Fig. 1. Over a specified period of time and unit of space, fluctuations in an ecosystem state or process occur, and, despite these fluctuations, many species persist or recolonize the area over time. When either an extreme discrete disturbance or novel stressor occurs, a departure from the historical range of variability (HRV) can be seen in one of two ways. Either the system can remain within the HRV but have reduced variability, or a shift to an entirely new state may be observed. The degree of departure from HRV conditions is an index to the risk of losing species or processes (figure from McComb 2007).