Ecologists have long debated the relationship between net primary productivity and species richness with the intent of explaining the processes that structure communities. Recent work has shown that experimentally increasing productivity can shift the roles of stochastic and deterministic processes in community assembly thereby explaining how regional diversity varies with productivity. We use a global experiment to investigate the effect of increasing productivity on community assembly. Within the Nutrient Network (Nutnet) we added nutrients to sites with background productivity levels that range from low to high. We asked: how does nutrient addition alter 1) site-to-site variation in species composition (beta diversity), and 2) how much control and experimental communities deviate from a null model of stochastic assembly across the productivity gradient?
Nutrient addition homogenized species composition (reduced beta diversity), but only at productive sites across the global gradient. Further, communities in the nutrient-addition treatment deviated further from the null model than control communities. This effect varied with baseline productivity so that at low productivity sites, nutrient addition did not change community structure, while at productive sites, nutrient addition created communities that were more deterministic.