Figure 2: Phase portraits of model (1) with the parameters , , , , , and . In this case, is a nodal source point; is a saddle point; is a nodal sink point, which is locally asymptotically stable; is a spiral sink, which is locally asymptotically stable; and is a saddle point. There exists a separatrix curve determined by the stable manifold of . The dashed curve is the -nullcline , and the dotted curve is the -nullcline .