Disturbance, patchiness, and diversity in streams

Perturbations in ecosystems consist of a sequence of 2 events: the disturba nce, marked by the application of the disturbing forces, and the response s hown by the biota to the damage inflicted by the disturbance. The disturban ce must be effectively characterized, without confounding it with the respo nse, for progress to be made in the study of the disturbance ecology of str eams. A disturbance may take the form of a pulse, a press, or a ramp, and t he consequent trajectory of the response may be a purse, a press, or a ramp . Floods and droughts are the major forms of natural disturbance in Bowing wa ters and, although the effects of floods have been relatively well studied, those of droughts have been largely neglected. Floods accentuate downstrea m and lateral transport links, often with damaging consequences, whereas dr oughts fragment the continuity of streams. Both floods and droughts destroy and generate habitat patchiness and patchiness of the biota. During recove ry, there are changes in the biotic composition and spatial configuration i n patches. Resistance and resilience of the biota to disturbance may be fac ilitated by the use of refugia. The characterization of flood refugia is mu ch more advanced than that of drought refugia. Recovery from floods is marked by the rapid attainment of relatively consta nt levels of diversity at the local scale of individual patches. At the reg ional scale of streams and their catchments, several studies have reported negative correlations between diversity and levels of flood disturbance, wh ereas other studies have reported unimodal diversity-disturbance curves con sistent with patterns expected of the intermediate disturbance hypothesis. Such a unimodal relationship may be generated in several ways that await te sting. In flowing waters at the regional scale, disturbance may play a cent ral role in regulating species diversity. A predicted increase in the sever ity and frequency of disturbances with global climate change requires a com prehensive understanding of the disturbance ecology of running waters.