Reevaluating trophic discrimination factors (Δδ13C and Δδ15N) for diet reconstruction

Stable isotope analysis is increasingly being used to assess diet and trophic positions of animals. Such assessments require estimates of trophic discrimination factors (TDFs)-offset between the isotopic composition of diet and animal tissues-with imprecise applications of TDFs leading to biased conclusions in resource use. Because TDFs are unavailable for most species, ecologists often apply values from taxonomically similar species or use trophic step increases of approximately 1 parts per thousand for carbon (TDF-delta C-13) and 3 parts per thousand for nitrogen (TDF-delta N-15). Such practices may yield inaccuracies since TDFs vary greatly, even within a species. To better understand the factors that influence TDFs, we conducted a meta-analysis of TDF-delta C-13 and TDF-delta N-15 for mammals and quantified variation in relation to consumer type (herbivore, omnivore, carnivore) and diet source (C-3-based, C-4-based, marine-based, mixture). Additionally, to guide TDF choice, we used an isotopic data set of small mammal tissues and diet items to assess how predicted dietary contributions vary with TDFs estimated using (1) taxonomic relatedness, (2) consumer type and diet source, or (3) values derived from wild animals eating natural diets. Our meta-analysis revealed that metabolic routing and interactions between consumer class, dietary source, and the protein versus energy content of diets best explained variation in TDF-delta C-13 values (-1.5 parts per thousand to 7.3 parts per thousand), whereas consumer class best explained variation in TDF-delta N-15 values (-0.5 parts per thousand to 7.1 parts per thousand). Our test of methods to estimate TDFs indicated that ecologists should avoid relying on taxonomic relatedness when selecting TDF-delta C-13 because mixed-diet lab studies may produce misleading results for herbivores and omnivores. Additionally, field-derived estimates could help fill TDF gaps where diets within a consumer class are absent. Overall, we suggest that using standard TDF trophic step values should be abandoned, because feeding studies are often poor proxies for natural diets, particularly for herbivores and omnivores. Instead, we make recommendations on how to select TDFs, along with a range of TDF-delta C-13 and TDF-delta N-15 values depending on diet source, consumer class, and tissue type. Use of these more refined recommendations and TDF values in isotopic assessments will improve estimates of diets and trophic interactions in natural systems, leading to a better understanding of ecological interactions and communities.