Stochastic processes govern invasion success in microbial communities when the invader is phylogenetically close to resident bacteria.

Despite recent efforts in identifying the determinants of invasion in microbial communities, experimental observations across different ecosystems are inconclusive. While relationships between resident community diversity and invasion success are often noted, community diversity says little about community assembly processes. Community assembly processes may provide a more inclusive framework to explain-and potentially prevent or facilitate-invasion. Here we let replicate nitrite-oxidizing bacterial guilds assemble under different conditions from a natural source community and study their compositional patterns to infer the relative importance of the assembly processes. Then, an invader strain from that same guild was introduced at one of three propagule pressures. We found no significant correlation between community diversity and invasion success. Instead, we observed that the effect of selection on invasion success was surpassed by the effect of drift, as inferred from the substantial influence of propagule pressure on invasion success. This dominance of drift can probably be generalized to other invasion cases with high phylogenetic similarity between invader and resident community members. In these situations, our results suggest that attempting to modulate the invasibility of a community by altering its diversity is futile because stochastic processes determine the invasion outcome. Increasing or reducing propagule pressure is then deemed the most efficient avenue to enhance or limit invasion success.