An empiricist's guide to modern coexistence theory for competitive communities

While many ecological theories have historically invoked niche differences as the primary mechanism allowing species coexistence, we now know that coexistence in competitive communities depends on the balance of two opposing forces: niche differences (ND) that determine how species limit their own growth rate versus that of their competitor, and relative fitness differences (RFD) that establish competitive hierarchies among species. Several different empirical methods have been proposed for quantifying ND and RFD in order to make predictions about coexistence between pairs of species, yet it remains unclear which method(s) are appropriate for a given empirical study and whether or not those methods yield the same information. Here we compare six different empirical methods and provide a practical guide to help empiricists determine which method(s) are best for their study system. We show that there are important tradeoffs between mechanistic methods, which require detailed understanding of species’ resource use and physiology but are more tractable experimentally, and phenomenological methods which do not require this detailed information but can be impractical for some study designs. Finally, we show that the methods do not always yield the same values and therefore we caution against future syntheses that compile these estimates from different empirical studies.