Alien Invasion

Once upon a time, the hot, humid forests on the small Pacific island of Guam were filled with the songs of birds. While blue-tailed skink lizards darted under ferns to snap up crickets, greenish blue kingfishers called to each other. At night, tiny flycatchers slept peacefully as bats swooped among breadfruit and tangantangan trees to feed on nectar and fruit. When the bats pooped, they spread the breadfruit's seeds and those of other plants. In a nearby clearing, where all the plants had been uprooted by a stormy typhoon, new trees and bushes would soon sprout.

But then—The Brown Tree Snake! Because of this intruder, the forests of Guam are now very different than they were fifty years ago.

How could one species of snake make such a difference? Simple. It upset nature's balance. It disrupted the web of relationships among the animals and plants on Guam that had developed over millions of years.

An Alien in Paradise

Brown tree snakes probably came to Guam hiding among boxes of cargo brought to the island in airplanes. Guam was a paradise for the brown tree snake—with all its favorite kinds of foods and no enemies that could harm it. The animals on Guam never had to develop defenses against a nighttime hunter. So catching food was a snap for the night-hunting brown tree snake. It could swallow a sleeping bird and then move on to feast on a young bat. Gulp—gone!

But while the snake thrived, the original inhabitants suffered. Flycatchers, honey-eaters, kingfishers, and more have all been devoured…nest by nest, bird by bird. The bats? Nearly all gone. The native lizards? Five types are extinct on the island.

Today scientists estimate there may be up to two million snakes on Guam. The forest is strangely quiet. With fewer fruit bats and nectar-feeding birds, pollination and the spreading of seeds is down. With fewer insect-eating lizards and birds, pest insects have increased.

Because of the brown tree snake, the variety of life forms on Guam—its biodiversity—has nosedived.

Nature Can Fool You

Not all change is bad. It is normal for ecosystems like Guam's tropical forest to change and develop over time. Some species die out and new species arrive, but usually it happens very, very slowly—over millions of years. But thanks to humans who crisscross the world in planes and ships, the arrival of alien animals or plants in ecosystems where they don't belong is now lightning fast. Scientists say that today, after habitat destruction, the biggest threat to biodiversity around the world is the spread of alien species.

For more cool examples of invasive species, read Aliens from Earth by Mary Batten.

BIG Mistakes!

Often humans don't understand that a change they make in the environment may have a very different result from what they expected. The web of nature is complicated!

In the 1930s the government wanted to control soil erosion in the southern United States. So it hired hundreds of workers to plant fields of kudzu, a beautiful vine that is native to Japan. Big mistake! Without Japan's cooler climate and kudzu-munching beetles to keep the plant in check, kudzu grew out of control. Today it's an expensive problem. Kudzu covers millions of acres of land, smothering native trees and plants.

In the late 1880s someone brought mongooses to Hawaii to control the rats (which had also been brought by humans). Nice try! But mongooses hunt during the daytime, while rats are active at night. So what's a hungry mongoose to do? Prey on birds, including the nene, a wild goose that is Hawaii's state bird. Once there were 25,000 nenes on Hawaii. Now, thanks to human interference, there are less than 500 nenes left in the wild.


All the living and nonliving things that exist and interact in one place.

The process of being worn away bit by bit, such as by water or wind.

The place where an animal or plant naturally lives and grows.

Back to Top


  1. How has the brown tree snake changed the community of animals on the island of Guam?
    [anno: The brown tree snake has hunted and killed many animals that have not developed natural defenses against the snake. Many species of animals are declining on Guam because of this invasive snake species.]
  2. Do you think the government of Guam should introduce an animal that is a natural predator of the brown tree snake? Why or why not?
    Explain your answer in a few sentences.
    [anno: Answers will vary. Students may believe it is a good idea because it would help other species make a comeback on the island. Students may believe it is a bad idea because the introduction of a brown tree snake predator could bring other problems as well.]