Litter identity mediates predator impacts on the functioning of an aquatic detritus-based food web
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During past decades, several mechanisms such as resource quality and habitat complexity have been proposed to explain variations in the strength of trophic cascades across ecosystems. In detritus-based headwater streams, litter accumulations constitute both a habitat and a resource for detritivorous macroinvertebrates. Because litter edibility (which promotes trophic cascades) is usually inversely correlated with its structural complexity (which weakens trophic cascades), there is a great scope for stronger trophic cascades in litter accumulations that are dominated by easily degradable litter species. However, it remains unclear how mixing contrasting litter species (conferring both habitat complexity and high quality resource) may influence top–down controls on communities and processes. In enclosures exposed in a second-order stream, we manipulated litter species composition by using two contrasting litter (alder and oak), and the presence–absence of a macroinvertebrate predator (Cordulegaster boltonii larvae), enabling it to effectively exert predation pressure, or not, on detritivores (consumptive versus non-consumptive predation effects). Leaf mass loss, detritivore biomass and community structure were mostly controlled independently by litter identity and mixing and by predator consumption. However, the strength of predator control was mediated by litter quality (stronger on alder), and to a lesser extent by litter mixing (weaker on mixed litter). Refractory litter such as oak leaves may contribute to the structural complexity of the habitat for stream macroinvertebrates, allowing the maintenance of detritivore communities even when strong predation pressure occurs. We suggest that considering the interaction between top–down and bottom–up factors is important when investigating their influence on natural communities and ecosystem processes in detritus-based ecosystems.
KeywordsTrophic cascades Litter mixing Litter decomposition Shredder Cordulegaster boltonii
We are grateful to Mark Gessner, Markus Schindler and Brendan McKie who designed and built the enclosures used in this experiment, André Frainer Barbosa for constructive discussion about the experimental design, as well as Barbara Downes and several anonymous referees for their very helpful comments and suggestions on the manuscript. We also greatly appreciate the technical assistance of Sylvain Lamothe and Didier Lambrigot in the field and laboratory. Finally, we thank Jean-Claude Arnaud, Président du syndicat des riverains et pêcheurs de Roquefère et de Labastide, who kindly allowed us access to the Rieutort.
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