MECHANISMS OF IMPACT OF AN INTRODUCED CRAYFISH (ORCONECTES-RUSTICUS) ON LITTORAL CONGENERS, SNAILS, AND MACROPHYTES

Since its introduction 30 yr ago, Orconectes rusticus has displaced O. propinquus and O. virilis and reduced macrophytes in several northern Wisconsin lakes. Since 1973, rate of displacement by O. rusticus varied among lakes, but relative abundance of O. rusticus increased in all nine lakes sampled in 1988 (eight of which were sampled in 1973 or 1975 and 1980). Using long-term field data and laboratory experiments, we explored mechanisms of differential impact of O. rusticus (relative to congeners) on benthic community structure. Displacement of O. propinquus (but not O. virilis) by O. rusticus increased mean crayfish size. Congener feeding preference did not differ among morphologically different macrophytes or between different-sized snails; however, weight-specific consumption and destruction rates did. Orconectes rusticus consumed more snails than its congeners and sublethally damaged more snails than O. virilis. Orconectes rusticus consumed more macrophyte biomass than O. propinquus; however, O. rusticus did not destroy (consumption plus fragmentation) more macrophyte biomass than its congeners. Changes in benthic community structure after displacement of congeners by O. rusticus may result from larger size (which probably lowers resource depression in the presence of size-selective fish predators) and greater weight-specific reductions of prey.