We use degree results of Dancer and Du () and bifurcation and monotonicity techniques of Brown and Du () to study a competitor-competitor-mutualist model with diffusion. We obtain rather complete results on the existence and nonexistence of positive periodic solutions. It turns out that this model behaves largely as the two-species competition model.
"Positive periodic solutions of a competitor-competitor-mutualist model." Differential Integral Equations 9 (5) 1043 - 1066, 1996.