having any snakes, either.

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