Fig. 4
Pathogen fitness as a function of defensive microbe density. Fitness under costs of reduced infectivity and varying strength of interspecific competition (i) β 1=0.001, (ii) β 1=0.01, (iii) β 1=0.1. Competition has an increasing effect on fitness at high defensive microbe density. The only ESS is when the density of defensive microbes is zero. Otherwise fitness declines (Eq. 13) and the strength of selection against the pathogen accelerates (Eq. 14). (Other parameter values: γ=50, μ P =0.0)