Ecology Letters (2011) 14: 36–41
One robust result from many small-scale experiments has been that plant community productivity often increases with increasing plant diversity. Most frequently, resource-based or competitive interactions are thought to drive this positive diversity–productivity relationship. Here, we ask whether suppression of plant productivity by soil fungal pathogens might also drive a positive diversity–productivity relationship. We created plant assemblages that varied in diversity and crossed this with a ± soil fungicide treatment. In control (non-fungicide treated) assemblages there was a strong positive relationship between plant diversity and above-ground plant biomass. However, in fungicide-treated assemblages this relationship disappeared. This occurred because fungicide increased plant production by an average of 141% at the lower ends of diversity but boosted production by an average of only 33% at the higher ends of diversity, essentially flattening the diversity–productivity curve. These results suggest that soil pathogens might be a heretofore unappreciated driver of diversity–productivity relationships.