Emergent coexistence and the limits of reductionism in ecological communities.

Understanding the extent to which the species composition of a community can be explained by pairwise interactions is a long-standing question in ecology. Recent observations have revealed that stable microbial communities contain a high number of species that cannot coexist in pairs, providing new empirical elements to explore this question from a fresh perspective. Here, using species-rich models of ecological communities with pairwise interactions alone, we show that emergent coexistence arises naturally in an extent that is consistent with empirical observations. Interestingly, this phenomenon does not require additional mechanisms like intransitive or higher-order interactions; rather, coexistence can arise from the dense networks of indirect effects. As diversity increases, we show that indirect effects can become so intricate that pairwise interactions decouple from community composition, revealing a fundamental limit to reductionist explanations of coexistence. Our findings provide the theoretical foundations to understand how simple pairwise interactions can lead to emergent coexistence patterns in ecological communities.