Omnivory is prevalent in terrestrial and aquatic food webs. However, the extent and seasonality of predatory feeding by omnivores in stream food webs is largely unknown. To understand better these aspects of omnivory in stream food webs, we investigated seasonal changes in the trophic positions of 2 omnivores in a small forested stream. We selected the amphipod, Gammarus pulex, and the caddisfly larvae, Hydropsyche spp., as key organisms because both taxa are common, reach high biomasses in many stream ecosystems, and have broad food spectra. We used stable-isotope analysis of the most prevalent taxa in the benthic macroinvertebrate community to assess the trophic positions of the 2 omnivorous taxa in different seasons. We estimated the degree of predatory feeding by quantifying the importance of different food resources with a stable-isotope mixing model (IsoSource). The predation capacity of omnivores, defined as the fraction of omnivore biomass associated with predation, was compared to the biomass of strictly predatory invertebrates. Our analysis indicated a predation capacity of Hydropsyche spp. similar to that of strict invertebrate predators, whereas the predatory biomass of G. pulex was even higher than those of the other invertebrate predators because of a combination of high biomass and large proportions of animal prey (50–90%) in the diets of both omnivores. The trophic position of omnivores in winter was comparable to that of invertebrate predators (trophic level 3), whereas in summer, their trophic position was comparable to primary consumers (level 2). These shifts might be caused by seasonally varying prey availability for Hydropsyche spp. and life-cycle patterns for G. pulex. Predation by omnivores was generally high in our study stream and probably has been underestimated in other stream ecosystems where omnivores, such as Hydropsyche spp. and G. pulex, are common.
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