The species are carried in on animals, vehicles, ships, commercial goods, produce, and even clothing. Some exotic introductions are ecologically harmless and some are beneficial. But other exotic introductions are harmful to recreation and ecosystems. They have even caused the extinction of native species - especially those of confined habitats such as islands and aquatic ecosystems. The recent development of fast ocean freighters has greatly increased the risk of new exotics in the Great Lakes region.
Ships take on ballast water in Europe for stability during the ocean crossing.
This water is pumped out when the ships pick up their loads in Great Lakes ports. Because the ships make the crossing so much faster now, and harbors are often less polluted, more exotic species are likely to survive the journey and thrive in the new waters. Many of the plants and animals described in this guide arrived in the Great Lakes this way. But they are now being spread throughout the continent's interior in and on boats and other recreational water-craft and equipment. This guide is designed to help water recreationists recognize these exotics and help stop their further spread.