Invasion-Induced Phase Transitions in Microbial Ecosystems*

It is a challenge to predict the outcome of introducing new species to a microbial community, especially those with poorly defined ecological interactions. Therefore, we studied a model community of two cross-feeding species, with small molecule resources in the system explicitly modeled. We challenged this community with invaders displaying three strategies: complete competitors for the common energy source, parasites that use shared resources produced by native species, and a third adopting both strategies. With analytical and numerical analysis, we defined phase diagrams of post-invasion community compositions in the space of community mutuality and invader fitness. As invader fitness increases, complete competitors cannot coexist with the native community, resulting in first-order transitions across phase boundaries, while parasites can be integrated into the community, leading to second-order phase transitions. We tested other communities, some with nonlinear interactions, and observed this correspondence between phase transition order and outcome of invasions (species loss vs. integration) is general. Our work can serve as a theoretical basis for understanding biodiversity changes in a community upon invasion by foreign species