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The waters of western North America are being transformed by invasive aquatic plants, fish, and other animals from around the world. These plants and animals, which have been introduced both intentionally and accidentally by humans, can irrevocably alter our native ecosystems. While they may be harmless in their own waters, once brought into a new ecosystem where their native predators do not exist, they can harm native species by eating their food, preying on them, transmitting diseases to which the natives have no defenses, or (like many invasive aquatic plants) simply outgrowing them. Not all non-native species cause serious problems, but some do, disrupting entire ecosystems by destroying habitat and altering food chains. These plants and animals are known as aquatic nuisance species. Aquatic nuisance species not only threaten the natural environment, they also cause serious economic damage. Each year, the United States alone spends billions of dollars attempting to control or slow the spread of these plants and animals. Recreational activities are impacted when boating and swimming areas become clogged with invasive plants, and human health can even be affected as some aquatic nuisance species carry parasites and diseases.