Facilitation Thinking for Coexistence Theory.

Species interactions are foundational to biodiversity maintenance. Facilitation, a common outcome of species interactions, occurs among and between a wide variety of organisms yet its treatment in the theory and models used to predict species coexistence is underdeveloped. We ask why this is and speculate about how to address this apparent discrepancy. We first evaluate a persistent ambivalence to facilitation in the context of population and community ecology, particularly in contemporary coexistence theory. We then propose 'facilitation thinking' to remedy the gap between empirical evidence of facilitation and mathematical theory of coexistence. We briefly discuss how a holistic treatment of facilitation in theory has the potential to reconfigure our basic understanding and definition of coexistence. Ultimately, we argue for an expanded theory of coexistence that accounts for a diversity of species interaction outcomes, allowing for the study of interactions and diversity maintenance beyond the war of all against all.