Disentangling the effects of external perturbations on coexistence and priority effects

Abstract

A major challenge in ecological research is to identify the tolerance of ecological communities to external perturbations. Modern coexistence theory (MCT) has been widely adopted as a framework to investigate the tolerance to perturbations in relative reductions of per capita growth rates, often using metrics that explicitly eliminate the independent role of intrinsic growth rates. More recently, the structural approach (SA) was introduced to investigate the tolerance of communities to perturbations in intrinsic growth rates as a function of the strength of intraspecific and interspecific competition. Because the external perturbations are likely to happen in both intrinsic growth rates and competition strengths, no framework alone can fully disentangle the effects of external perturbations. Here we combine MCT and SA to disentangle the tolerance in coexistence and priority effects of a pair of competing species when subject to perturbations in intrinsic growth rates and competition strengths. Through this combination, we reveal the emergence of a key trade‐off: increasing the tolerance to perturbations in intrinsic growth rates typically decreases the tolerance in competition strengths, and vice versa. Furthermore, this trade‐off is stronger under coexistence than under priority effects. We test this combined framework on competing pairs of 18 California annual plant species. For both coexistence and priority effects, we find that the tolerance to perturbations in intrinsic growth rates is maximized instead of that to perturbations in competition strengths in the studied annual plant communities. Synthesis. Our combined framework of modern coexistence theory and structural approach illustrates that it is possible to disentangle the impact of different external perturbations on the persistence of species. Importantly, our findings show that species interactions may reveal whether communities are dominated either by changes in intrinsic growth rates or by competition strengths. Overall, this combined framework can open a new perspective to understand and predict the response of populations to changing environmental conditions.

Understanding the emergence of contingent and deterministic exclusion in multispecies communities

“…The importance of the orientation is that stable feasibility leads to coexistence, whereas unstable feasibility leads to contingent exclusion (Case, 1999; Fukami et al ., 2016). The orientation of the feasibility domain is mainly driven by the ratio of intra-to interspecific interactions (Song et al ., 2020a). In sum, following the structural approach, whether competitive exclusion is deterministic or contingent should be expected to be mainly driven by the match between the observed intrinsic growth rates (mainly constrained by life-history processes) with the shape and the orientation of the feasibility domain (both of which are determined by the observed competition structure).…”

“…By way of example, focusing on two-species communities (see Figure 1 for a graphical illustration), one can establish three key intuitions about competitive exclusion derived from the structural approach (Song et al ., 2020a): (i) For contingent exclusion to occur, it is necessary that species depress their competitor’s per capita growth rate more than their own (changing the orientation of the feasibility domain). (ii) The larger the intrinsic growth rate of the competitively inferior species, the more likely contingent exclusion is to occur.…”

“…This theoretical development started by focusing on the conditions leading to deterministic exclusion (Chesson, 2000; Adler et al ., 2007), and then it was extended to investigate the conditions for contingent exclusion (Mordecai, 2011; Fukami et al ., 2016; Ke & Letten, 2018). Similarly, extensive empirical research started to examine the sources of deterministic exclusion (Mayfield & Levine, 2010; Violle et al ., 2011; Adler et al ., 2010), and more recently it has moved to the analysis of contingent exclusion (Grainger et al ., 2018, 2019; Song et al ., 2020a). Focusing on competition between two species, this body of work has shown that deterministic exclusion is more likely to occur when the competitively inferior species has a lower intrinsic growth rate and when intraspecific interactions are stronger than interspecific interactions.…”

“…Focusing on competition between two species, this body of work has shown that deterministic exclusion is more likely to occur when the competitively inferior species has a lower intrinsic growth rate and when intraspecific interactions are stronger than interspecific interactions. By contrast, greater similarity in species intrinsic growth rates and stronger interspecific relative to intraspecific interactions promote contingent exclusion (Ke & Letten, 2018; Song et al ., 2020a).…”

“…In general, the structural approach posits that how likely a particular outcome of competition is to occur can be understood through the full range of environmental conditions (contexts) compatible with that qualitative outcome. While the structural approach was initially devised to investigate species coexistence as the qualitative outcome (Rohr et al ., 2014; Saavedra et al ., 2017), it can also be extended to study competitive exclusion (Song et al ., 2020a). Here, we apply the structural approach to investigate the emergence and sources of competitive exclusion in multispecies communities as a function of species’ intrinsic growth rates, community size (number of competing species), and competition structure (i.e., the interaction matrix).…”