Fig. 5.
Colony size as a function of plant reward level. Relations among reward level R, ant colony size S of species i, and the efficiency Ei at which rewards are converted into workers were inferred from models based on Michaelis–Menten functions (SI Text) with the intrinsic maximum colony size being Smaxi, and Rmini describing the minimum reward level at which a colony can exist. When both species compete for a high-reward host, the positive feedback of rewards to protecting behavior (Fig. S3) allows the mutualist (i = m) to displace the parasite (i = p), as indicated here by a term that exponentially increases with the mutualists' colony size, where a and C are constants. Black line indicates P. ferrugineus; dotted gray line indicates P. gracilis; gray line indicates P. gracilis in the presence of a competitively superior P. ferrugineus colony.