Study on geographical differences in American mink diets reveals variations in isotopic composition of potential mink prey

Invasive alien predators pose a threat to native fauna and the studies of their feeding habits are crucial to understanding their impact on prey populations. Diet of the American mink Neovison vison, an invasive species in Europe, is relatively well studied based on scat analysis, however, the use of other methods of diet analysis enables a better overview of this issue. We analyzed the isotopic composition of carbon and nitrogen in the livers, which reflects the diet from about 30–40 days (scat analysis provides information on the diet over 1–2 days only) of the American mink from four national parks in Poland and in the muscles of three types of mink prey (root voles, common frogs, and roach) in order to estimate their contribution to the mink diet. Mink in Biebrza and Narew National Parks fed mainly on frogs and fish, in Drawa National Park on voles and fish, and in Warta Mouth National Park almost exclusively on fish—as shown by Bayesian mixing models calculated for three selected groups of prey. There was no isotopic evidence for differences between the diets of male and female mink. In all groups of prey, we found surprisingly high differences between individuals of the same species from different study sites in the isotopic composition of both δ13C (up to 3‰) and δ15N (up to 6‰). Based on a detailed literature review, we predict that the main reasons for these variations are differences in abiotic environment, food availability, and trophic position. We also indicate a lack of data on trophic discrimination in fish and amphibians, which makes it hard to assess the influence of differences in trophic position on isotopic variations. We suggest caution to authors who plan to study geographical variations in diet of animals using stable isotope analysis without acknowledging that taxonomically and ecologically similar prey can differ in isotopic composition between studied areas.