Species coexistence is driven by niche difference, rather than fitness difference across multiple pairwise community.

Background/Question/Methods

Understanding the drivers of species coexistence is an important objective in ecology. Yet, the multitude of methods to study coexistence has hampered cross-community comparisons. Niche and fitness differences, central concepts of modern coexistence theory, hold the potential to unify insights across methodological differences. They respectively measure how species limit themselves compared to others and their competitive ability. A first outstanding question is if large niche differences, small fitness differences, or both, explain coexistence. Here, we standardized niche and fitness differences across 1018 species pairs to investigate species coexistence across ecological groups (annual plant, perennial plant, phytoplankton, bacteria or yeast) and methodological settings (field experiments, laboratory, or greenhouse). Using data gathered from 29 empirical papers, we present the first analysis of species interacting mechanisms across multiple ecological groups. Moreover, to understand whether coexistence is driven by different underlying mechanisms we performed an automated clustering algorithm. Finally, we tested whether these clusters are driven by external factors (i.e. ecological group, experimental setup, natural co-occurrence, population model used, and growth method).

Results/Conclusions

We find that, first, coexistence is driven by niche differences, not fitness differences. Indeed, coexisting and non-coexisting species are segregated along the niche differences axis rather than the fitness differences axis. Our clustering algorithm support this result as species group into clear clusters of coexisting and non-coexisting species and these clusters are driven by niche differences. We found that these clusters are not driven by specific factors, rather they highlight underlying differences in their interacting mechanisms from coexisting and non-coexisting communities that transcend all these empirical systems. The stark difference between coexisting and non-coexisting species hints at the presence of clearly different underlying species interactions. Our results show that, at a local scale, coexistence in the examined communities was primarily driven by mechanisms promoting niche differences, regardless of ecological group types. This highlights the importance of sustaining mechanisms that promote niche differences in order to maintain species coexistence.