Bacteriophage richness reduces bacterial niche overlap in experimental microcosms

Antagonistic interactions such as competition and predation shape the structure and dynamics of ecological communities. Their combined effects can affect the species richness within a particular trophic level. Despite theory linking the complementarity of interactions across trophic levels and ecosystem functioning, there is a shortage of empirical tests of such predictions. We present an experimental investigation of these combined effects within a bacteria-phage interaction network. We measured the biomass yield of combinations of bacterial strains under increasing levels of bacteriophage richness. Our results show an increasing impact of phage on bacteria with increasing phage diversity. In contrast, no combination of phages significantly changed the overall productivity of bacterial mixed cultures when compared with expectations based on bacterial monocultures. Finally, we found that the addition of phages decreases the realized niche overlap among pair of bacterial species with the greatest reduction occurring when all phages were present. Our results show that the productivity of this system is the results from the combined effects of exploitative (shared resources between bacteria) and apparent (shared phages between bacteria) competition.