In San Francisco Bay, virtually every organism comes from someplace else. Alien species can displace native species, destroy an ecosystem's food web, and bring with them disease-causing bacteria and viruses that local flora and fauna do not have immunity to. Even humans are not immune - new outbreaks of cholera around the world are blamed on contaminated water discharged from ships.
Aliens can wreak economic havoc, too.
Ballast water is used to balance the loads in ships while they are at sea and it is discharged when it is no longer needed. Porcelain crabs, Petrolisthes, from the Gulf of Mexico, are known to be in the Savannah River. No one knows for sure right now. But a two-year study paid for by the Georgia Ports Authority to inventory fish species in the river as part of the harbor deepening project may begin to give scientists a hint of who the invaders might be. There is general knowledge of the types of alien species that might be here but no baseline study to say exactly what is here.