Spatial exclusion leads to "tug-of-war" ecological dynamics between competing species within microchannels

Competition is ubiquitous in microbial communities, shaping both their spatial and temporal structure and composition. Classical minimal models of competition, such as the Moran model, have been employed in ecology and evolutionary biology to understand the role of fixation and invasion in the maintenance of population diversity. Informed by recent experimental studies of cellular competition in confined spaces, we extend the Moran model to incorporate mechanical interactions between cells that divide within the limited space of a one-dimensional open microchannel. The model characterizes the skewed collective growth of the cells dividing within the channel, causing cells to be expelled at the channel ends. The results of this spatial exclusion model differ significantly from those of its classical well-mixed counterpart. The mean time to fixation of a species is greatly accelerated, scaling logarithmically, rather than algebraically, with the system size, and fixation/extinction probability sharply depends on the species' initial fractional abundance. By contrast, successful takeovers by invasive species, whether through mutation or immigration, are substantially less likely than in the Moran model. We also find that the spatial exclusion tends to attenuate the effects of fitness differences on the fixation times and probabilities. We find that these effects arise from the combination of the quasi-neutral "tug-of-war" diffusion dynamics of the inter-species boundary around an unstable equipoise point and the quasi-deterministic avalanche dynamics away from the fixed point. These results, which can be tested in microfluidic monolayer devices, have implications for the maintenance of species diversity in dense bacterial and cellular ecosystems where spatial exclusion is central to the competition, such as in organized biofilms or intestinal crypts. Competition for territory between different species has far reaching consequences for the diversity and fate of bacterial and other ecological communities. In this study, we theoretically and computationally study the competitive dynamics of two bacterial populations competing for space in one-dimensional confined environments, extending classical models that serve as paradigms for competitive dynamics but do not explicitly include spatial exclusion. We find that spatial effects can drastically change the population outcomes by changing the probabilities that one species excludes the other from the environment the mean times to such exclusion. In comparison to the predictions of population models that neglect spatial aspects of the competition, species with higher relative fitness differences are less heavily favoured to outcompete their rival species. Spatial exclusion also changes the system stability with respect to invasion: there is a reduction in the effectiveness of invader's ability to take over the population. Our results show that spatial exclusion has rich and repercussions on species dominance and the long-time composition of populations. These must be considered when trying to understand complex bacterial ecosystems such as biofilms and intestinal flora.