The River Continuum Redux: Aquatic Insect Diets Reveal the Importance of Autochthonous Resources in the Salmon River, Idaho

Along a river network, changes in the physical characteristics of a stream - changes articulated by the River Continuum Concept (RCC) - have been predicted to influence stream food webs, particularly the resources supporting aquatic macroinvertebrates. Although diets of macroinvertebrates can track resource availability along the longitudinal gradient of streams, insect diets were not assessed in the original RCC study; rather, insects were grouped into functional feeding groups (FFGs). In addition, global climate change is increasing the frequency and spatial extent of wildfire and beetle outbreaks throughout the western United States and could affect riparian vegetation and the amount of allochthonous inputs entering the system. Based on gut-content analysis of insects collected and archived from the Salmon River (SR), ID in 1976 and used in the development of the RCC, I first tested whether insect diets followed the original predictions of the RCC. Based on the predictions of RCC, I predicted that consumption of allochthonous inputs of terrestrial leaves would be greatest in the headwaters and decline along the continuum. In addition, by resampling these sites in 2009, I assessed whether the relative importance of allochthonous and autochthonous resources have shifted along this continuum, indicating potential shifts in riparian vegetation since 1976. Insect diets in 1976 and 2009 were similar across years and seasons, and these data did not indicate changes in riparian vegetation. In addition, insect diets did not follow the predictions of the RCC, as SR insects consumed primarily autochthonous material in the forested headwaters. Allochthony decreased along the longitudinal gradient, as predicted by the RCC; however, this trend was not as dramatic as predicted by FFGs alone. Insects in the headwaters, including the dominant shredder Yoraperla (Insecta: Plecoptera), consumed mostly autochthonous material. Autotrophic production, then, was shown to be an important food resource, even in this forested headwater stream. As this reliance on autochthonous resources was high for those invertebrates (e.g. shredders) thought to rely mostly on allochthonous material, my results indicate the importance of gut content analysis and cautions against relying on FFGs to explore which resources fuel stream food webs