When people sail the sea, marine organisms tag along. If carried long distances, these hitchhikers can invade and disrupt ecosystems far from their natural homes, pushing out the local species. Some invaders catch a ride by attaching themselves to the sides or bottoms of boats. But many more stow away in ships' "ballast tanks."

Busy shipping areas, like the Port of Oakland, are especially vulnerable to invading species that can be carried in ballast water.  Scientists from the Smithsonian's Environmental Research Center and elsewhere are studying techniques such as filtering and treating ballast water before it is flushed in an effort reduce the risk of intoducing new species through ballast water.

Read about 5 Invasive Species You Should Know.