The spread of non-indigenous species over the last century is unprecedented and far surpasses the speed and scale of natural invasions. On any one day, more than 3,000 species may be in motion around the world in ships' ballast water. Approximately 1,000 marine species may already be resident in environments where they are not indigenous. In San Francisco Bay, there are over 200 alien species, while the origin of a further 125 has yet to be determined. Predicting the likely impacts of an introduced species is extremely difficult.
Some species quietly assimilate into the recipient regions without causing any noticeable impact.
Others thrive and outcompete native species. The Solution Controlling marine invaders after they have successfully colonized is, in most cases, extremely difficult and highly costly. Eradication is generally considered impossible. Most marine invaders have a planktonic larval stage which allows for wide dissemination. Introducing other predators or diseases to control the invaders can lead to further ecological damage.