Trophic Dynamics in the Riverscape: Unifying Food Web and Ecosystem Approaches to Measure the Strength of Species Interactions in Habitats Linked by Fish Movement

A more spatially continuous perspective on the ecology of stream fishes in riverscapes has emerged in the past decade, but much of this progress has been made in the realm of fish-habitat relationships, with fewer such advances regarding the trophic ecology of fishes and their role in food webs.  There is a special need for such efforts, as the movements of fishes often link local food webs in an array of habitats spread throughout river networks.  Thus, understanding the trophic ecology of individual fish species requires knowledge of each of these linked food webs, and, reciprocally, understanding the dynamics of local food webs requires knowledge of the life histories and feeding of fishes that participate in those food webs for portions of their life cycle.  We have conducted an intensive study of fish in food webs in the Glen and Grand Canyon segments of the Colorado River that provides an illustration.  By combining measures of production at multiple trophic levels with diet analysis of invertebrates and fishes, we characterized the trophic basis of production of the entire assemblage of fishes but also estimated the strength of interactions along individual food web pathways.  In this fashion, we monitored food webs in Glen and Grand Canyon river segments over a 3-yr period that encompassed a 2008 high flow experiment (HFE) release from Glen Canyon Dam.  Following the HFE, nonnative rainbow trout in the tailwater segment below the dam exhibited a dramatic increase in production, apparently facilitated by increases in midges and blackflies with whom they interact strongly.  Subsequently, rainbow trout moving from Glen to Grand Canyon caused increases in demand for their prey in those habitats.  In Grand Canyon, rainbow trout also prey on a suite of small fishes, including juveniles of the endangered humpback chub.  Increased trout numbers in Grand Canyon were accompanied by changes in the strengths of a variety of food web interactions, including intensification of piscivory and potential for competition among fishes for limited insect prey.  Thus, our approach allowed us to detect and quantify the effect of fish produced in one river segment on the strength of food web interactions in another to which they moved.  Such studies that explicitly partition the production and prey demand of mobile fishes into different habitats are needed if ecologists are to achieve a riverscape perspective on the trophic dynamics of fishes and their role in food webs.