The introduction of exotic species into modern aquatic communities provides a model for studying rates of colonization among fossil macro- and microorganisms. Recent examples from the San Francisco Bay estuarine system show how exceptionally quickly this may occur and how profound its impact can be on local populations and ecosystems.

The introduced Asian clam Potamocorbula amurensis (Schrenck) was first discovered in Grizzly and Suisun bays in October 1986, and other workers have shown that by December 1987 it had increased in abundance to densities >1,000 individuals/m2. By June 1987, it had spread throughout the entire estuarine system. This particular species of clam is such an efficient filter-feeder that, shortly after its introduction, the phytoplankton population in the bay began to decline steadily to the point where, four years later, the spring bloom in the region had virtually disappeared.

We have similar evidence that a rapid faunal takeover recently occurred in the microfauna of San Francisco Bay. The exotic Japanese foraminifer Trochammina hadai Uchio, first discovered in the bay in 1995, has now been shown to have been introduced sometime between 1981 and 1986. Its earliest appearance is documented in 23 samples collected in the southern part of the bay in 1986, where it comprised from 5-11% of the foraminiferal population at the extreme southern end and 46-89% in the region from Hayward to Alameda. Attaining such dominant species frequencies in such a short amount of time after its introduction is truly a noteworthy phenomenon. We have since found T. hadai throughout San Francisco Bay in salt marsh to fully marine environments, commonly at frequencies of 20-60% of the foraminiferal population, peaking at 93% in the central part of the bay at Point Isabel. Whereas the Asian clam's remarkable invasion of the bay took less than a year, T. hadai took no more than five (the timing of which is not further constrained due to a lack of available samples). Additionally, T. hadai's dominance of San Francisco Bay's foraminiferal assemblage is no isolated event, as it has proliferated in other Pacific Coast estuaries as well: San Diego Bay, Newport Bay, Long Beach Harbor, Marina Del Rey, Santa Barbara Yacht Harbor and Willapa Bay.

75/125 YEARS