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

Abstract

The niche and fitness differences of modern coexistence theory separate mechanisms into stabilizing and equalizing components. Although this decomposition can help us predict and understand species coexistence, the extent to which mechanistic inference is sensitive to the method used to partition niche and fitness differences remains unclear. We apply two alternative methods to assess niche and fitness differences to four well known community models. We show that because standard methods based on linear approximations do not capture the full community dynamics, they can sometimes lead to incorrect predictions of coexistence and misleading interpretations of stabilizing and equalizing mechanisms. Conversely, a more recently developed method to decompose niche and fitness differences, that accounts for the full nonlinear dynamics of competition, consistently identifies the correct contribution of stabilizing and equalizing components. This approach further reveals that when the true complexity of the system is taken into account, essentially all mechanisms comprise both stabilizing and equalizing components. Amidst growing interest in the role of non-additive and higher-order interactions in regulating species coexistence, we propose that the effective decomposition of niche and fitness differences will become increasingly reliant on methods that account for the inherent non-linearity of community dynamics.

Different measures of niche and fitness differences tell different tales

“…1, red block). These equations have been applied to similar community models, by rephrasing these similar community models in the terms of the inter and intra-specific interaction coefficients α ij (Spaak et al 2021a). Specifically, this method has been used in community models including competition for predators (Chesson and Kuang 2008) or mutualists (Johnson 2021), a time discrete annual plant model (Godoy and Levine 2014), resource competition models with Holling type II response (Letten et al 2017), inclusion of facilitation between species (Bimler et al 2018) and plant-soil feedbacks (Kandlikar et al 2019).…”

Modeling phytoplankton-zooplankton interactions: opportunities for species richness and challenges for modern coexistence theory

“…The relative importance of different coexistence mechanisms might depend on species richness. For example, Spaak et al (2021a) have shown that increasing species richness increases the importance of fitness differences compared to niche differences. Third, the trophic structure was simple and excluded any higher trophic level and any mixotrophs.…”

“…These communities behaved similarly to communities driven by Allee effects or communities driven by obligatory mutualism. It was not previously recognized that these communities can arise from simple resource competition, a well understood mechanism that is key for modern coexistence theory (Chesson & Kuang, 2008; Chesson, 1990; Letten et al ., 2017; Letten & Stouffer, 2019; Spaak et al ., 2021a,b).…”

“…But 60 years of research has produced a long list of the different limiting factors that could promote stable coexistence in plankton communities and the diversity of primary producers in general. For example, we expect trade-offs between resource affinities for light, nitrogen or phosphorus (Litchman et al ., 2007; Litchman & Klausmeier, 2008; Edwards et al ., 2011; Kraft et al ., 2015) or for different wavelengths of light (Stomp et al ., 2004; Spaak et al ., 2021a). Moreover, we can expect some species to specialize on low resource conditions and others on high resource conditions, the “gleaner-opportunist” trade-off (Litchman & Klausmeier, 2001; Kiørboe et al ., 2018).…”

“…As a result, we have little understanding of coexistence mechanisms in the highest trophic level or, more generally, how the entire community coexists (Godoy et al ., 2018). Additionally, modern coexistence theory is often applied to models with phenomenological or linear terms describing species interactions, excluding higher order interactions (Chesson, 2018; Germain et al ., 2016; Pérez-Ramos et al ., 2019) but see (Spaak et al ., 2021a; Shoemaker et al ., 2019; Spaak & De Laender, 2021; Spaak et al ., 2021b; Singh & Baruah, 2020; Letten & Stouffer, 2019). Consequentially, we have only limited understanding of the drivers and mechanisms of species coexistence in communities including multiple trophic levels and non-linear species interactions.…”