STRUCTURE: The Enemy of My Enemy is My Friend

� Abstract Mutualisms occur when interactions between species produce reciprocal benefits. However, the outcome of these interactions frequently shifts from positive, to neutral, to negative, depending on the environmental and community context, and indirect effects commonly produce unexpected mutualisms that have communitywide consequences. The dynamic, and context dependent, nature of mutualisms can transform consumers, competitors, and parasites into mutualists, even while they consume, compete with, or parasitize their partner species. These dynamic, and often diffuse, mutualisms strongly affect community organization and ecosystem processes, but the historic focus on pairwise interactions decoupled from their more complex community context has obscured their importance. In aquatic systems, mutualisms commonly support ecosystem-defining foundation species, underlie energy and nutrient dynamics within and between ecosystems, and provide mechanisms by which species can rapidly adjust to ecological variance. Mutualism is as important as competition, predation, and physical disturbance in determining community structure, and its impact needs to be adequately incorporated into community theory.