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Plants and animals that are not native to an ecosystem can wreak havoc on the naturally occurring species within that system. These invasive, non-native species often out-compete native species for resources and occupy much of the available habitat. Purple loosestrife and kudzu are examples.
Other invasive species prey on native species or usurp or destroy their habitats. For example, brown snakes have eradicated virtually all bird life on Guam , and zebra mussels prey on and replace native mussel beds. The impact of invasives on agriculture, fisheries, and other aspects of our economy is estimated in the billions of dollars each year; the impact on biodiversity is immeasurable. | 
