Throughout history, humans traveling the world have brought plants and animals around with them—often intentionally, sometimes unwittingly. When these non-native species take root in new ecosystems and start causing trouble, they become known as invasive species.

Many species that are introduced to a region, such as U.S. food crops including wheat, tomatoes, and rice, are not invasive. It is only when an alien species damages the surrounding ecosystem by crowding out other organisms and altering their habitat that it becomes invasive. Surrounding native species that have evolved to defend themselves primarily from other native predators can be ill equipped to handle attacks from new, unfamiliar enemies.

All kinds of plants and animals, including trees, fish, rodents, insects, and fungi, can