 Micro-organisms interact with one another in diverse communities which are shaped by inter-specific interactions. While these interactions between individuals are usually additive, when multiple species each have a negative effect on a focal species, their combined effect is often stronger than the sum of the effects of individual species. This means that even though the effects of individual species may be small individually, they can still have a large collective impact on the focal species. This finding is important for understanding the fate of species introduced into an occupied environment, as well as for applications in medicine and agriculture, such as probiotics and biocontrol agents. It is also relevant for ecological questions surrounding migrating and invasive species. This article was authored by Amicai Bachman-Cas, Tingting Song, and Jonathan Friedman.