Trophic complexity enhances ecosystem functioning in an aquatic detritus-based model system. 
1.  Understanding the functional significance of species interactions in ecosystems has become a major challenge as biodiversity declines rapidly worldwide.  Ecosystem consequences arising from the loss of diversity either within trophic levels (horizontal diversity) or across trophic levels (vertical diversity) are well documented.  However, simultaneous losses of species at different trophic levels may also result in interactive effects, with potentially complex outcomes for ecosystem functioning.  2.  Because of logistical constraints, the outcomes of such interactions have been difficult to assess in experiments involving large metazoan species.  Here, we take advantage of a detritus-based model system to experimentally assess the consequences of biodiversity change within both horizontal and vertical food-web components on leaf-litter decomposition, a fundamental process in a wide range of ecosystems.  3.  Our concurrent manipulation of fungal decomposer diversity (0, 1 or 5 species), detritivore diversity (0, 1 or 3 species), and the presence of predatory fish scent showed that trophic complexity is key to eliciting diversity effects on ecosystem functioning.  Specifically, although fungi and detritivores tended to promote decomposition individually, rates were highest in the most complete community where all trophic levels were represented at the highest possible species richness.  In part, the effects were trait-mediated, reflected in the contrasting foraging responses of the detritivore species to predator scent.  4.  Our results thus highlight the importance of interactive effects of simultaneous species loss within multiple trophic levels on ecosystem functioning.  If a common phenomenon, this outcome suggests that functional ecosystem impairment resulting from widespread biodiversity loss could be more severe than inferred from previous experiments confined to varying diversity within single trophic levels.