Evolution of facilitation requires diverse communities.

Diverse experimental plant communities are more productive than monocultures. The increase of this biodiversity effect over time has been attributed to evolutionary selection for complementarity in mixtures. Here we show that evolutionary selection for enhanced net facilitative plant interactions occurred only in mixtures, while evolutionary selection for reduced net competition occurred in mixtures with mixture coexistence history and monocultures with monoculture coexistence history. Widespread declines in natural and agricultural biodiversity could therefore compromise potential evolution of facilitative interactions, that is, cornerstone processes in nature conservation and the development of sustainable agriculture.