Geographic distribution of feather ³⁴S in Europe

Geographic distribution models of environmentally stable isotopes (the so-called "isoscapes") are widely employed in animal ecology, and wildlife forensics and conservation. However, the application of isoscapes is limited to elements and regions for which the spatial patterns have been estimated. Here, we focused on the ubiquitous yet less commonly used stable sulfur isotopes (delta S-34). To predict the European delta S-34 isoscape, we used 242 feather samples from Eurasian Reed Warbler (Acrocephalus scirpaceus) formed at 69 European wetland sites. We quantified the relationships between sample delta S-34 and environmental covariates using a random forest regression model and applied the model to predict the geographic distribution of delta S-34. We also quantified within-site variation in delta S-34 and complementarity with other isotopes on both individual and isoscape levels. The predicted feather delta S-34 isoscape shows only slight differences between the central and southern parts of Europe while the coastal regions were most enriched in S-34. The most important covariates of delta S-34 were distance to coastline, surface elevation, and atmospheric concentrations of SO2 gases. The absence of a systematic spatial pattern impedes the application of the delta S-34 isoscape, but high complementarity with other isoscapes advocates the combination of multiple isoscapes to increase the precision of animal tracing. Feather delta S-34 compositions showed considerable within-site variation with highest values in inland parts of Europe, likely attributed to wetland anaerobic conditions and redox sensitivity of sulfur. The complex European geography and topography as well as using delta S-34 samples from wetlands may contribute to the absence of a systematic spatial gradient of delta S-34 values in Europe. We thus encourage future studies to focus on the geographic distribution of delta S-34 using tissues from diverse taxa collected in various habitats over large land masses in the world (i.e., Africa, South America, or East Asia).