Interspecific plant competition mediates the metabolic and ecological signature of a plant-herbivore interaction under warming and elevated CO2

Biotic interactions shape community evolution, but we lack mechanistic insights on how metabolic and ecological processes under climate change are altered by biotic interactions. We used a two-trophic model community consisting of the aphid Dysaphis plantaginea feeding on the forb Plantago lanceolata , and a grass competitor Lolium perenne that does not experience herbivory by the aphid. Monocultures and mixtures were exposed to the herbivory treatment and to three relevant simulated environmental changes as prevalent under current climate change (increased temperature, CO 2 , and increased temperature and CO 2 ) Elevated CO 2 reduced the nitrogen content of P. lanceolata , while simultaneous increases of CO 2 and temperature modified the plant metabolic component and the magnitude of these responses in different directions. Elevated CO 2 enhanced defence systems in P. lanceolata , but these effects were not altered by warming. Interspecific plant competition did, however, neutralise these responses. There were no indirect effects of climate change on aphid population growth despite changes in plant defense, nutritional quality and biomass induced by our environmental change scenarios. We thus demonstrate interactions between abiotic and biotic processes on plant metabolite profiles, but more importantly, that climate change effect on a selection of the metabolic pathways are altered by herbivory and competition. Our experiment under semi-natural conditions thus demonstrates the non-additive and often neutralizing effects of biotic interactions on plant metabolism and species performance under climate-associated environmental change.