How community size affects survival chances in cyclic competition games that microorganisms play.

Cyclic competition is a mechanism underlying biodiversity in nature and the competition between large numbers of interacting individuals under multifaceted environmental conditions. It is commonly modeled with the popular children's rock-paper-scissors game. Here we probe cyclic competition systematically in a community of three strains of bacteria Escherichia coli. Recent experiments and simulations indicated the resistant strain of E. coli to win the competition. Other data, however, predicted the sensitive strain to be the final winner. We find a generic feature of cyclic competition that solves this puzzle: community size plays a decisive role in selecting the surviving competitor. Size-dependent effects arise from an easily detectable "period of quasiextinction" and may be tested in experiments. We briefly indicate how.