Fluctuation-independent niche differentiation and relative non-linearity drive coexistence in a species-rich grassland

Despite the advances in ecological theory, evidence for the relative importance of the different mechanisms that promote species coexistence is lacking. Some mechanisms depend on the presence of interannual fluctuations in the environment combined with interspecific differences in the responses to such fluctuations. Among coexistence mechanisms, niche differentiation and storage effects have received much attention, whereas relative non-linearity (RNL) has been thought to be an unlikely and weak mechanism for multi-species coexistence and remains untested in nature. We quantified the relative contribution of different mechanisms to the coexistence of 19 grassland species by using field-parameterized population models and invasion analysis. Our results showed that 17 out of 19 species had the potential to coexist stably. Species diversity was maintained by RNL and large fluctuation-independent niche differences, i.e., between-species differentiation that is unrelated to interannual variations in environmental factors. Moreover, RNL increased the fitness of species that were less favored by niche differentiation, contributing to their persistence in the community. Storage effect was negligible or destabilizing, making no contribution to stable coexistence. These results, altogether with recent theoretical developments and indirect evidence in published data, call for a reassessment of RNL as a relevant mechanism for multi-species coexistence in nature.