Resources that cross ecosystem boundaries (subsidies) are common in nature, but we have little knowledge of how such resources affect individual traits and, consequently, ecological interactions. In wetlands, leaf litter is an important resource subsidy that regulates ecological processes including the production of consumer biomass. However, litter quality is highly variable and depends on local plant species diversity. Many aquatic consumers are highly nutrient limited, so differences in nutrients and other chemical components in litter are likely to affect numerous individual traits of consumers including life-history and sexually selected traits. We tested whether the species of tree leaf litter consumed by freshwater amphipods affected survival, growth, and development of sexually selected traits. Amphipods had higher survival, attained larger body sizes, and had larger male sexual traits when reared on litter that was relatively nutritious and low in plant secondary and structural compounds. Such widespread trait changes suggest that changing forest composition could have substantial effects on wetland ecology and the evolution of sexual traits over ecological time scales.