Amphipods (Gammarus pseudolimnaeus) prefer but are harmed by a diet of non-native leaves

The strong dependence of headwater streams on allochthonous inputs of energy and nutrients from riparian plant communities predicts that the introduction of non-native tree species should significantly affect these stream systems. The response of macroinvertebrate shredders to native and non-native leaves entering the stream will affect the transfer of energy and nutrients to other components of the ecosystem, but the specifics of such a response appear to be highly variable across systems. We investigated whether a locally abundant shredder, the amphipod Gammarus pseudolimnaeus, preferred stream-conditioned native (green ash, Fraxinus pennsylvanica) or non-native (common buckthorn, Rhamnus cathartica) leaves and how an exclusive diet of conditioned leaves of each species influenced amphipod survival, body mass, behavior, and energetics in a laboratory setting. Amphipods significantly preferred leaves of common buckthorn to those of ash. However, a 24-d diet of buckthorn leaves resulted in significantly higher mortality and significantly lower body mass compared to a diet of ash leaves. Lipid (but not glycogen) concentrations were significantly lower for experimental amphipods fed either diet compared to field-collected amphipods, but neither lipid nor glycogen concentrations differed between amphipods fed ash or buckthorn. Riparian ecosystems throughout eastern and central North America are experiencing a significant expansion of common buckthorn. Concurrently, the spread of the non-native emerald ash borer insect (Agrilus planipennis) is causing a regional decline in green ash and other native ash species. Our results suggest that these changes in riparian plant communities are likely to alter the processing of allochthonous litter, population dynamics of shredders, and broader community dynamics of regional headwater streams and associated downstream ecosystems.