FIG. 4. The phase plane for grass and trees, according to Eqs. (17)-(19). The null and isoclines are plotted with dashed lines. These are the curves where or , respectively. The equilibria are points where the isoclines cross. The stable equilibria are labelled as "G'' and "W," respectively. The unstable equilibrium is labelled as "S." The trajectories that enter S form a separatrix: they are plotted with dotted lines. Trajectories to the right of the separatrix eventually reach G, and those to the left eventually reach W.