Understanding the role of species as drivers of ecosystem processes is imperative to preserve, utilize, and sustain ecosystems throughout the globe. The addition of species through invasion and the subtraction of species through extirpation and/or extinction can have profound effects on primary production, community respiration, and nutrient cycling. This is especially true for freshwater ecosystems in which a preponderance of native species are threatened with extirpation and/or extinction and where non-native species are frequently introduced. Commonly, anthropogenic activities result in the loss of biodiversity and enhance the ability of exotic species to invade and persist in novel habitats. These activities are expected to increase through time and advances in understanding the consequences of species loss and addition on ecosystem function are needed to guide appropriate management and conservation decisions. The loss and addition of organisms may result in a reduced capacity to support processes of self-purification of natural freshwater ecosystems and may render many habitats functionally impaired. The goal of this session is to use theoretical and applied work from studies of the addition of species and the loss of species from freshwater ecosystems to create a conceptual framework to quantify the effect of organisms on ecosystem processes. This goal will be achieved by addressing the following questions: 1. What patterns emerge from studies examining changes in ecosystem function after species invasion and species loss? 2. Are these patterns consistent across study sites and study organisms? 3. Are there specific character traits that make a species more apt to be an important driver of ecosystem function? The session will begin by a talk summarizing what has been learned from studies examining the effects of species loss and species gain on ecosystem processes. This talk will be followed by a series of theoretical and applied talks examining the effects of species addition and species loss from a broad range of freshwater ecosystems. For example, one of the talks will discuss changes in nutrient recycling after the loss of native bivalves in river ecosystems. We will also feature talks focused on the context-dependent nature of ecosystem change in response to increasing populations of organisms. We will close the session with a brief summary of findings that highlights the emerging patterns from the two types of studies and begin to outline a framework for understanding and predicting the potential effects of species gain and species loss on ecosystem processes in freshwater ecosystems.