Evolution of One Species Increases Resistance to Invasion in a Simple Synthetic Community

The species that make up a microbial community determine its potential function. A major goal of microbial ecology is to make assemblages of microbes - synthetic communities - with targeted applications. Replacing a dysfunctional community with a synthetic microbial community can have transformative impacts upon a host or ecosystem, yet the introduced community may be outcompeted by local species or communities, resulting in transient effects. Here, we study a simple synthetic community comprised of two species - E. coli and S. cerevisiae - that have coevolved for either 0, 1000 or 4000 generations, and evaluate the potential for 12 bacterial strains, from five species, to invade. We find that the dominant species (E. coli) in the community protects the less dominant species from being outcompeted during an invasion, and that this effect is strengthened by longer periods of coevolution. Using a mathematical model, we show how prolonged co-evolution leads to protective effects for a community member sensitive to displacement.