River networks are characterized by predictable downstream changes in environmental conditions such as habitat structure, temperature, disturbance frequency, solar radiation, and the degree of terrestrial influence. These gradients present species with physiological and behavioral opportunities and challenges, and can influence their resource use, movement, survival, population dynamics, and ultimately the ecological role species play in river food webs. Despite nearly two decades of research demonstrating the primacy of predation by stream-dwelling fishes, we have little ability to predict how the role of such top predators should change across these gradients within a river network. Over the past 5 years we have combined manipulative experiments of stream food webs with broad-scale surveys and models to estimate how the strength of trophic interactions between juvenile steelhead (Oncorhynchus mykiss) and aquatic consumers change with the degree of terrestrial subsidy across river networks. Here we synthesize the results of these diverse approaches to provide an overview of an emerging conceptual model.
Results/Conclusions
Bayesian multi-source mixing models of juvenile steelhead diets and tissue stable isotope (d13C, d15N, dD) composition predict strong changes in the reliance on terrestrial-derived carbon sources at drainage areas of 20-100km2, a position previously predicted to be a key threshold in algal productivity and the hypothesized transition from heterotrophic to autotrophic stream ecosystems. Field-based experiments in small tributaries (<20km2) suggest that terrestrial invertebrates directly subsidize juvenile steelhead growth and abundance at the top of aquatic food webs, but do not intensify top-down control of aquatic invertebrates or primary production in these locations. These results are in contrast to experiments in rivers >100km2 demonstrating that juvenile steelhead initiate cascading strong interactions leading to changes in ecosystem productivity in years with seasonal winter disturbance. Our results collectively suggest that two key attributes that change down river networks control the importance of juvenile steelhead interactions in stream food webs, 1) the magnitude of direct terrestrial subsidy (in the form of terrestrial invertebrate prey), and 2) the proportion of aquatic consumers that are vulnerable to predation by fish. We conclude that the ubiquitous distribution of juvenile steelhead in many river networks provides a unique opportunity to make spatially explicit predictions about top-down control in river food webs.