Co-occurring soil bacteria exhibit a robust competitive hierarchy and lack of non-transitive interactions

Microbial communities are typically incredibly diverse, and this diversity is thought 14 to play a key role in community function. However, explaining how this diversity 15 can be maintained is a major challenge in ecology. Temporal fluctuations and 16 spatial structure in the environment likely play a key role, but it has also been 17 suggested that the structure of interactions within the community may act as a 18 stabilizing force for species diversity. In particular, if competitive interactions are 19 non-transitive as in the classic rock-paper-scissors game, they can contribute to the 20 maintenance of species diversity; on the other hand, if they are predominantly 21 hierarchical, any observed diversity must be maintained via other mechanisms. 22 Here, we investigate the network of pairwise competitive interactions in a model 23 community consisting of 20 strains of naturally co-occurring soil bacteria. We find 24 that the interaction network is strongly hierarchical and lacks significant non25 transitive motifs, a result that is robust across multiple environments. Moreover, in 26 agreement with recently proposed community assembly rules, the full 20-strain 27 competition resulted in extinction of all but three of the most highly competitive 28 strains, indicating that higher order interactions do not play a major role in 29 structuring this community. The lack of non-transitivity and higher order 30 interactions in vitro indicates that other factors, such as temporal or spatial 31 heterogeneity, must be at play in enabling these strains to coexist in nature. 32