Biases in the analysis of stable isotope discrimination in food webs

1. Recently, Caut, Angulo & Courchamp (2009, Journal of Applied Ecology) published a review on diet-tissue isotopic shifts in animals. They concluded that diet-tissue shifts are influenced by the isotopic composition of the diet for both 13C and 15N in a wide range of animal taxa. 2. We suggest that the conclusion of Caut, Angulo & Courchamp is in error, and provide a discussion of sources of error in the assessment of diet-tissue discrimination. 3. Errors in the derivation of diet-tissue shifts include imprecise definitions, mathematical artefacts and invalid statistical analysis. It is likely that the work also suffers from experimental bias. The mathematical artefacts and statistical invalidity result from using the same variable (diet isotopic composition) in the independent and dependent variable for regression analysis and failure to correct for the resulting bias. Experimental bias can result from the incomplete turnover of body pools after diet switches or during natural fluctuations in diet isotope composition. Unfortunately, the main sources of error work in the same direction, strengthening the biased relationship between the diet-tissue shift and diet isotope composition. Therefore, the analysis of Caut et al. (2009) does not provide proof of a relationship between diet-tissue shift and diet isotope composition. 4.Synthesis and application. Future work on diet-tissue discrimination factors should (i) follow the mathematical rules resulting from how isotope data are presented, (ii) be based on appropriate statistics analysis that avoids or corrects for spurious self-correlations, and (iii) consider possible complications associated with the presence of slowly turning-over stores and non-equilibrating 'dead' body pools.