Fig. 1. Schematic representation of possible ways in which the equilibrium state of a system can vary with conditions, such as nutrient loading, exploitation, or a rise in temperature. In panels a and b, only one equilibrium exists for each condition. However, if the equilibrium curve is folded backward (panel c), three equilibria can exist for a given condition. The arrows in the graphs indicate the direction in which the system moves if it is not in equilibrium (i.e., not on the curve). It can be seen from these arrows that all curves represent stable equilibria, except for the dashed middle section in panel c. If the system is pushed away a little bit from this part of the curve, it will move further away instead of returning. Hence, equilibria on this part of the curve are unstable and represent the border between the basins of attraction of the two alternative stable states on the upper and lower branches.