Intermediate phenotypic heterogeneity creates an optimal coexistence window in bacteria-phage communities
Phenotypic heterogeneity can expand ecological niche space even within a genetically homogeneous population. Recent experiments showed that growth-rate heterogeneity in a clonal bacterial host can stabilize coexistence among multiple phages. Yet this discovery leaves unresolved whether increasing host heterogeneity monotonically promotes coexistence. Here we show that it does not. Using a trait-based bacteria--phage coexistence framework, we find that phage richness is maximized at an intermediate scale of host growth-rate heterogeneity, determined by the match between host phenotypic dispersion and phage niche width. Our results show that optimal phage diversity emerges from an ecological trade-off between two competing effects: increasing phenotypic heterogeneity enables niche differentiation but eventually reduces the host abundance in the preferred niche of each phage. Therefore, our findings predict that experimentally increasing host growth-rate variance at a fixed mean growth rate should produce a nonmonotonic phage richness response, providing a quantitative principle for how intraspecific phenotypic variation shapes biodiversity in microbial ecosystems.