Additivity and asymmetricity in microbial community coalescence

Community coalescence, a process of mixing between two or more pre-existing ecological communities, plays an important role in assembly of natural and engineered microbiomes. Despite the large importance of this process, the quantitative patterns of community coalescence remain elusive. Using both theory and experiment with bacterial microcosms, we show that species pool size and interspecies interaction strengths determine the additivity and asymmetricity in community coalescence. Additivity decreases and eventually reaches a plateau as species pool size and interspecies interactions increase, providing evidence of community maturation in niche theory. Asymmetricity increases as interspecies interaction strengths increase, providing evidence of emergent cohesiveness. We further show that such features are well matched with population dynamics modelling. Taken together, our results demonstrate that the outcome of coalescence exhibits predictable statistical features depending on the species pool size and interspecies interaction strength.