Food Source Determines Stable Isotope Discrimination Factors ΔN and ΔC in Tadpoles

Analyses of stable isotope ratios are widely applied in studies on a large variety of aspects in trophic ecology. Most studies rely on a precise estimation of the relevant discrimination factor Delta (also called the fractionation factor), that reflects the fractionation or differences in isotope ratios of a certain element (mainly nitrogen N and carbon C) between an animal's diet and its tissue and is used to identify one step in the food web. We experimentally determined Delta N and Delta C of two species of widespread amphibians in Europe, Rana temporaria and Bufo bufo. and tested for the effect of food source (cyanobacteria Spirulina vs. zooplanktonic Daphnia) on Delta and for interspecific differences. Our study shows high variation in Delta in relation to the food source, but low interspecific differences. Tadpoles that were fed with Spirulina did have considerably lower Delta N than tadpoles fed with Daphnia in both species, and lower Delta C only in R. semporaria. The range of Delta obtained here can be a useful baseline for future trophic studies on tadpoles of Rana and Bufo. The strong diet-dependency of Delta, however, argues strongly against the use of a fixed discrimination factor in future isotope studies.