Eco-evolutionary interactions with multiple evolving species reveal both antagonistic and additive effects.

Most eco-evolutionary research focuses on ecological effects of single-species evolution. We therefore know little of eco-evolutionary dynamics when multiple species evolve simultaneously. We quantified evolution-mediated ecological effects in communities equivalent in genetic diversity and starting biomass, but different in selection background (heatwave exposure) of one or all four zooplankton species (three Daphnia and one Scapholeberis species). We observed transient eco-evolutionary effects that differed depending on which species in the community had evolved. Evolution did not always lead to higher abundances of the evolved species. Indirect effects on species abundances caused by evolution of another species could be as strong as direct effects mediated by its own evolution. The cumulative effect of evolution in multiple species was antagonistic for community composition and grazing pressure but additive for community-wide biomass. Our results imply that focusing on single species’ evolutionary effects on ecology may lead to unreliable predictions when multiple species evolve simultaneously.