Diminishing species richness of mollusks in Oneida Lake, New York State, USA

Between 1915 and 1917, F. C. Baker studied the molluscan fauna of Oneida La ke, a shallow, eutrophic lake in central New York State. He listed 41 livin g taxa, of which two gastropods, Bithynia tentaculata (Linnaeus, 1758) and Pleurocera acuta (Rafinesque, 1829), were then recent introductions. In 196 7-68, John Forney and I (Harman and Forney, 1970) surveyed Oneida Lake, vis iting Baker's original collecting sites. We tallied 35 living taxa dominate d by the introduced European B. tentaculata. In 1992-95, I again made exten sive collections of mollusks finding a total of 24 living taxa dominated by Dreissena polymorpha (Pallas, 1771), the then recently introduced Eurasian zebra mussel. Fifty percent of Baker's original eulittoral collection site s could not be found. An average 1 m(2) area in 1992-95 included 6708 indiv iduals of D. polymorpha, 60 of B. tentaculata and 2 of the unionid bivalve Elliptio complanata (Lightfoot, 1786). Molluscan species richness was reduc ed by 15% between 1917 and 1968, a trend concurrent with the increase in ab undance of Bithynia. Species richness was further reduced by 31% between 19 68 and 1995, as human activities impacted eulittoral habitats and D. polymo rpha colonized Oneida. The total decrease in species richness between 1917 and 1995 was of 42%. Since 1996 no living unionids have been observed in th e lake.