Temporal shifts in intraspecific and interspecific diet variation: effects of predator body size and identity across seasons in a stream community

Intraspecific variation is increasingly recognized as an important factor in ecological interactions, sometimes exceeding the role of interspecific variation. Few studies, however, have examined how intra-versus interspecific variation affect trophic interactions over time within a seasonally dynamic food web. We collected stomach contents from 2028 reticulate sculpin (Cottus perplexus), 479 cutthroat trout (Oncorhynchus clarkii clarkii), and 107 Pacific giant salamanders (Dicamptodon tenebrosus) in western Oregon streams and compared diets among predator species and size classes over three seasons. Predator body size and species identity both showed strong effects on dietary niche breadth, proportional prey composition, and prey size, with seasonal variation in the relative magnitudes of intraspecific and interspecific diet variation. Size-associated diet variation was high in summer and fall but was heavily outweighed by species-associated diet variation in spring. This pattern was driven primarily by a 50-fold increase in the consumption of terrestrial thrips (Order: Thysanoptera) by cutthroat trout in spring compared to summer and fall. Mean dietary niche breadth generally increased with body size and was roughly half as wide in sculpin as in trout and was intermediate in salamanders. Predator-specific responses to the seasonality of terrestrial prey availability were associated with interspecific differences in foraging mode (e.g., benthic vs drift-feeding) and contributed to temporal variation in the roles of predator size and identity in trophic niche differentiation. Our results thereby demonstrate that intraspecific and interspecific diet variation can exhibit strong seasonality in stream predators, emphasizing the dynamic nature of food webs and the need to incorporate sampling over relevant temporal scales in efforts to understand species interactions.