Freshwater studies have contributed significantly to understanding the relationship between biodiversity and ecosystem function.  Much of the evidence comes from studies of species composition of leaf litter and fungi, in which both types of organisms can play non-additive roles on breakdown rates of organic matter, an important ecosystem process.  Furthermore, global change phenomena such as elevated temperature, CO₂ or nutrients also change the rates of important ecosystem processes.  Comparisons of the magnitude of the effects of these factors versus diversity show that, for example, nutrient enrichment can have greater effects than diversity on ecosystem function.  To study the interactive effects of global change factors and species loss on ecosystem function, we conducted a study of how nutrient enrichment affected the relationship between leaf litter diversity and organic matter breakdown. The effects of litter diversity on breakdown rate observed in our reference stream were subdued under nutrient-enriched conditions.  Whereas mixtures of species had generally slower breakdown rates than predicted based on rates from single species in the reference, such mixing effects were negated under nutrient enrichment.  Our work suggests that the effect of species loss might interact strongly with other aspects of global change to alter important ecosystem processes in streams.