Scale-dependent associations between native freshwater mussels and invasive Corbicula

In North America there is conflicting evidence concerning whether the invasive Asian clam, Corbicula fluminea, and native mussels (Unionidae), can successfully co-exist. One reason underlying disparate conclusions may be the different spatial scales at which data have been collected. We compared the distribution and abundance of native unionid mussels and Corbicula at two spatial scales, stream reaches and 0.25 m² patches, within one biogeographic region, the Ouachita Highlands, of the south central U.S. We found that Corbicula abundance was negatively related to native mussel abundance at small spatial scales. While Corbicula densities varied widely in patches without native mussels, and in patches where mussels occurred at low abundance, Corbicula density was never high in patches where mussels were dense. We hypothesize that the likelihood of successful Corbicula invasion decreases with increasing abundance of adult native mussels. Several mechanisms may potentially drive this pattern including lack of space for Corbicula to colonize, physical displacement by actively burrowing mussels, and locally reduced food resources in patches where native mussels are feeding. In addition, Corbicula may be unable to withstand environmental bottlenecks as readily as unionids. When patch-scale density and biomass information were pooled to represent entire stream reaches, the negative relationship between native mussels and Corbicula was no longer as apparent, and there was not a significant relationship between native mussels and Corbicula. These results point to the importance of appropriate sample scale in examining potential associations between species.