The effects of species pool, dispersal and competition on the diversity-productivity relationship

Aim The diversity-productivity relationship is a controversial issue in ecology. Diversity is sometimes seen to increase with productivity but a unimodal relationship has often been reported. Competitive exclusion was cited initially to account for the decrease of diversity at high productivity. Subsequently, the roles of evolutionary history (species pool size) and dispersal rate have been acknowledged. We explore how the effects of species pool, dispersal and competition combine to produce different diversity-productivity relationships. Methods We use a series of simulations with a spatially explicit, individual-based model. Following empirical expectations, we used four scenarios to characterize species pool size along the productivity gradient (uniformly low and high, linear increase and unimodal). Similarly, the dispersal rate varied along the productivity gradient (uniformly low and high, and unimodal). We considered both neutral communities and communities with competitive exclusion. Results and main conclusions Our model predicts that competitive interactions will result in unimodal diversity-productivity relationships. The model often predicts unimodal patterns in neutral communities as well, although the decline in richness at high productivity is less than in competing communities. A positive diversity-productivity relationship is simulated for neutral communities when the species pool size increases with productivity and the dispersal rate is high. This scenario is probably more widespread in nature than the others since positive diversity-productivity relationships have been observed more frequently than previously expected, especially in the tropics and for woody species. Our simulated effects of species pool, dispersal and competition on diversity patterns can be linked to empirical observations to uncover mechanisms behind the diversity-productivity relationship.