Flow rate mediates the competitive influence of a grazing caddisfly on mayflies

The Glossosomatid caddisflies, which are stream‐insect grazers, have been found to negatively affect the densities of mayfly grazers through intensive periphyton exploitation. The grazing efficiency of a Glossomatid caddisfly, however, is known to decrease with increasing current velocity. A manipulative field experiment was conducted to test the hypotheses that the presence of Glossosoma sp. decreased the densities of three mayfly grazers, Baetis thermicus , Cinygmula sp. and Paraleptophlebia chocorata , and that the decrease was stronger in slower stream‐flow conditions. The experiment followed a two‐factorial design with flow conditions and Glossosoma densities as the main factors. The periphyton biomass in conditions of reduced Glossosoma density was greater than in natural Glossosoma densities, but was not affected by flow conditions. In partial contradiction of the experimental hypothesis, the experimental reduction in Glossosoma density resulted in a higher density of Baetis , irrespective of flow conditions. The grazing activity of Glossosoma would not be affected by the flow variations that were employed in the present experiment. The reduction in Glossosoma density resulted in higher densities of Cinygmula and Paraleptophlebia only in fast‐flow conditions, a result that did not support the experimental hypotheses. The competitive influence of Glossosoma on these mayflies might be altered by flow conditions causing differences in the resource structure.