Higher-order interaction inhibits bacterial invasion of a phototroph-predator microbial community

In nature, the composition of an ecosystem is thought to be important for determining its resistance to invasion by new species. Studies of invasions in natural ecosystems, from plant to microbial communities, have found that more diverse communities are more resistant to invasion. It is thought that more diverse communities resist invasion by more completely consuming the resources necessary for would-be invaders. Here we show that Escherichia coli can successfully invade cultures of the alga Chlamydomonas reinhardtii (phototroph) or the ciliate Tetrahymena thermophila (predator), but cannot invade a community where both are present. The invasion resistance of the algae-ciliate community is due to a higher-order (3-way) interaction that is unrelated to resource consumption. We show that the mechanism of this interaction is the algal inhibition of bacterial aggregation which leaves bacteria vulnerable to ciliate predation. This mechanism of invasion resistance requires both the algae and the ciliate to be present and provides an example of invasion resistance through a trait-mediated higher-order interaction.