Relationships between community composition, productivity and invasion resistance in semi-natural bacterial microcosms

Experiments with artificial communities have suggested that invasion resistance in microbial communities is often a side-effect of community members contribution towards overall community productivity (broadly defined as cumulative cell density and/or growth rate). However, few experiments have investigated this in natural microbial communities. We conducted experimental invasions of two bacterial species (Pseudomonas fluorescens and Pseudomonas putida) into laboratory microcosms inoculated with 680 different mixtures of bacteria derived from naturally-occurring microbial communities collected in the field. Using 16S amplicon sequencing to characterise microcosm starting composition, and high-throughput assays of community phenotypes including productivity and invader survival, we determined that productivity is a key predictor of invasion resistance in natural microbial communities, substantially mediating the effect of composition on invasion resistance. The results suggest that similar general principles govern invasion in artificial and natural communities, and that factors affecting resident community productivity should be a focal point for future microbial invasion experiments.