Global patterns and knowledge gaps in rare earth element bioaccumulation in wildlife: A systematic review.

Rare earth elements (REEs) are increasingly recognized as emerging environmental contaminants due to their expanding use in electronics, renewable energy, medical devices, and fertilizers. This systematic review synthesizes current knowledge on REE concentrations in free-ranging wildlife from aquatic and terrestrial ecosystems. We analyzed 95 peer-reviewed studies (1976-2025) selected from Scopus and Web of Science using PRISMA 2020 guidelines, evaluating geographic distribution, taxonomic coverage, trophic level, tissue type, analytical methods, and data reporting. The number of publications increased sharply after 2011, with most studies focused on aquatic organisms and European countries. Notably, countries with large REE reserves, such as Vietnam, Russia and India, were underrepresented. Research concentrated on three aquatic taxa: Actinopterygii (ray-finned fishes), Bivalvia (clams and mussels), and Malacostraca (crabs and shrimp). REE concentrations were typically higher in organisms at lower trophic levels and in benthic species, supporting trophic dilution. In vertebrates, liver, kidney, and gills showed the highest REE concentrations, while non-invasive samples such as feathers and feces, although lower concentrations, demonstrated potential for biomonitoring. Methodological inconsistencies in subgroup classification, normalization, and statistical reporting limit cross-study comparability. This review highlights major knowledge gaps, especially in terrestrial ecosystems and agricultural areas exposed to REEs via fertilizers. Future research should prioritize broader taxonomic and geographic coverage and standardized methods. Given their persistence and dual role as both tracers and contaminants, REEs warrant integration into long-term monitoring to clarify their ecological and toxicological effects.