Long-term responses of native bivalves (Unionidae and Sphaeriidae) to a Dreissena invasion

A long-term (28-year) study documented complicated responses of populations of native bivalves (Unionidae and Sphaeriidae) to the dreissenid invasion of the freshwater tidal Hudson River. Early in the invasion (1993–2000), formerly large populations of all native species declined steeply, reaching 0 to 35% of preinvasion densities by the year 2000. These declines in population density were accompanied by large declines in body mass, body condition, and recruitment of natives. Since the year 2000, the population of sphaeriids has recovered to preinvasion densities, and body condition, recruitment, and juvenile growth of unionids have recovered substantially, even though the number of dreissenids in the river has not declined. However, population densities of unionids have not recovered, and 2 of 3 formerly common unionid species (Anodonta implicata and Leptodea ochracea) have not been collected in several years. The 3rd unionid species (Elliptio complanata) also appears to be on the verge of disappearing from the river. Statistical models suggest that the dynamics of native populations were more likely driven by exploitative competition than by fouling. They also suggest that changes in dreissenid body size may have modulated the strength of this exploitative competition over the course of the invasion. The lack of recovery of unionid populations despite substantial recovery of juvenile recruitment and growth, as well as adult body condition, appears to be a result of failure of juveniles to reach adulthood, probably because of high mortality from predators.