Contrasting effects of water and nutrient additions on grassland communities: A global meta-analysis

Aim: Changes in global climate and land use are expected to alter water and nutrient availability. Various meta-analyses and large-scale experiments show that increasing nutrient availability is expected to decrease the diversity of ecological communities, but so far, no study has attempted to provide a global-scale perspective of diversity responses to water manipulation. Location: Global. Methods: We conducted a meta-analysis focusing on the effects of water and nutrient additions both on species richness and on biomass of herbaceous plant communities. We identified 41 water addition experiments, of which 19 experiments manipulated both water and nutrients. Results: Although both water and nutrient additions increased biomass (by c. 15 and 34%, respectively), only the latter consistently decreased richness (by c. 23%). Biomass responses to water addition were mainly derived from an increase in forb biomass (by c. 37%), whereas corresponding responses to nutrient addition were derived from an increase in graminoid biomass (by c. 56%). Addition of both water and nutrients led to larger biomass responses compared with the addition of each resource alone (by c. 69%), but the negative effect on species richness was similar to nitrogen addition alone. None of these responses could be explained by general (resource-independent) theories, such as the productivity–diversity hypothesis or the niche dimension hypothesis. Main conclusions: While highlighting overlooked patterns, this meta-analysis reveals a fundamental knowledge gap in our ability to predict biodiversity responses to global change and demonstrates that future theories attempting to explain and predict such changes must take into account the potential implications of resource-specific and functional group-specific responses.