Flow-related disturbance in streams: an experimental test of the role of rock movement in reducing macroinvertebrate population densities

Densities of hydropsychids (Trichoptera) on different-sized stones were compared before and after a winter flood, and the effects of rock movement (a likely form of disturbance during floods) on these organisms were tested. Before the flood, the density of hydropsychids was an order of magnitude higher on large than on small stones; after the flood, densities on larger stones had fallen to levels similar to those on small stones. In a four-week colonization experiment over the period during which the flood occurred, densities of hydropsychids were compared on bricks of two sizes, which were either fixed to the bed of the stream so that they could not move, or unfixed and able to move with changes in flow. Disturbance rates generally differed between small and large unfixed bricks, but all unfixed bricks moved during a large flood, regardless of size. The densities of hydropsychids on fixed and unfixed bricks were similar, and they reflected densities on natural stones after the flood, suggesting that even substrata that do not move during floods may fail to provide a refuge from the effects of high flows.