What Have We Learned From Empirical Applications of Modern Coexistence Theory?

Since the early 2000's, Modern Coexistence Theory (MCT) has guided empirical investigations of coexistence. We reviewed empirical applications of MCT to answer two questions: (1) What have we learned about the strength of temporal, spatial and variation-independent mechanisms of coexistence in nature? (2) How have studies of niche and fitness differences advanced our understanding of coexistence? With respect to the first question, we now have evidence that the temporal storage effect and variation-independent mechanisms help stabilize coexistence in many systems. We have not learned more due to the low number of tests of these mechanisms (24 out of 84 MCT studies), the narrow focus on mechanisms involving temporal variation and grassland communities, and lack of rigorous model validation. With respect to our second question, studies of niche and fitness differences (61 of 84 studies) have answered fundamental questions about the role of phylogenetic relatedness, plant-soil feedbacks, and functional traits in coexistence. We recommend that future empirical MCT studies (1) match hypotheses to MCT quantities, (2) address gaps in empirical studies, (3) avoid known biases in estimating competition coefficients, and (4) validate models with independent data.