Biodiversity is an essential determinant of ecosystem functioning. Numerous studies described positive effects
of diversity on the functioning of communities arising from complementary resource use and facilitation. However, high biodiversity may also increase competitive interactions, fostering antagonism and negatively affecting community performance. Using experimental bacterial communities we differentiated diversity effects based on genotypic richness and dissimilarity. We show that these diversity characteristics have opposite effects on ecosystem functioning. Genotypic dissimilarity governed complementary resource use, improving ecosystem functioning in complex resource environments. Contrastingly, genotypic richness drove allelopathic interactions, mostly reducing ecosystem functioning. The net biodiversity effect on community performance resulted from the interplay between the genetic structure of the community and resource complexity. These
results demonstrate that increasing richness, without concomitantly increasing dissimilarity, can decrease
ecosystem functioning in simple environments due to antagonistic interactions, an effect insufficiently
considered so far in mechanistic models of the biodiversity–ecosystem functioning relationship.