Approach

First, we will manipulate a wide range of leaf species (including native and exotic plant species) along a gradient of eutrophication to test if effects of riparian plant diversity on leaf litter decomposition can be predicted, and if these effects are modulated by environmental context. The experiment will be done in Autumn, when leaf litter input from deciduous trees peaks, and in Spring when litter input from riparian trees is scarce. The impacts will be examined at multiple response levels, namely litter decomposition, microbial decomposers and invertebrates. Aquatic fungi are the major microbial decomposers and have complex interactions with invertebrates, making leaf-litter more palatable to shredders. A large number of studies have tested diversity effects of fungal decomposers or invertebrate shredders on plant litter decomposition focusing on just one trophic level, but the trophic complexity in nature requires studies across trophic levels to determine top-down and bottom-up effects. This question will be challenged in the FreshBioFun by simultaneously manipulating fungal and shredder diversity. The experiment will be conducted at different levels of eutrophication; this will allow us to ascertain at which level diversity is most critical for leaf decomposition, and if the pattern change under eutrophic conditions. Then, we will test how the introduction of an omnivorous predator interferes with invertebrates and leaf decomposition. The omnivorous may induce top-down cascade effects on plant litter dynamic by feeding on invertebrates or may compete with invertebrates by consuming plant litter. Finally, we will use data available in literature and gathered during the FreshBioFun to adjust statistical parametric models in order to i) predict how biodiversity loss affects the functioning and stability of freshwater ecosystems, and ii) extend our studies to time periods and community sizes not possible within the constraints of the experimental set ups. This will allow us to develop a conceptual framework integrating novel information on the relationship between biodiversity and ecosystem functioning in detritus food-webs with implications to stream management.