Structured environments fundamentally alter dynamics and stability of ecological communities

Abstract

The dynamics and stability of ecological communities are intimately linked with the specific interactions—like cooperation or predation—between constituent species. In microbial communities, like those found in soils or the mammalian gut, physical anisotropies produced by fluid flow and chemical gradients impact community structure and ecological dynamics, even in structurally isotropic environments. Although natural communities existing in physically unstructured environments are rare, the role of environmental structure in determining community dynamics and stability remains poorly studied. To address this gap, we used modified Lotka−Volterra simulations of competitive microbial communities to characterize the effects of surface structure on community dynamics. We find that environmental structure has profound effects on communities, in a manner dependent on the specific pattern of interactions between community members. For two mutually competing species, eventual extinction of one competitor is effectively guaranteed in isotropic environments. However, addition of environmental structure enables long-term coexistence of both species via local “pinning” of competition interfaces, even when one species has a significant competitive advantage. In contrast, while three species competing in an intransitive loop (as in a game of rock−paper−scissors) coexist stably in isotropic environments, structural anisotropy disrupts the spatial patterns on which coexistence depends, causing chaotic population fluctuations and subsequent extinction cascades. These results indicate that the stability of microbial communities strongly depends on the structural environment in which they reside. Therefore, a more complete ecological understanding, including effective manipulation and interventions in natural communities of interest, must account for the physical structure of the environment.

Spatial covariation of competing species in a fluctuating environment

“…This is in accordance with previous theoretical studies on effects of dispersal (Engen et al 2002, Walter et al 2017. Underlying spatial structure, caused by, for example, heterogeneity in resource availability, can have an additional impact on how organisms distribute themselves in space (Cohen andLevin 1991, Vallespir Lowery andUrsell 2019), which we have not considered in the spatially homogenous model presented here. Studies from several taxa have shown that environmental fluctuations can have a synchronizing effect on population dynamics of different species (Myers 1998, Post and Forchhammer 2002, Hansen et al 2013, Lahoz-Monfort et al 2013.…”

Community coalescence: an eco-evolutionary perspective

“…In homogeneous (and disturbed) environments, species which monopolize resources fastest will probably dominate, resulting in a chimeric outcome composed of the most competitive species from input communities [25,26]. By contrast, more structured environments can promote coexistence between species by limiting interaction intensity as niche boundaries are greater [36,37]. In these environments, coalescence outcomes are expected to be more modular as mixing between communities is limited and interactions become more localized.…”

Increasing growth rate slows adaptation when genotypes compete for diffusing resources

“…Here, ̂ is the scaled distance parameter, is the unscaled distance variable, is the diffusion constant of the resource, and µ is the organism's growth or uptake rate. A similar natural length scale was recently observed in a Lotka-Volterra model (Vallespir Lowery & Ursell, 2018), but its relevance to adaptation has not been explored. Does increasing ̂ increase the amount of structured interactions?…”