Fig. 2. Two ways to shift between alternative stable states. (a) If the system is on the upper branch, but close to the bifurcation point F2, a slight incremental change in conditions may bring it beyond the bifurcation and induce a catastrophic shift to the lower alternative stable state (“forward shift”). If one tries to restore the state on the upper branch by means of reversing the conditions, the system shows hysteresis. A backward shift occurs only if conditions are reversed far enough to reach the other bifurcation point F1. (b) A perturbation (heavy arrow) may also induce a shift to the alternative stable state, provided that it is sufficiently large to bring the system over the border of the attraction basin.