Tipping points emerge from weak mutualism in metacommunities

Abstract

The coexistence of obligate mutualists is often precariously close to tipping points where small environmental changes can drive catastrophic shifts in species composition. For example, microbial ecosystems can collapse by the decline of a strain that provides an essential resource on which other strains cross-feed. Here, we show that tipping points, ecosystem collapse, bistability and hysteresis arise even with very weak (non-obligate) mutualism provided the population is spatially structured. Based on numeric solutions of a metacommunity model and mean-field analyses, we demonstrate that weak mutualism lowers the minimal dispersal rate necessary to avoid stochastic extinction, while species need to overcome a mean threshold density to survive in this low dispersal rate regime. Moreover, we show that, starting with a randomly interacting species pool, metapopulation structure tends to select for an ecosystem with mutualistic interactions. Bistable metacommunities could, therefore, be a natural outcome of evolutionary dynamics in structured ecosystems.

Mutualism Destabilizes Communities, but Competition Pays the Price

“…A recent modeling study of assembly processes in communities also suggests that when species sequentially invade a community, a balance of interaction types is vital for community stability, with higher fractions of mutualistic interactions corresponding to increased species persistence as well as increased stability of the community as a whole to external invasions (Qian and Akçay, 2020). Such so-called 'ecological selection' (Qian and Akçay, 2020) for community structure during assembly has also been observed in dispersal models (Denk and Hallatschek, 2023) and eco-evolutionary community models (Nell et al, 2022). Our study highlights that ecological selection of this form can operate not only through a sequential assembly of communities but also through extinctions from initially assembled unstable communities.…”