"Exotic" species are plants and animals that have been introduced into habitats where they are non-native. They are also known as "alien," "pest plants," "weeds," and "problem species." Most of the exotic species in our area have been introduced from Europe and Asia. Some, like the Zebra Mussel, have come in the water via the St. Lawrence Seaway and Great Lakes, others from ports of call, like the Emerald Ash Borer, and still others via land and air, like the Gypsy Moth.

Exotic species are a problem because they have high productivity, a higher growth rate, a lack of natural controls, and are severe agents of habitat alteration and degradation. They choke out the native species and destroy natural habitat.

Control of any exotic species begins with identification and understanding. Means of control include suppressing the species, reducing the population, preventing the spread, eradicating it where possible, and restoration of native species and habitats.

more info: http://forestry.msu.edu/mipc/aboutInvasive.htm