One of the most ubiquitous patterns in plant ecology is species loss following nutrient enrichment. A common explanation for this universal pattern is an increase in the size asymmetry of light partitioning (the degree to which large plants receive more light per unit biomass than smaller plants), which accelerates the rates of competitive exclusions. This 'light asymmetry hypothesis' has been confirmed by mathematical models, but has never been tested in natural communities due to the lack of appropriate methodology for measuring the size asymmetry of light partitioning in natural communities. Here, we use a novel approach for quantifying the asymmetry of light competition which is based on measurements of the vertical distribution of light below the canopy. Using our approach, we demonstrate that an increase in light asymmetry is the main mechanism behind the negative effect of nutrient enrichment on species richness. Our results provide a possible explanation for one of the main sources of contemporary species loss in terrestrial plant communities.

Summary Plant communities show two general responses to gradients of soil resources: a decrease in species richness at high levels of resource availability and an associated shift in species composition from small and slow‐growing species to large and fast‐growing species. Models attempting to explain these responses have usually focused on a single pattern and provided contradicting predictions concerning the underlying mechanisms. We use an extension of Tilman's resource competition model to investigate the hypothesis that both patterns may originate from the size‐asymmetric nature of light exploitation by competing plants. The only mechanism producing changes in species richness and species composition in our model is mortality due to competition. Under the framework of the model, asymmetric light exploitation is a necessary and sufficient condition to obtain the empirically observed responses of species richness and species composition to soil resource gradients. This theoretical result is robust to relaxing the simplifying assumptions of the model. Our model shows that the traits enhancing competitive superiority depend on the mode of resource exploitation: under symmetric exploitation, competitive superiority is achieved by tolerance of low resource levels, while under asymmetric exploitation, it is achieved by the ability to grow fast and attain a large size. This result indicates that a long‐standing debate concerning the traits that enhance competitive superiority in plant communities (the ‘Grime–Tilman debate’) can be reduced into a single parameter of our model – the degree of asymmetry in resource competition. The model also explains the observed shift from below‐ground to above‐ground competition with increasing productivity, the associated increase in the asymmetry of competitive interactions and the increasing likelihood of competitive exclusion under high levels of productivity. None of these patterns could be obtained under symmetric competition in our model. Synthesis. The ability of the model to explain a wide range of observed patterns and the robustness of these predictions to its simplifying assumptions suggest that the size asymmetry of competition for light is a fundamental factor in determining the structure and diversity of plant communities.

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.

Significance Nutrient enrichment of natural ecosystems is a primary characteristic of the Anthropocene and a known cause of biodiversity loss, particularly in grasslands. In a global meta-analysis of 630 resource addition experiments, we conduct a simultaneous test of the three most prominent explanations of this phenomenon. Our results conclusively indicate that nitrogen is the leading cause of species loss. This result is important because of the increase in nitrogen deposition and the frequent use of nitrogen-based fertilizers worldwide. Our findings provide global-scale, experimental evidence that minimizing nitrogen inputs to ecological systems may help to conserve the diversity of grassland ecosystems.

A fundamental notion in community ecology is that local species diversity reflects some balance between the contrasting forces of competitive exclusion and competitive release. Quantifying this balance is not trivial, and requires data on the magnitude of both processes in the same system, as well as appropriate methodology to integrate and interpret such data. Here we present a novel framework for empirical studies of the balance between competitive exclusion and competitive release and demonstrate its applicability using data from a Mediterranean annual grassland where grazing is a major mechanism of competitive release. Empirical data on the balance between competitive exclusion and competitive release are crucial for understanding observed patterns of variation in local species diversity and the proposed approach provides a simple framework for the collection, interpretation, and synthesis of such data.