Despite the advances in ecological theory, evidence for the relative importance of the different mechanisms that promote species coexistence is lacking. Among coexistence mechanisms, storage effects have received much attention, whereas relative non-linearity has been thought to be an unlikely and weak mechanism for multi-species coexistence. Additionally, studies of the maintenance of species diversity have usually focus on one coexistence mechanisms at time, when in fact multiple coexistence mechanism are likely to operate simultaneously. Quantitative measurements are required to assess the relative importance of each mechanism, the way they relate to each other, and the frequency with which they co-occur in different ecosystems.
Here we quantified the relative contribution of fitness differences, fluctuation-independent niche differences, relative non-linearity and storage effect to the coexistence of 19 grassland species. We fitted population growth models using 13 years of field data and used them to analyze coexistence through invasion analysis.
Results/Conclusions
Our results showed that 17 out of 19 species had the potential to coexist stably. Species diversity was maintained mainly by large fluctuation-independent niche differences. Relative non-linearity was the second coexistence mechanism that contributed the most to diversity maintenance. Moreover, relative non-linearity increased the fitness of species that were less favored by niche differentiation, contributing to their persistence in the community. As in other studies, storage effect was negligible or destabilizing, making no contribution to stable coexistence. The fact that temporal fluctuations have small effects on coexistence in many systems highlights the need for a more detailed understanding of fluctuation-independent mechanisms and its drivers. These results, altogether with recent theoretical developments and indirect evidence in published data, call for a reassessment of relative non-linearity as a relevant mechanism for multi-species coexistence in nature.