In recent years, the introduction rate of plants, animals, and protists and other microorganisms has been accelerating.
The invasion rate of freshwater cladocera, small crustaceans such as the water flea Daphnia,
is now 50,000 times higher than the background level before humans played a dominant role in species transport.
In San Francisco Bay, a new aquatic species becomes established every 14 weeks.
(Before 1960, the rate was approximately once every 55 weeks.)
This acceleration is likely because of a rise in propagule pressure, the number of individuals released
in a particular area, and human disturbance to aquatic systems.
Propagule pressure has risen as a result of increased shipping traffic and aquacultural activities.
Humans have also changed aquatic systems through eutrophication
(the increase of nutrients such as nitrogen and phosphorus), the removal of top predators, and other modifications.