Quantifying the relative importance of competition, predation, and environmental variation for species coexistence

Abstract

1Coexistence theory and food web theory are two cornerstones of the longstanding effort 2 to understand how species coexist. Although competition and predation are known to act 3 simultaneously in communities, theory and empirical study of the two processes continue to 4 be developed independently. Here, we integrate modern coexistence theory and food web 5 theory to simultaneously quantify the relative importance of predation, competition, and 6 environmental fluctuations for species coexistence. We first examine coexistence in a classic 7 multi-trophic model, adding complexity to the food web using a novel machine learning ap-8 proach. We then apply our framework to a parameterized rocky intertidal food web model, 9 partitioning empirical coexistence dynamics. We find that both environmental fluctuation 10 and variation in predation contribute substantially to species coexistence. Unexpectedly, co-11 variation in these two forces tends to destabilize coexistence, leading to new insights about 12

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

“…With few exceptions, the investigated communities consisted of basal species competing for abiotic resources. It is entirely possible that our findings do not apply to higher trophic levels, as research hints to the possibility that higher trophic levels are driven by different coexistence mechanisms (Shoemaker et al, 2019). A deeper understanding of how two-species coexistence links to multi-species coexistence might give us a better understanding of why niche differences are important in two-species communities.…”

“…Second, we have little mechanistic understanding of why coexisting species exhibit higher niche differences leading to a prevalence of negative frequency dependence. Various mechanisms can be responsible for this result, including mechanisms driven by specific organismal traits (Gallego et al, 2019; Kraft et al, 2015), phenological traits (Godoy and Levine, 2014; Farrer et al, 2010; Martorell and Freckleton, 2014; Adler, HilleRisLambers, et al, 2009), and these mechanisms can be fluctuation-independent (Armitage and Jones, 2019) or fluctuation-dependant (Hallett, Shoemaker, et al, 2019). To provide such understanding here would be purely speculative, given our data.…”

Emerging Frontiers in the Study of Molecular Evolution

“…Indeed, considering the influence of competition, predation or co-evolution in addition to the spatial arrangement of populations can provide new insights into the evolutionary processes that shape species' distributions (Hand et al 2015). Explicit models for species interactions in communities that interface with metagenomic and ultimately full genomic data are a futuristic area (Aldebert and Stouffer, 2018;Shoemaker et al 2019).…”

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

“…Additionally, we have fluctuation dependent mechanisms, storage effects and relative non-linearities (Letten et al ., 2018; Zepeda & Martorell, 2019; Litchman, 2003; Ellner et al ., 2019), the interaction of evolution and fluctuations (Yamamichi et al ., 2020), and internally generated fluctuations (Huisman et al ., 2006). We now may have too many explanations for diversity rather than too few, as we know little about the relative importance of all these mechanisms (Shoemaker et al ., 2019).…”

“…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.…”