Variation in discrimination factors (Δ15N and Δ13C): the effect of diet isotopic values and applications for diet reconstruction.
The use of stable isotopic techniques to study animal diets and trophic levels requires a priori estimates of discrimination factors (Δ13C and Δ15N, also called fractionation factors), which are the differences in isotopic composition between an animal and its diet. Previous studies have shown that these parameters depend on several sources of variation (e.g. taxon, environment, tissue) but diet as a source of variation still needs assessment. We conducted an extensive review of the literature (66 publications) concerning estimates of animal-diet Δ13C (n=290) and Δ15N (n=268). We analysed this data set to test the effect of diet isotopic ratio on the discrimination factor, taking into account taxa, tissues, environments and lipid extraction treatments. Our results showed differences among taxonomic classes for Δ13C, but not for Δ15N, and significant differences among tissues for both Δ13C and Δ15N. We found a significant negative relationship between both, Δ13C and Δ15N, with their corresponding diet isotopic ratios. This relationship was found also within taxonomic classes for mammals (Δ13C and Δ15N), birds (Δ13C), fishes (Δ13C and Δ15N) and invertebrates (Δ13C and Δ15N). From these relationships, we propose a method to calculate discrimination factors based on data on diet isotope ratios (termed the 'Diet-Dependent Discrimination Factor', DDDF). To investigate current practice in the use of discrimination factors, we reviewed studies that used multi-resource isotopic models. More than 60% of models used a discrimination factor coming from a different species or tissues, and in more than 70% of models, only one Δ13C or Δ15N was used for all resources, even if resources had very different isotopic ratios. Also, we estimated DDDFs for the studies that used isotopic models. More than 40% used Δ15N values and more than 33% used Δ13C values differing >2 per mil from estimated DDDFs. Synthesis and applications. Over the last decade, applied ecologists have discovered the potential of stable isotopes for animal diet reconstruction, but the successful adoption of the method relies on a good estimation of discrimination factors. We draw attention to the high variability in discrimination factors, advise caution in the use of single discrimination factors in isotopic models, and point to a method for obtaining adequate values for this parameter when discrimination factors cannot be measured experimentally. Future studies should focus on understanding why discrimination factors vary as a function of the isotopic value of the diet.