Coexistence and competitive exclusion in mutualism

Abstract

The competitive exclusion principle is fundamental to understanding coexistence. Well‐established theories predict the conditions for coexistence in consumer–resource interactions. Given that species often compete for commodities offered by mutualists, competitive exclusion theory should also be critical to understanding how mutualisms function. We explicitly apply the competitive exclusion principle to mutualism and derive a rule analogous to Tilman's R* rule for exploitative competition. Coexistence is impossible when competitors compete solely for a shared partner‐provided commodity because superior competitors deplete that commodity sufficiently to exclude inferior competitors. We then investigate how competition between two guild members for a partner‐provided commodity and a resource external to the mutualism affects competitor coexistence. There are three key results. First, coexistence is possible via partitioning of a partner‐provided commodity and another resource. Second, unlike in classic R* Theory, competitive outcomes are influenced both by species’ abilities to obtain commodities and their mutualisms with the shared commodity‐providing partner, which can indirectly alleviate competitors’ commodity limitation. Third, the outcome of competition has important consequences for the commodity‐providing partner, which depend on the type of mutualism and the competitive abilities of competing mutualists. This theory provides a novel framework for investigating how competitors for mutualistic commodities coexist in nature.

Pollination outcomes reveal negative density‐dependence coupled with interspecific facilitation among plants

“…Facilitation can only foster coexistence if rare species benefit disproportionately in relation to the abundant species (Soliveres et al ). Thus, plants’ pollination niches may represent an axis stabilising plant interspecific competition (Benadi & Pauw ; Lanuza et al ; Johnson & Bronstein ). Nevertheless, landscape‐level NDD was only marginally significant for pollen tubes.…”

Intuitive and broadly applicable definitions of niche and fitness differences

“…This may lead to the omission of mechanisms contributing to N (Chu & Adler, 2015). For example, niche partitioning could arise at different life stages of a species (Moll & Brown, 2008), or through its interactions with resources (Chesson, 1990), predators (Chesson & Kuang, 2008) or mutualists (Johnson & Bronstein, 2019) and will be affected by environmental change (Rey et al, 2017;Wainwright et al, 2018). An important advantage of the definitions is that they do not require analytical solutions of a community model or even a community model at all: one can simply simulate or perform the experiments described in the section "Application to experiments" and measure the resulting growth rates to compute N and F .…”

“…Natural communities host a multitude of mechanisms that can lead to frequency dependence. Well-known examples include resource partitioning (Adler et al, 2007;Levine & HilleRisLambers, 2009), differential vulnerability to predators (Allan et al, 2010;Carson & Root, 2000;Chesson & Kuang, 2008), differential associations with mutualists (Johnson & Bronstein, 2019;Siefert et al, 2018) or occupation of distinct microhabitats (Silvertown, 2004). These mechanisms have been collectively coined as 'niche differences' (Chesson, 2000;HilleRisLambers et al, 2012;Letten et al, 2017).…”

A perspective on stage‐structured mutualism and its community consequences

“…To my knowledge, existing models of mutualistic community dynamics are almost all non-structured (Valdovinos et al 2013, Bascompte and Jordano 2014, Mougi 2016a, Revilla and Křivan 2016, Johnson and Bronstein 2019. Recently, community ecologists have integrated different interaction types into so-called hybrid community networks (Melián et al 2009, Allesina and Tang 2012, Mougi and Kondoh 2012, Sauve et al 2014, Suweis et al 2014, Lurgi et al 2016.…”