Forage ecology of Neotropical fish in Brazilian biomes using stable isotopes

Neotropical regions are responsible for harboring most of the global diversity of freshwater fish, providing essential ecosystem services for society. Human activities (e.g., land use changes) jeopardize aquatic ecosystems as well as species, communities, etc., impairing ecosystem services. We investigate the impact of human disturbance on the foraging ecology of Neotropical freshwater fishes across five Brazilian biomes by stable isotope analysis. We analyzed carbon (δ¹³C) and nitrogen (δ¹⁵N) isotopic compositions of Neotropical fishes, sourced from the SIA-BRA dataset. Fishes were categorized into herbivores, omnivores, and carnivores. We tested the correlation between human disturbances, indicated by the human disturbance index (hdi), and changes in fish diets. We expected that the assimilation of C₄‑carbon from exotic forage would increase with higher disturbance levels, while δ¹⁵N levels would similarly rise due to nitrogen input from anthropogenic sources. We found increases in fish δ¹³C with human disturbance increases, suggesting greater assimilation of C₄ carbon in places where native vegetation was replaced by C₄ sources, confirmed by isotopic mixing models. In contrast, δ¹⁵N values did not show a significant relationship with human disturbance, probably due to the complex interactions and multiple sources of nitrogen in disturbed environments. Our finds suggest that stable isotope analysis provides a powerful tool for monitoring the effects of landscape changes on aquatic food webs. Particularly, the δ¹³C values of detritivorous fish that feed on C₄ plant detritus could serve as bioindicators of environmental degradation. However, a specific isotopic characterization of each site would be valuable for more accurate niche information.