Invasion by mobile aquatic consumers enhances secondary production and increases top-down control of lower trophic levels
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Increased biological diversity due to invasion by non-indigenous species (NIS) is a global phenomenon with potential effects on trophic interactions and ecosystem processes in the invaded habitat. We assessed the effects of resource availability and invasion of three non-indigenous invertebrate grazers (two crustaceans and a snail) on secondary production, relative dominance of NIS grazers and resource depletion in experimental freshwater mesocosms. The relative dominance of NIS grazers increased with increasing initial resource availability, although the effect was largest for one of the three species. The effect was due to the fact that all the included non-indigenous grazers were able to expand their populations quickly in response to resource addition. For the most dominating species, the increased grazer diversity due to invasion in turn resulted in higher production of grazer biomass and a more efficient depletion of the periphyton resource. The effect was largest at high initial resource availability, where NIS dominance was most pronounced. Our results show that an invasion-induced increase in species diversity can increase resource depletion and consequently production, but that the effect depends on identity of the introduced species. The results also suggest that properties of the recipient system, such as resource availability, can modulate ecosystem effects of NIS by affecting invader success and dominance.
KeywordsGrazer Resource depletion Gammarus tigrinus Limnomysis benedeni Physella acuta
We thank Cordula Krücken, Melinda Molnar and Viola Lehmpfuhl for field and laboratory assistance and the staff and colleagues at the Institute for Botany (University of Cologne) and the Grietherbusch field station for advice and logistical support. Jost Borcherding, Andreas Scharbert and Ulrich Werneke shared their knowledge on the Rhine floodplain ecosystem and provided fruitful discussions in the planning of the study. Thanks also to Jost Borcherding and Peter Hambäck for discussions and helpful comments on earlier versions of the manuscript and to Sarah C. Lee and one anonymous referee for constructive reviews. The project was financed by the Swedish Research Council Formas (post-doc grant to S.A.W.) and additional funding was provided by C.F. Lundströms foundation and the Royal Swedish Academy of Science.
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