Box 6: Interaction of anthropogenic effects and natural factors have led to complex changes in ecosystem structure and function.


The Invasion of the Chinese Clam Potamocorbula amurensis in California:

The Chinese clam Potamocorbula, transported in ships' ballast water, was discovered in San Francisco Bay in 1986. Within several years densities reached 20,000/m2, with the clam making up greater than 95 percent of the biomass, displacing much of the former benthic community. The invasion of Potamocorbula appears to have been facilitated by a 1986 flood that depressed the brackish northern Bay fauna, followed by the intrusion of high-salinity waters during an ensuing drought that prevented the return of the preflood fauna. Potamocorbula has persisted even after the return of normal salinities. Potamocorbula is in the process of completely altering Bay food webs: the clam has eliminated large summer phytoplankton blooms in the northern Bay. Clams continue to be supported in part by bacterioplankton. Cascading effects are expected as these trophic alterations affect zooplankton that, in turn, may impact fish populations.


Key References: Carlton et al., (1990); Nichols et al., (1990); Alpine and Cloem (1992); Werner and Hollibaugh (1993).

The Deterioration of Coral Reefs in the Florida Keys and Caribbean Basin:

Since 1982 natural factors enhanced by human activities have led to dramatic coral loss on many Florida and Caribbean reefs and to its replacement by algae. These natural phenomena include hurricanes, increased relative abundance of coral predators, and a massive pathogen-caused mortality of the algae-eating sea urchin Diadema antillarum. These events were greatly intensified because the reefs already had been altered by overremoval of herbivorous fish, magnifying the consequences of sea urchin loss. Eutrophication and nearshore development may have also tipped the balance in favor of algae. Additionally, coral loss due to extensive ''coral bleaching" of uncertain origin—corals losing their essential, symbiotic algae—may be accentuated by eutrophication interacting with global warming and increased ultraviolet-B exposure. Reefs have proven resilient to local damage when the stress is ameliorated, but the cumulative and synergistic nature of all stresses may be irreversible on the scale of the duration of a human life.


Key References: Lessios (1988); Knowlton et al., (1990); Knowlton (1992); Porter and Meier (1992); Brown and Ogden (1993); Gleason and Wellington (1993); Jackson (1994); Sebens (1994).

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