Remarkable invasion of San Francisco Bay (California, USA), by the Asian clam Potamocorbula amurensis. II, Displacement of a former community

Long-term macrobenthic sampling at a site in northern San Francisco Bay has provided an unusual opportunity for documenting the time course of an invasion by a recently introduced Asian clam Potamocorbula amurensis. Between 1977, when sampling began, and 1986, when t~e new clam was first discovered, the benthic community varied predictably in response to river inflow. Dunng years of normal or high river inflow, the community consisted of a few brackish or freshwater species. During prolonged periods of low river inflow, the number of species doubled as estuarine species re.g. NIya arenaria) migrated up the estuary. In June 1987, at the beginning of the longest dry period in recent decades, large numbers (> 12 000 m-2) of juvenile P. amurensis were discovered at the site. By midsummer 1988 the new clam predominated (> 95 %) in both total number of individuals and biomass, and the expected dry-period estuarine species did not become re-established. The rapid rise of P. amurensis to numerical dominance throughout the region of the original introduction was probably facilitated by the fact that this region of the bay had been rendered nearly depauperate by a major flood in early 1986. Once introduced, the clam had sufficient time (> 1 yr) to become well established before the salinity regime was appropriate for the return of the estuarine species. Subsequently, the new clam was apparently able to prevent the return of the dry-period community. Its ability to live in low salinity water (< 1 ~) suggests that P. amurensis may not be displaced with the return of normal winter river flow and, therefore, may have permanently changed benthic community dynam,cs in this region of San Francisco Bay. INTRODUCTION cisco Bay by Potamocorbula amurensis allows us to examine these factors. The explosive population growth and spread of Our study of this invasion has greatly benefited from the euryhaline Asian corbulid clam Potamocorbula the fact that the initial colonization occurred in Suisun amurensis in northern San Francisco Bay soon after its Bay (Fig. 1), a region of San Francisco Bay that has arrival in 1986 (Carlton et el. 1990) raises fundamental been the focus of routine water column and sediment questions: What were the conditions at the initial invasamphng (including quantitative macrobenthos sampsion site that permitted the invading species to become ling) since 19.77. This is also the region of t.he estuary successfully established? What has been the effect of where the clam’s influence on the existing benthic the invasion on the pre-existing community? community has been most marked. The data from the Detailed analyses of species invasions (e.g. Elton long-term sampling effort provided us the opportunity 1958, Mo9ney & Drake 1986, Drake et el. 19891 suggest to study benthic community dynamics both.before and that, while the success of any given species introducafter the introduction of Potamocorbula amurensis, and tion is not very predictable (Simberloff 1986), the to examine the circumstances under which the new important factors to be considered are the characterisspecies thrived. tics of that species, the availability of suitable habitat, The first purpose of this report is to describe the and the nature of the community p~:esent in the invaded season-to-season and year-to-year patterns of variation area. Our early detection of the invasion of San Franin community structure that were characteristic of the © Inter-Research/Printed in F. R. Germany 0171-8630/901006610095/$ 03.00