My research focuses on the roles of animals in influencing the structure and function of ecosystems, and the consequences of changes in biodiversity in ecosystems. My current work aims to integrate these themes by asking questions about ecosystem-level effects of species losses and additions. Are natural systems ecologically redundant such that species deletions and additions are buffered by other community members? Or do changes in the numbers of species and their relative abundance generally have broad ecosystem consequences? I have focused much of my research on the ecology of tropical river systems, where I am trying to understand the strength of species interactions within hyper-diverse food webs and the influence of species on ecosystem processes such as nitrogen cycling and organic matter dynamics.  One of the most striking findings in our work is that grazing and sediment-feeding fishes play dominant roles in these systems, and their selective removal results in dramatic changes in community structure and ecosystem function. Furthermore, fishes that feed on sediments are important agents of biological disturbance and can introduce significant spatial and temporal heterogeneity with important ecosystem consequences.  Some of the current projects being pursued in my lab include the role of migratory fishes as food web subsidies and as major functional drivers in tropical river systems, the importance of consumptive versus nutrient recycling effects of animals in aquatic ecosystems, top-down and bottom-up controls in ecosystems across productivity gradients, and stoichiometric relationships among consumers and resources in stream food webs.