Exotic species of animals and plants.
Exotic species are either deliberately or accidentally imported.
- Goldfish were deliberately imported as a colorful pet, but few people have the heart to get rid of them, so live ones sometimes get flushed down the toilet. The migration of goldfish in the early 20th century throughout Lake Erie has been documented.
- Zebra mussels were accidentally imported in the ballast of a ship - now cause multiple problems.