Species-specific ontogenetic isotopic niche dynamics in Amazon River turtles

Abstract The partitioning of the available resources is a key to species’ coexistence. Ontogenetic changes in resource use can alter ecological roles within species, yet such trajectories remain poorly resolved for Amazonian freshwater turtles. We used stable carbon and nitrogen isotope ratios (δ 13 C and δ 15 N) to evaluate ontogenetic and spatial variation in the isotopic niches of Podocnemis expansa and P. sextuberculata in floodplain lakes of the Middle Juruá River, Brazilian Amazon. P. expansa showed substantial isotopic niche overlap among ontogenetic classes (57.1–70.8%) and little evidence of size-related change in δ 13 C or δ 15 N. In contrast, P. sextuberculata showed lower overlap between sampled age classes (49.1%) and a size-related shift in δ 13 C, indicating stronger ontogenetic differentiation in assimilated resource pathways. P. sextuberculata also exhibited higher δ 15 N values than P. expansa . These results indicate that trophic ecology in Podocnemis emerges from the interaction among species identity, ontogenetic stage, and the spatial and temporal structure of floodplain resource landscapes. Moreover, the conservation of Amazonian turtles is centred on ontogenetic resource landscapes with its unique habitats, hydrological and ecological processes and resource pathways that support individuals as they grow.