Explaining the large numbers of species that coexist within communities is a long-standing challenge in biology. Ecological theory suggests that coexistence requires negative frequency-dependent feedbacks to prevent exclusion of the least fit species. For plant communities, empirical evidence of negative frequency-dependence is rapidly accumulating and increasingly identified as a potential driver of species coexistence and diversity patterns. Until now, however, connecting these empirical findings with ecological theory has been hampered by the corresponding theoretical framework of frequency-dependent feedback not being developed beyond a small number of interacting species.
Results/Conclusions
Using a generic model framework, we obtained an exact result revealing under which conditions frequency-dependent feedback constrains coexistence at the community level. Regardless of the number of species in a community and inherent fitness inequalities between them, the interaction network can be characterized by a single community interaction coefficient, IC, which determines whether community-level feedback is positive or negative. Negative feedback is a necessary (but not sufficient) condition for persistence of the entire community. Even in cases where the coexistence equilibrium state cannot recover from perturbations, IC < 0 can enable species persistence via cyclic succession. The number of plant species coexisting in a community is predicted to increase with the average strength of negative feedback. This theoretical prediction is supported by patterns of tree species diversity in more than 200,000 deciduous forest plots in the eastern United States, which can be reproduced in simulations that span the observed range of community feedback. By providing a quantitative metric defining the amount of negative feedback needed for coexistence, we can now integrate theory and empirical data to test whether observed feedback-diversity correlations are strong enough to infer causality.