Aliens have landed around the world and are taking over Earth. Not extraterrestrial monsters--nonnative or exotic species. The kind introduced by thoughtless people. The kind you, your neighbors or somebody in your town may bring home from vacation, justifying your act by saying, "It's only one. What can it hurt?" Once these aliens--plants and animals--take root or get a foothold, they crowd out or otherwise destroy native species that can't compete. Consider these examples, a few among many, and the ways these species became endangered.
In Yellowstone Lake, cutthroat trout have long served as food for bald eagles, river otters, grizzly bears and other animals. In 20 years, however, the cutthroats could be gone; crowded out by lake trout introduced by fishermen around 1994. Because lake trout live in deeper water than cutthroats, they don't serve as food for animals and birds. They do compete with cutthroats for food. What's going to stop them from taking over the lake? Nothing, unless the U.S. Fish and Wildlife Service successfully limits the population with an annual netting attempt.
On San Clemente Island, off the coast of California near San Diego, only 15 San Clemente loggerhead shrikes are left. The rest are gone because goats introduced in 1875 got the run of the island and ruined the shrike's habitat. The U.S. Navy, which reportedly has mismanaged the island for many years, could have undertaken a goat control program but didn't do so until recently. The result: no native vegetation and nesting habitat; so no shrikes.
On another island, Guam, in the western Pacific, the flightless Guam rail managed for years to hide from introduced dogs, cats and rats, but couldn't hide from the million or so brown tree snakes descended from stowaways believed to have come by ship after World War II. The rails were gone from the wild by the mid 1980s but rescuers have been breeding captives and releasing them on a nearby snake-free island--Rota. Residents now have to keep snakes from stowing away on boats and aircraft.
In New Zealand, the giant scarlet-flowered mistletoe plant is threatened by aliens. Mistletoes, which can grow 9 feet in width and length and live a century have flower petals sealed together "in the floral equivalent of a childproof cap." Some native birds twist off the petal ends, thereby aiding pollination. Enter the destructive aliens: rats, cats, ferrets and others that eat birds. Scientists hope that a species of bee, which is able to bite, push and pull open some flower buds, will ensure the plant's survival.
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