Abstract: A major challenge in biodiversity research is this incorporation of species interactions into population and community response to global environmental change (GEC). Facultative mutualisms are a type of species interaction especially sensitive to changing environmental conditions, and the breakdown of these mutualisms could amplify negative impacts of novel climate regimes on focal species. However, facultative mutualisms are poorly studied in the context of GEC compared with other types of species interactions. Here, we investigate how the breakdown of a facultative mutualism between two coastal foundation species impacts community response to ocean warming (+1.5°C) and acidification (-0.4 pH) via a manipulative co-culture experiment in mesocosms. Under ambient environmental conditions, species interactions were mutualistic with eelgrass enhancing oyster shell growth by 17% and oysters increasing eelgrass leaf growth rates by 35% and clonal reproduction by 38%. Varying sensitivity of each species to ocean warming and/or acidification treatments led to complex shifts in species interactions that were trait dependent. As such, community outcomes under future conditions were the result of species interactions that amplified, mitigated, or did not impact species response to environmental change. Given that species interaction effect sizes could be twice the magnitude of warming or pH treatments, our results underscore the need to identify key species and interaction types that strongly influence community response to GEC. Predictive species distribution models and management plans that incorporate species interactions could benefit by incorporating how sensitive the strength and outcome of key species interactions are to GEC.
