THE INTERMEDIATE DISTURBANCE HYPOTHESIS, REFUGIA, AND BIODIVERSITY INSTREAMS

The intermediate disturbance hypothesis has been influential in the de velopment of ecological theory and has important practical implication s for the maintenance of biodiversity but has received few rigorous te sts. We tested the hypothesis that maximum taxon richness of macroinve rtebrates will occur in communities subject to intermediate levels of disturbance at 54 stream sites that differed in the frequency and inte nsity of flood-related episodes of bed movement. Our results support t he intermediate disturbance hypothesis, with both highly mobile and re latively sedentary taxa conforming to the predicted bell-shaped curve. Taxon richness was not related to habitat area (stream width), distan ce from the headwater, or the diversity of microhabitats (particle siz e categories) but was significantly and negatively related to the prop ortion of the substratum made up of small particles. Of all the factor s measured, however, bed disturbance was by far the best at accounting for variation in taxonomic richness. We also quantified several kinds of potential refugia for invertebrates and found a positive relations hip between richness and a refugia axis that combines amount of dead s pace with proportion of large substratum particles.