Why Disturbances Can Be Predictable: A Perspective on the Definition of Disturbance in Streams

Resh et al. (1988, "J. N. Am. Benthol. Soc." 7:433) assert that lotic ecosystems are unique in that disturbances are necessarily "unpredictable" events. Acceptance of this "predictability clause" in the definition of disturbance introduces problems that influence our ability to identify, describe, and discuss important aspects of disturbance in streams. I focus on four specific aspects of Resh et al.'s treatment of predictability, particularly as it relates to hydrologic disturbance (e.g., spates) First, Resh et al. offer only purely statistical criteria for identifying hydrologic disturbance, an approach that has important limitations for characterizing predictability of the disturbance regime. I suggest adoption of more physically based measures of disturbance (e.g., flow at which movement of bed is initiated) to which specific ecological responses can be determined. Second, lotic species are assumed to be "adapted" to predictable hydrological events, despite the general absence of supporting empirical evidence. I suggest that this assertion be reframed as a testable hypothesis. Third, defining disturbances as necessarily unpredictable can result in a tautology, in which biological response to disturbance and predictability of disturbance are expressed in terms of one another. Fourth, a more explicit distinction between ecological and evolutionary time scales of response to disturbance allows predictability to be properly considered as a separate (and non-tautological) component of the disturbance regime. In sum, disturbances (including predictable ones) always have ecological effects; however, the magnitude of ecological response to a particular disturbance may be constrained by evolutionary (historical) adjustments of the biota if the disturbance regime is characterized by high predictability.