Intraguild omnivory in predatory stream insects

1. The veracity of food webs and other trophic structures derived from empirical data sets depends ultimately on our ability to identify species' diets and, thus, to classify organisms according to their major energy source or functional feeding groups. True omnivory (mixing plant and animal food) is common among terrestrial and marine arthropods, but poorly documented in freshwater systems. In a field study, we examined a guild of six large-bodied, lotic insects (Plecoptera and Trichoptera) that are generally considered to be predatory and that coexist in one stream. Gut contents analysis was used to characterize prey in the diet and to quantify the consumption of plant material. Nitrogen isotope ratios (delta N-15) were measured in whole-animal samples to determine whether any algae were assimilated into body tissues. 2. Gut contents analysis showed that all six study taxa were polyphagous with respect to their animal prey. All taxa also consumed substantial amounts of algae, but little true detritus (terrestrial plant material). Contrary to previous suggestions, an ontogenetic shift from carnivory to algivory was apparent in two species; a shift from algivory to carnivory was not observed. 3. Stable isotope analysis indicated that some predators assimilated algae: delta N-15 was lower than expected if predators were wholly carnivorous. Using a stable isotope mixing model to estimate the proportion of algae assimilated into body tissues, we suggest that three taxa are true omnivores and estimate that 40-55% of body nitrogen is derived from algae. The remaining three taxa appear to be primarily carnivorous. 4. Intraguild omnivory is a distinctive feature of the food web in this stream, and probably many others, and needs to be incorporated explicitly into models of stream systems. The food limitation hypothesis may explain, in part, true omnivory at this site. However, the nutritional value of algae and its role in the growth and fecundity of predatory stream insects requires further investigation before underlying mechanisms can be identified with confidence.