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Whether they are called invasive, nonnative alien, exotic, or non-indigenous, introduced species are those that evolved elsewhere and have been purposely or accidentally relocated . While some species have invaded habitats on their own (e.g., migrating wildlife, plants and animals rafting on floating debris), human exploration and colonization have dramatically increased the diversity and scale of invasions by exotic species. Introduced species often find no natural enemies in their new habitat and therefore spread easily and quickly.
Invasive aquatic species have threatened the Great Lakes ever since Europeans settled in the region. Since the 1800s, more than 140 exotic aquatic organisms of all types - including plants, fish, algae and mollusks - have become established in the Great Lakes. As human activity has increased in the Great Lakes watershed, the rate of introduction of exotic species has increased. The following three invasive aquatic species have been encountered in CLOCA studies:
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