The interplay between species' positive and negative interactions shapes the community biomass-species richness relationship

We used an individual-based spatially-explicit model to assess the role of facilitation and plant strategies in shaping the 'community biomass-species richness' relationship. Facilitation had few impacts on community's richness under both the most benign (high community biomass) and the most severe (low community biomass) environments where its intensity was weak. From medium to high environmental severity, facilitation increased community richness, because all plant strategies were facilitated. In contrast, from low to medium environmental severity facilitation decreased community richness, because only the most competitive species were facilitated, which induced a decrease in the richness of the stress-tolerant species overwhelming the increase in richness of the competitive species. Above all, our simulations show how 'strategy-dependent' interactions among species combine to shape the humped-back biomass-species richness relationship. It also demonstrates that facilitative effects might have long-term negative effects on species richness, which result is not included in current facilitation models.