From competition to facilitation and mutualism: a general theory of the niche

Abstract

Niche Theory is a central framework in ecology based on the recognition that most interactions between organisms are indirect, mediated by the biotic and abiotic dynamical environment these organisms live in. Despite its potential generality, the theory still mostly focuses on how resource–consumer dynamics mediate competition in ecological communities. However, it is being increasingly recognized that positive interactions between organisms also play an important role in driving the structure and functioning of ecological communities, from plants to microbes. In this paper, we present a unified theory of the niche that applies to both positive and negative interactions between organisms, mediated by one or two environmental factors. We show that classical concepts such as niche differences and fundamental and realized niches can naturally be expanded to facilitative and mutualistic interactions. In addition, we introduce and formalize new general niche concepts that appear exclusively in the presence of positive interactions: (1) the Allee niche, a region of environmental conditions for which a species can persist but not invade from low densities and (2) niche facilitation, when the presence of a species expands the set of environmental conditions under which a second species can invade and/or persist. To show the broad applicability of this theory, we illustrate these concepts using a diverse set of theoretical examples, from bacteria feeding on an inhibiting substrate, to nitrogen‐fixing plants and the indirect mutualism between a plant and a carnivore species. In sum, our work shows how Niche Theory provides a natural framework for positive interactions in ecology, bringing a unified perspective and new conceptual tools to study ecological systems where these positive interactions occur.

Species richness increases fitness differences, but does not affect niche differences

“…Importantly, the definition of Spaak et al (2020) for niche and fitness differences capture the intuitive concepts of niche and fitness differences known from the Lotka–Volterra community model (Chesson, 1990; Chesson & Kuang, 2008). Specifically, negative niche differences imply positive frequency dependence (Ke & Letten, 2018), niche differences exceeding 1 imply facilitation (Koffel et al, 2021) and intermediate niche differences imply negative frequency dependence. Stronger intrinsic strength is associated with lower fitness differences (more positive ) and species persist, that is have a positive invasion growth rate, if niche differences overcome fitness differences ().…”

Stability criteria for the consumption and exchange of essential resources

“…Our results can also be seen as a multi-species generalization of classic stability results for species competing for two resources (termed contemporary niche theory) where instability occurs because of the difference between impact and sensitivity vectors [17,[46][47][48]. In contemporary niche theory, there are three criteria which must be satisfied for a stable equilibrium to exist.…”

Niche differences, not fitness differences, explain coexistence across ecological groups

“…One way to overcome this limitation is by applying the concepts of niche and fitness difference across different ecological groups (Chesson, 2000b; J. W. Spaak and De Laender, 2020; Koffel et al, 2021b; J. W. Spaak, Godoy, et al, 2021).…”

“…However, their use comes with an important limitation: there is not a generally accepted method on how to measure niche and fitness di erences (Godwin et al, 2020;Song et al, 2019). Rather, there are as many as eleven di erent methods to assess niche and fitness di erences (J. W. Spaak and De Laender, 2020;Ko el et al, 2021a). Many of these methods are tailored to a specific community model and, consequentially, have only been applied to ecological groups which are well described by these models (Bimler et al, 2018;Godoy, 2019).…”

“…Rather, there are as many as eleven different methods to assess niche and fitness differences (J. W. Spaak and De Laender, 2020; Koffel et al, 2021a). Many of these methods are tailored to a specific community model and, consequentially, have only been applied to ecological groups which are well described by these models (Bimler et al, 2018; Godoy, 2019).…”