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
(likely influenced by amino acid composition) and between trophic levels
and diet sources (likely a result of dietary protein content and metabolic
routing). Comparatively, for Δ15N, vertebrates showed considerable
variation among tissue types and trophic levels, likely due to differences
in tissue synthesis and physiological capabilities. Overall, Δ13C ranged
from -5.1‰ to 9.1‰ and Δ15N from -3.3‰ to 9.7‰, underscoring that 1.0‰ for
Δ13C and 3.4‰ for Δ15N are not universally appropriate. Moreover, both
Δ13C and Δ15N varied by more than 9‰ within a single species and tissue
type, demonstrating that using TDF values from the same, or similar,
species may not be appropriate if diet and trophic level are not
considered. Despite the importance of diet source on TDF values, most
trophic ecology studies did not account for it. Further, most fish studies
relied on literature review values that failed to account for tissue type,
trophic level, and diet source. To aid ecologists in diet and trophic
assessments of vertebrates, we used our meta-analysis to model
taxon-specific TDF estimates (mean ± SD) for each tissue type, trophic
level, and diet source combination. These more refined TDF values should
improve ecological assessments that use stable isotopes.