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Abstract The body size of aquatic vertebrates is declining across populations and ecosystems worldwide owing to warmer water temperature and changing streamflow. In freshwaters, the effects of stream network position and density‐dependent factors on body size are less understood. We used an extensive dataset spanning 41 stream sites over 7 years to evaluate how density‐dependent and density‐independent factors influence the size of two top predators in small watersheds, Coastal Cutthroat TroutOncorhynchus clarkii clarkiiand Coastal Giant SalamandersDicamptodon tenebrosus. We tested three hypotheses of body‐size variation for trout and salamanders, including intraspecific density dependence, interspecific density dependence, and resource availability, using empirical observations in hierarchical linear mixed models in a model‐selection framework. In our best‐supported models, the strongest predictors of size were conspecific negative density dependence, as expected, suggesting greater intraspecific interactions probably owing to conspecific individuals having similar requirements. We reveal a biogeographic pattern in which body size peaks in middle stream‐network positions and plateaus or declines at lower and upper locations, proposing that stream network position also plays a role in determining body size in small watersheds. Salamander density also has a quadratic effect on adult trout size, with salamanders having a greater overall effect on the body size of both species than trout, suggesting that salamanders might be more dominant than trout in some interactions. Collectively, we found that biotic interactions, mainly conspecific but also interspecific, and stream‐network position affect trout and salamander body sizes in small watersheds. 

Abstract Food webs show the architecture of trophic relationships, revealing the biodiversity and species interactions in an ecosystem. Understanding which factors modulate the structure of food webs offers us the ability to predict how they will change when influential factors are altered. To date, most of the research about food webs has focused on species interactions whereas the influences of surrounding environments have been overlooked. Here, using network analysis, we identified how the structure of aquatic food webs varied across a range of geophysical conditions within a whole stream system. Within a headwater basin in the Cascade Mountain Range, Oregon, USA, macroinvertebrate and vertebrate composition was investigated at 18 sites. Predator–prey interactions were compiled based on existing literature and dietary analysis. Several structural network metrics were calculated for each food web. We show that the structure of food webs was predictable based on geophysical features at both local (i.e., slope) and broader (i.e., basin size) spatial extents. Increased omnivory, greater connectance, shorter path lengths, and ultimately greater complexity and resilience existed downstream compared to upstream in the stream network. Surprisingly, the variation in food web structure was not associated with geographic proximity. Structural metric values and abundance of omnivory suggest high levels of stability for these food webs. There is a predictable variation in the structure of food webs across the network that is influenced by both longitudinal position within streams and patchy discontinuities in habitat. Hence, findings illustrate that the slightly differing perspectives from the River Continuum Concept, Discontinuity Patch Dynamics, and Process Domains can be integrated and unified using food web networks. Our analyses extend ecologists’ understanding of the stability of food webs and are a vital step toward predicting how webs and communities may respond to both natural disturbances and current global environmental change.