Insect production from temporary and perennially flowing headwater streams in western Oregon

To evaluate the contribution of headwater streams to the forest ecosystem of the Pacific Northwest, we used emergence traps to examine the effect of stream flow (perennial vs. dry-season temporary) on emergent aquatic insect fauna at three sites in the conifer forests of western Oregon. Total density and biomass of aquatic insects were higher in temporary streams than in perennial streams. Taxonomic richness was consistently higher in perennial streams. The number of Chironomidae exceeded those of all other taxa during the spring, but the chironomids were largely replaced by Mycetophilidae as the most abundant taxon during the summer, especially in temporary streams. Trichoptera and Ephemeroptera emerged in higher numbers from perennial than from temporary streams. These results are consistent with our hypothesis that the absence of vertebrate predators (fish and giant salamanders) allows the populations of arthropods in temporary streams to flourish, serving as: (1) a potential source of colonization and (2) an important role in the terrestrial food web as an abundant food source for terrestrial insectivores.