Chesson (2000) showed how many previous studies on specific coexistence mechanisms, such as resource partitioning or temporal niche partitioning, could be unified into a general framework often called Modern Coexistence Theory (MCT). MCT has inspired new basic research about the mechanisms maintaining species coexistence, with important implications for understanding how high-diversity communities persist and for predicting impacts of environmental changes such as habitat fragmentation, nutrient enrichment, and changes in climate variability. However, MCT has limitations that have restricted empirical applications: new study systems often require a new model and a new, technically complex mathematical analysis; the standard categorization of coexistence mechanisms is not detailed or flexible enough for the possible coexistence mechanisms in many communities; and a key assumption of the underlying mathematical theory fails to hold in many data-driven models.
We present first steps towards a general simulation-based approach to MCT that will remove these obstacles. The key ingredients are (i) a general method for partitioning population growth rates into contributions from different mechanisms and their interactions, and (ii) evaluating the contributions through counterfactual simulations rather than analytic small-variance approximations. We apply this new approach to an experimental study by Descamps-Julien and Gonzalez (2005) on diatom species coexistence under periodic variation in temperature. Our approach diagnoses that persistence depends on fluctuation-dependent mechanisms, as was shown experimentally. For one of the two diatom species, the dominant fluctuation-dependent contributions are terms that are absent from MCT’s standard partitioning of non-spatial mechanisms: relative nonlinearity in response to temperature (analogous to, but distinct from, relative nonlinearity of competition), and the interaction between the effects of variance in competition and in temperature.