A unified definition of niche and fitness differences

Explaining nature’s biodiversity is a key challenge for science1. To persist against a background of interacting species with potentially higher fitness, populations must be able to grow faster when rare, a feature called negative frequency dependence2–4. Coexistence theory quantifies this dependence as niche differences (), but the available definitions differ greatly in how should vary with the type of species interactions, and often apply to specific community types only. Here, we present a new definition of that is consistent with biological intuition and can be applied to communities driven by both negative and positive species interactions, filling a main gap in the literature5,6. We also derive a definition for the corresponding fitness differences () among species, and illustrate how and jointly determine coexistence for various community types. We demonstrate that our definitions can be operationalised with theoretical models and experimental data. Our definition qualifies as the first method to quantify and in a standardised way across theoretical and empirical communities, facilitating comparison and fostering synthesis in community ecology.