Abstract
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.
Significance statement
Genetically homogeneous bacterial populations can contain phenotypically distinct subpopulations, creating different ecological opportunities for the bacterio-phages that infect and prey on them. However, we show that this heterogeneity does not simply increase phage diversity. Instead, phage diversity is greatest at an intermediate level of bacterial phenotypic heterogeneity. Too little heterogeneity leaves phages competing for similar bacteria, whereas too much spreads bacterial biomass across phenotypic space, leaving phages with too few suitable hosts to persist. The optimum arises when host phenotypic dispersion matches phage infection breadth. This scale-matching principle links variation within a single population to biodiversity among interacting species and provides a direct experimental prediction for how microbial communities should respond to changes in phenotypic heterogeneity.