Non-native wildlife and plant species like quagga mussels, sea lampreys, alewives, nutria, Burmese pythons, brown tree snakes, mitten crabs, ruffe, purple loosestrife, water hyacinths, piranhas, alligators, and so on are among us. Some wreak havoc on ecosystems, and some can be a blessing.
Sometimes non-native species arrive by accident. Quagga and zebra mussels hitchhiked into the Great Lakes in the bilge of ships. Some come as pets that are freed and then take root, like mynah birds, parrots, and red-eared slider turtles. Others just get out of control, such as Asian carp, which were introduced to Mississippi to clean algae out of sewage treatment ponds before a flood enabled them to escape into natural waterways. Since then they have taken hold in many areas of the Mississippi River system and are poised to enter the Great Lakes.
People may also smuggle non-native species for profit. The international illegal trade in wildlife is estimated to be second only to drugs in value. California game wardens tell me that drug dealers, for example, have a liking for pet tigers, lions, and wolves.