Niche differentiation in the dynamics of host-symbiont interactions: symbiont prevalence as a coexistence problem

Heritable symbioses can have important ecological effects and have triggered important evolutionary innovations. Current predictions for long-term symbiont prevalence are based on their fitness benefits and vertical transmission rates but ignore non-linear competitive feedbacks among symbiotic and symbiont-free hosts. We hypothesized that such feedbacks function as stabilizing mechanisms, promoting coexistence of host types and maintaining intermediate symbiont frequency at the population scale. Using a model grass / endophyte symbiosis, we manipulated competition within and between endophyte-symbiotic (E+) and endophyte-free (E-) hosts and fit competition models to experimental data. We show for the first time that symbiont-structured competition can generate stable coexistence of E+ and E- hosts, even under perfect vertical transmission. Niche differentiation was the key to coexistence, causing hosts of each type to limit themselves more strongly than each other. These results establi...