Competition for shared resources increases the impact of propagule size during coalescence of gut microbial communities.

The outcomes of species introductions depend on both propagule size and resource competition, but the effects of these factors are inconsistent across community contexts. Here, we investigate how propagule size and resource competition interact to shape colonization during community coalescence by mixing pairs of in vitro gut microbial communities at ratios that vary over seven orders of magnitude. Each resulting co-culture contained species whose relative abundance depended on inoculation dose, causing the composition of some co-cultures to vary substantially across mixture ratios, even after >30 generations of growth. Using a consumer-resource model, we show that dose-dependent colonization can arise when niche overlap is high and species compete near-neutrally for shared resources. This model successfully predicts the outcomes of mixtures of strain isolates, in which propagule size has larger, longer-lasting effects in more diverse communities. Our study establishes a framework for predicting the context-dependent impacts of propagule size, providing principles for the design of microbiome therapeutics.