Different ways in which an ecosystem can respond to change in conditions. Although dynamic systems can respond smoothly to change in external conditions (a), they can sometimes change profoundly when conditions approach a critical level (b), or have more than one stable state over a range of conditions (hysteresis) (c), in which case it may be impossible to reverse from one state back to the other (d). The lines represent equilibrium states. The arrows indicate the direction of change when the system is out of equilibrium. After Scheffer and Carpenter 2003 in Trends in Ecology and Evolution 18: 648-656, and Scheffer et al. 2000 in Ecosystems 3: 451-471.