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In the most comprehensive study ever made of threats to vanishing species, scientists from the Environmental Defense Fund, The Nature Conservancy, and the Smithsonian Institute ranked the threats to 1,880 rare species and subspecies of birds, mammals, reptiles, amphibians, fish, mussels, butterflies, beetles, dragonflies, other invertebrates, and plants. The report, published in July 1998, linked alien species to the disappearance of nearly half of the imperiled species in the United States. For 18 percent of endangered plants and animals, according to an analysis of U.S. government data done by The Nature Conservancy, exotics are the major cause of endangerment. Only habitat loss ranks as a more serious threat, affecting 85 percent of our vanishing species.
Often introduced species are unable to cope with their "new" environment and do not flourish. Others have little effect on local ecosystems. But about 15 percent find themselves unrestrained by predators, parasitoids, parasites, and competition from other species, in which case their populations often increase exponentially. Ecologists will never be able to predict with any degree of accuracy which species will react in what way. And that 15 percent causes enormous ecological and economic damage.
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