Meta-analysis and critical review of trophic discrimination factors (Δ13C and Δ15N): importance of tissue, trophic level, and diet source

Robustly quantifying dietary resource use and trophic position using stable isotopes requires accurate trophic discrimination factors (TDF; Δ13C and Δ15N for carbon and nitrogen, respectively), defined as the isotopic difference between consumer and diet. Early TDF studies converged on values of around 1.0‰ for Δ13C and 3.4‰ for Δ15N, but more recent work indicates that TDF values may be more nuanced, depending on taxa, tissues, trophic level, and diets. Yet, the relative importance of these factors remains unclear.
Focusing on vertebrates (birds, fish, herptiles, and mammals), we conducted a literature review of 279 studies that estimated TDF values and used a Bayesian framework to determine how tissue type, trophic level, and diet source influence variation in Δ13C and Δ15N. Additionally, we reviewed 358 trophic ecology studies to determine if studies accounted for these factors during their TDF selection process.
For Δ13C, vertebrates showed consistent patterns among tissue types (l..., ,