Figure 1.

Impact of parasite‐conferred resistance (left) and tolerance (right) on the ecological dynamics. Dashed lines correspond to $c_1=0$ (no costs) and solid lines to accelerating transmission rate costs $(c_1=0.75,c_2=3)$. (Ai–ii) The basic reproductive ratio, $R_0(i,j)$, of parasite $j=1,2$ (black and gray, respectively) when parasite $i=2,1$ is at equilibrium (the dotted line shows the exclusion threshold). (Bi–ii) Parasite frequency at equilibrium. (Ci–ii) Host density at equilibrium (the protective parasite is a net mutualist when host density is above the dotted line). Parameters: $a=1$, $b=0.5$, $q=0.5$, ${\stackrel{\sim }{\alpha }}_1=0.5$, ${\stackrel{\sim }{\alpha }}_2=1$, ${\stackrel{\sim }{\beta }}_1=5$, ${\stackrel{\sim }{\beta }}_2=5$, ${\gamma }_1=0.1$, ${\gamma }_2=0.1$.