Transgenerational coexistence history attenuates negative direct interactions and strengthens facilitation

Abstract Background Interactions among species are a fundamental aspect of biodiversity and driving ecosystem functioning and services. Species interactions include direct (pairwise) interactions among two species and indirect interactions that occur when a third species interacts with the two others and changes the direct interactions between the two. In a three-species interaction network, these interactions can be transitive (where one species outperforms all others) or intransitive (where each species outperforms another). How direct and indirect interactions influence ecosystem functions in crop systems, and how diversification and evolutionary adaptation can influence those interactions and therefore ecosystem functions has not been studied. Methods A common garden experiment was conducted with crop communities in monocultures, 2- and 3-species mixtures that had either a common or no coexistence history (i.e. community adaptation) for three years. Net, direct and indirect interaction intensities were estimated and compared between the diversity levels and coexistence histories. Furthermore, species interaction networks were inspected for transitive/intransitive interactions. Results We found evidence for lower competition in mixtures and for reduced negative direct interaction intensity and enhance facilitative effects upon community adaptation. We could further show that indirect interactions were generally less important for community adaptation than direct interactions. Additionally, we showed that community adaptation has the potential to shift interactions in the species interaction networks from competitive intransitive into pairwise competitive interactions where interactions occurred mainly between two species. Synthesis Co-adapted crop species with reduced negative interactions might have the potential to enhance productivity especially in more diverse cropping systems. This supports the notion that intercropping is a vital part towards a more sustainable agriculture and one with further yield potential when developing cultivars adapted to grow in mixtures.