Toward a “modern coexistence theory” for the discrete and spatial

Abstract

The usual theoretical condition for coexistence is that each species in a community can increase when it is rare (mutual invasibility). Traditional coexistence theory implicitly assumes that the invading species is common enough that we can ignore demographic stochasticity but rare enough that it does not compete with itself, even after it has reached a stationary spatial distribution. However, short‐distance dispersal of discrete individuals leads to locally dense population clusters, and existing theory breaks down. We have an intuition that when we account for invader–invader competition, shorter‐range dispersal should reduce the invader's ability to escape competition, but exactly how does this translate into lower population growth? And how will invader discreteness affect outcomes? We need a way of partitioning the contributions to coexistence, but current modern coexistence theory (MCT) does not apply under these conditions. Here we present a computationally based partitioning method to quantify the contributions to coexistence from different mechanisms, as in MCT. We also build up an intuition for how invader clumping and discreteness will affect these contributions by analyzing a case study, a lattice‐based spatial lottery model. We first consider fluctuation‐dependent coexistence, partitioning the contributions of variable environment, variable competition, demographic stochasticity, and their correlations and interactions. Our second example examines fluctuation‐independent coexistence maintained by a fecundity–survival trade‐off, and partitions the contributions to coexistence from interspecific differences in fecundity, in mortality, and in dispersal. We find that demographic stochasticity harms an invader, but only slightly. Localized invader dispersal, on the other hand, can have a strong effect. When invaders are more clumped, they compete with each other more intensely when rare, so they too become limited by environment‐competition covariance. More invader clumping also means that variation in competition changes from helping the invader to harming it. More broadly, invader clumping is likely to weaken any coexistence mechanism that relies on the invader escaping competition from the resident, because invader clumping means that the resident is no longer the only source of competition.

Latitudinal scaling of aggregation with abundance and its consequences for coexistence in species rich forests

“…Third, we found that the frequently used deterministic invasion criterion 1,5 did not yield useful predictions because it was satisfied for nearly all forests analysed here (equation 11) and failed to detect differences in the local extinction risk of species related to demographic stochasticity 14 . Additionally, it does not apply if rare species are more strongly aggregated than abundant species 9 , as observed here in temperate forests. This leads to conspecific population-level interaction coefficients that are not constant as commonly assumed 49 , but depend negatively on abundance (equation 7c).…”

“…The search for simple principles underlying the complex spatial structure and dynamics of plant communities is a long-standing challenge in ecology [1][2][3][4][5][6] . In particular, the relationship between the spatial distribution of plants and species coexistence is challenging to resolve in species-rich communities [7][8][9] . Analysing the spatial patterns of tree species in 21 large forest plots, we find that rare species tend to be more spatially aggregated than common species, and a latitudinal gradient in the strength of this negative correlations that increases from tropical to temperate forests.…”

The role of plasticity and stochasticity in coexistence

“…While the role of phenotypic plasticity in species coexistence or exclusion has attracted much attention in recent years (Berg & Ellers, 2010;Callaway et al, 2003;Miner et al, 2005;Turcotte & Levine, 2016), the major theoretical advancements with regard to species coexistence presume fixed traits on ecological time scales (Chesson, 2000;Ellner et al, 2022;Letten et al, 2017;Serván et al, 2018). In addition, the most illuminating studies on the interplay between phenotypic plasticity and coexistence are in plants, often focusing on plantspecific traits (reviewed in Hillerislambers et al (2012)).…”

Dispersal of a dominant competitor can drive multispecies coexistence in biofilms

“…Ecological theory predicts that for N genotypes (different strains and species) to coexist in an ecosystem, they must be constrained by at least N independent limiting factors. These limiting factors can be resource types, interactions within and among species, and myriad environmental features [19][20][21][22][23][24][25] . When an ecosystem varies in space or time, the number of limiting factors can increase, and additional genotypes can coexist in the heterogeneous ecosystem, when otherwise they would be lost in a corresponding homogeneously mixed system 24,26 .…”