Disturbance, patchiness, and diversity in streams

Perturbations in ecosystems consist of a sequence of 2 events: the disturbance, marked by the application of the disturbing forces, and the response shown by the biota to the damage inflicted by the disturbance. The disturbance must be effectively characterized, without confounding it with the response, for progress to be made in the study of the disturbance ecology of streams. A disturbance may take the form of a pulse, a press, or a ramp, and the consequent trajectory of the response may be a pulse, a press, or a ramp. Floods and droughts are the major forms of natural disturbance in flowing waters and, although the effects of floods have been relatively well studied, those of droughts have been largely neglected. Floods accentuate downstream and lateral transport links, often with damaging consequences, whereas droughts fragment the continuity of streams. Both floods and droughts destroy and generate habitat patchiness and patchiness of the biota. During recovery, there are changes in the biotic composition and spatial configuration in patches. Resistance and resilience of the biota to disturbance may be facilitated by the use of refugia. The characterization of flood refugia is much more advanced than that of drought refugia. Recovery from floods is marked by the rapid attainment of relatively constant levels of diversity at the local scale of individual patches. At the regional scale of streams and their catchments, several studies have reported negative correlations between diversity and levels of flood disturbance, whereas other studies have reported unimodal diversity–disturbance curves consistent with patterns expected of the intermediate disturbance hypothesis. Such a unimodal relationship may be generated in several ways that await testing. In flowing waters at the regional scale, disturbance may play a central role in regulating species diversity. A predicted increase in the severity and frequency of disturbances with global climate change requires a comprehensive understanding of the disturbance ecology of running waters.