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.