Building modern coexistence theory from the ground up: the role of community assembly

Modern coexistence theory (MCT) is one of the leading methods to understand species coexistence. It uses invasion growth rates - the average, per-capita growth rate of a rare species - to identify when and why species coexist. Despite significant advances in dissecting coexistence mechanisms when coexistence occurs, MCT relies on a "mutual invasibility" condition designed for two species communities, but poorly defined for species rich communities. Here, we review well-known issues with this component of MCT and propose a solution based on recent mathematical advances. We propose a clear framework for expanding MCT to species rich communities and for understanding invasion resistance as well as coexistence, especially for communities which could not be analyzed with MCT so far. Using two data-driven community models from the literature, we illustrate the utility of our framework and highlight the opportunities for bridging the fields of community assembly and species coexistence. Statement of authorship: Studied conceived jointly by JWS and SJS. JWS and SJS wrote the manuscript together. JWS wrote the python code and SJS wrote R code. Data accessibility: All computer code used in this manuscript will be made publicly available on figshare. Niche and fitness differences | Storage effect | Coexistence