PREDICTING THE INTENSITY AND IMPACT OF DREISSENA INFESTATION ON NATIVE UNIONID BIVALVES FROM DREISSENA FIELD DENSITY

Introduced dreissenid mussels (Dreissena polymorpha and Dreissena bugensis) foul native unionid bivalves by attaching to their shells in large' clusters and may critically impair many North American unionids that are already threatened by habitat degradation. Using literature and new field data, we examined patterns of Dreissena infestation on unionids, and the relationships between Dreissena field density, infestation intensity, and unionid mortality. Linear regression models showed that Dreissena field density strongly predicts (i) the proportion of unionids colonized by dreissenids (r(2) = 0.90, p < 0.0001) and (ii) the mean number of dreissenids attached to unionids (r(2) = 0.81, p < 0.0001). We fitted a compound Poisson model that accounts for dreissenid clustering and predicts both the proportion of colonized unionids and the mean infestation intensity as effectively as our empirically derived models. The proportion of unionids colonized by Dreissena follows a saturation curve, increasing rapidly with Dreissena densities up to 200/m(2), and reaching a plateau at 70-80% colonization. Unionid mortality (reflected by the proportion of dead unionids) is strongly correlated with Dreissena field density (r(2) = 0.82, p < 0.002) at densities above 1000/m(2). Our models predict that severe unionid mortality (>90%) occurs when Dreissena density and mean infestation intensity reach 6000/m(2) and 100 dreissenids/unionid.