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

Stable isotope analysis is increasingly used to assess diet and trophic
positions of animals. Such assessments require estimates of trophic
discrimination factors (TDF — 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 about 1‰ for carbon (TDF-δ13C) and 3‰ for
nitrogen (TDF-δ15N). Such practices may be inaccurate since TDFs vary
greatly, even within a species. To better understand the factors that
influence TDFs, we conducted a meta-analysis of TDF-δ13C and TDF-δ15N for
mammals and quantified variation in relation to consumer
type (herbivore, omnivore, carnivore) and diet
source (C3-based, C4-based, marine-based, mixture). Additionally,
to guide TDF choice, we used an isotopic dataset 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-δ13C values (-1.5 to 7.3‰), whereas
consumer class best explained variation in TDF-δ15N values (-0.5 to 7.1‰).
Our test of methods to estimate TDFs indicated that ecologists should
avoid relying on taxonomic relatedness when selecting TDF-δ13C, as
mixed-diet lab studies may cause 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, as feeding
studies are often poor proxies for natural diets, particularly for
herbivores and omnivores. Instead, we provide recommendations on how to
select TDFs, along with a range of TDF-δ13C and TDF-δ15N 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.