Throughout history, people have frequently moved species around the globe. Think of tomatoes or corn. Both of these plants originally grew in South America, but they were transported into new areas and are now common worldwide. We often transport species on purpose, such as tomatoes and corn, so they can be grown for food, or exotic plants and animals so that people can keep them as pets. But sometimes we also do it by accident. For example, an organism can be moved from its natural home as a stowaway on a boat—maybe it is accidentally stuck to the outside of the boat during the entire voyage! Chinese pond mussels, for instance, have very tiny babies that attach in high numbers to the gills of fish. Throughout human history, people grew fish for food and moved the fish, on purpose, around the world. That is how people accidentally transferred the tiny mussels, along with the fish, to the new environment. The mussels then detached from the fish and started their new life on the bottom of some lake or river on a new continent.

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