An invasion is under way that is undermining our economy and endangering our most precious natural treasures. The incursion comes not from foreign armies, political terrorists, or extraterrestrials. Instead, the stealthy invaders are alien species. These plants and animals have been introduced - either intentionally or by accident - into areas outside their natural ranges. Unchecked by natural controls, invasive species are spreading across our lands and through our waterways, and wreaking havoc with already fragile native species and ecosystems.
The growing attention to the problem of invasive species often focuses on their costs to agriculture, ranching, forestry, and industry. The price they exact from our natural forests, grasslands, and waterways, however, is at least as great. Invasive species are now regarded as the second-leading threat to imperiled species, behind only to habitat destruction. Not only do they pose a direct threat to native species by out-competing them for space or nutrients, but they also pose indirect threats. Invasive species can change the food web in an ecosystem by destroying or replacing native food sources. The invasive species may provide little or no food to value for wildlife. Some invasive species are capable of changing the conditions in an ecosystem, such as changing the soil chemistry.
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