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

Abstract

While most ecological theories have historically invoked niche differences as the primary mechanism allowing species coexistence, we now know that species coexistence in competitive communities actually 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 measuring ND and RFD in order to make predictions about coexistence of species, yet it remains unclear which method(s) are appropriate for a given empirical study and whether or not those methods actually yield the same information. Here we summarize and compare five different empirical methods, with the aim of providing a practical guide for empiricists who want to predict coexistence among species. These include two phenomenological methods that estimate ND and RFD based on observing competitive interactions among species; two mechanistic methods that estimate ND and RFD based solely on information about species' resource requirements; and a fifth method that does not yield ND and RFD but describes the impacts of those forces within communities. Based on the specific requirements, limitations, and assumptions of each approach, we offer a series of decision steps that can be used to determine which method(s) are best for a given study system.In particular, we show there are important tradeoffs between mechanistic methods, which require detailed understanding of species niches 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. Importantly, we show that although each method can be used to estimate ND and RFD, the methods do not always yield the same values. Therefore we caution against future syntheses that compile these estimates from different empirical studies. Finally, we .

Natural enemies have inconsistent impacts on the coexistence of competing species

“…The general approach of propagating joint-posteriors into derived metrics of coexistence from model terms could equally apply to other coexistence modelling frameworks (Godwin et al 2020;Spaak & de Laender 2020) or alternative mechanisms that can facilitate community-level coexistence, such as intransitive competition (non-hierarchical competition, akin to an extended rock-paper-scissors tournament). We demonstrate this in SI 6 with a simple analysis of the impact of natural enemies on intransitivity.…”

“…Although ecologists have been competing insect species (especially Drosophila) against each other for decades (Pearl 1932;Ayala 1969;Gilpin et al 1986;Worthen 1989;Davis et al 1998), very few studies have explicitly tested for mutual-invasibility in insect systems (Siepielski et al 2018). In particular, few studies have directly assessed coexistence per se, rather than particular conditions necessary for coexistence (Godwin et al 2020;Spaak & de Laender 2020). environmental changes will create novel communities (Alexander et al 2016).…”

The effect of non-linear competitive interactions on quantifying niche and fitness differences

“…While these simplifying assumptions will by design be violated in certain conditions, we do not know the implications of such violations for our capacity to understand coexistence. Such knowledge is important for application of coexistence theory, as is choosing the appropriate definition of niche and fitness difference (Godwin et al, 2020).…”

Species coexistence in resource‐limited patterned ecosystems is facilitated by the interplay of spatial self‐organisation and intraspecific competition

“…This mechanism only gives insights into the coexistence of both species groups. Species richness within those groups may occur due to other facilitative processes that are independent of spatial self‐organisation principles (Godwin et al 2020 review both historic and more recent advances). For brevity, I refer to such species groups as single species in the remainder of the paper.…”