Food‐web dynamics in a large river discontinuum

Nearly all ecosystems have been altered by human activities, and most communities are now composed of interacting species that have not co‐evolved. These changes may modify species interactions, energy and material flows, and food‐web stability. Although structural changes to ecosystems have been widely reported, few studies have linked such changes to dynamic food‐web attributes and patterns of energy flow. Moreover, there have been few tests of food‐web stability theory in highly disturbed and intensely managed freshwater ecosystems. Such synthetic approaches are needed for predicting the future trajectory of ecosystems, including how they may respond to natural or anthropogenic perturbations. We constructed flow food webs at six locations along a 386‐km segment of the Colorado River in Grand Canyon (Arizona, USA) for three years. We characterized food‐web structure and production, trophic basis of production, energy efficiencies, and interaction‐strength distributions across a spatial gradient of perturbation (i.e., distance from Glen Canyon Dam), as well as before and after an experimental flood. We found strong longitudinal patterns in food‐web characteristics that strongly correlated with the spatial position of large tributaries. Above tributaries, food webs were dominated by nonnative New Zealand mudsnails (62% of production) and nonnative rainbow trout (100% of fish production). The simple structure of these food webs led to few dominant energy pathways (diatoms to few invertebrate taxa to rainbow trout), large energy inefficiencies (i.e., <20% of invertebrate production consumed by fishes), and right‐skewed interaction‐strength distributions, consistent with theoretical instability. Below large tributaries, invertebrate production declined ∼18‐fold, while fish production remained similar to upstream sites and comprised predominately native taxa (80–100% of production). Sites below large tributaries had increasingly reticulate and detritus‐based food webs with a higher prevalence of omnivory, as well as interaction strength distributions more typical of theoretically stable food webs (i.e., nearly twofold higher proportion of weak interactions). Consistent with theory, downstream food webs were less responsive to the experimental flood than sites closest to the dam. We show how human‐induced shifts to food‐web structure can affect energy flow and interaction strengths, and we show that these changes have consequences for food‐web function and response to perturbations.