This study investigated whether interspecific competition affected the abundance and distribution of Hydropsyche oslari and Simulium virgatum. In addition, I studied the mechanisms of competition between the two taxa. H. oslari was the superior competitor when compared to S. virgatum because the presence of H. oslari in boulder habitats in Refugio Creek altered the microdistribution and depressed numbers of S. virgatum. Hydropsyche preempted space and was aggressive towards Simulium. Simulium avoided both hydropsychid larvae and their nets. This avoidance behaviour was reinforced by aggression from H. oslari. Possible immediate reasons why simuliids avoided Hydropsyche nets included 1) Hydropsyche attacked Simulium, 2) occupied nets interfered with feeding through increased turbulence or lower renewal rate for food, and 3) Simulium preferred to settle with conspecifics.
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