A key challenge in predicting the effects of global changes is determining how they may modify the influence of localised stressors, such that steps can be taken to minimise combined effects. Combined effects of global and local stressors can be difficult to predict as they are underpinned by influences on individual species and interactions between them, which in turn may be affected by absolute and relative densities. Here we tested experimentally the influence of increased temperature and/or nutrients on individual species, interactions between them and the consequences for ecosystem functioning. Elevated temperature had a more positive influence on functioning when species were combined in mixtures than when in monoculture. Responses to increased nutrients were positive irrespective of whether a species was in monoculture or in a mixture. Additionally, those effects were modified by changes in the relative density of the species, which in some cases resulted in shifts from negative interspecific interactions (competition) to positive interactions (facilitation) or vice versa.