When giant amphibians rule the river: Community trophic structure of Japanese giant salamander ecosystems

Top predators strongly influence food webs, often because they reach large body sizes. Amphibians are typically regarded as prey in such systems, but giant salamanders are a rare exception. Because gigantism entails extreme ontogenetic body size variation, it is expected to drive strong trophic niche shifts and reshape interactions within and among species as individuals grow. However, such trophic shifts in giant amphibians and relationships with other consumers remain largely unexplored, limiting our understanding of the contribution of this flagship species to river food web structure. Here, we used stable isotope analysis (δ¹³C-δ¹⁵N) to investigate the trophic structure of a freshwater community dominated by the Japanese giant salamander (Andrias japonicus). We assessed how extreme ontogenetic body size variation modifies intra- and interspecific interactions and contributes to overall community structure. We also quantified the dominant carbon sources fueling the community, and modeled trophic positions (TPs), niche metrics, and relationships of all major consumer species. By sampling 161 giant salamanders and approximately 700 other consumers for stable isotope analysis, we provide the first comprehensive characterization of food web structure in a freshwater community dominated by A. japonicus. We found clear trophic shifts: smaller individuals overlapped with mesopredators (fish, prawns, turtles), suggesting potential competition, whereas TP increased with size, placing larger individuals above other predators, at the top of the food web. Large salamanders broadened the community δ¹⁵N range, contributing significantly to the trophic evenness and total community niche space. Basal source tracing revealed that both aquatic and terrestrial energy pathways sustained the food web. Overall, these findings show that gigantism in A. japonicus shapes trophic hierarchies, contributing significantly to vertical complexity and trophic diversity of the community, and highlighting how ontogenetic shifts in giant predators can structure riverine food webs. By integrating isotopic niches, TP, and basal source contributions at the whole-community scale, this study provides a generalisable framework for linking predator ontogeny to food web structure, with direct relevance for conserving vulnerable freshwater ecosystems; endangered top predators such as A. japonicus may contribute disproportionately to food web resilience and community structure.