The long-term effects of an alien species may differ from transient effects that occur shortly after its invasion of a new ecosystem. Conservationists fear that the invasion of North America by the zebra mussel since 1985 may lead to the extinction of many populations and species of native bivalves. The appearance of zebra mussels in the Hudson River estuary in 1991 was followed by steep declines (65–100%) in population size of all species of native bivalves between 1992 and 1999. The body condition of all unionids and growth and recruitment of young unionids also declined significantly. Initial declines in population size and body condition were correlated primarily with the filtration rate of the zebra mussel population but not with fouling of native bivalves by zebra mussels. However, samples taken since 2000 have shown that populations of all 4 common native bivalves have stabilized or even recovered, although the zebra mussel population has not declined. The mechanisms underlying this apparent reversal of fortune are not clear. Recruitment and growth of young mussels have shown limited recovery, but the body condition of adults has not. We found no evidence that spatial refuges contributed to this reversal of population declines. Simple statistical models project now that native bivalves may persist at population densities about an order of magnitude below their preinvasion densities. These results offer a slender hope that zebra mussels may coexist with unionids and sphaeriids in North America, as they do in Europe.
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