Species interactions and colonization resistance in a spatially structured microbial community

Even though the majority of microbial communities are spatially structured, past studies have primarily focused on well-mixed communities. Here we examine how a spatial environment affects interspecies interactions and subsequently the ability of resident microbial communities to resist colonization by invaders—a property often called colonization resistance. In our study, we use a model in which (1) interactions are mediated by diffusible compounds in the environment and (2) species passively disperse in the environment. We find that in spatial communities, association with beneficial cross-feeding partners and isolation from detrimental partners emerge in the community and generally weaken colonization resistance compared to their well-mixed counterparts. This effect is stronger when cross-feeding is more prevalent and species dispersal is slower, driven by the localization of the interactions and the availability of more diverse niches for the invader to occupy. Previous reports often associate slower diffusion of mediators with higher coexistence and weaker colonization resistance. In contrast, we find a tradeoff: slower diffusion on the one hand can make interactions more localized and weaken colonization resistance. On the other hand, it can also limit the spread of mediators and weaken interactions that promote invasion, thereby strengthening colonization resistance. This tradeoff in our study leads eventually to a stronger colonization resistance in communities with slower diffusion of mediators. Overall, we find that by modulating the diversity of available niches and the strength of interspecies interactions, a spatially structured environment can influence colonization resistance.