Stable isotopes of a terrestrial amphibian illustrate fertilizer-related nitrogen enrichment of food webs in agricultural habitats

Abstract

To comprehensively assess the impacts of agricultural practices on biodiversity in complex landscapes mixing both agricultural habitats and remnants of other (presumably more favorable) types of habitats, a prerequisite is to evaluate to which extent agricultural habitats are actually used by a given species. Here, we tested whether the stable isotope method can help to discriminate habitat use of a wild vertebrate, the spined toad (Bufo spinosus). We expected habitat to influence their δ13C values and the use of fertilizers to increase δ15N values of individuals from agricultural landscapes. Based on 114 toads from seven sites characterized by contrasted habitats (agricultural, forest or mixed habitats), we found that toad blood δ15N values were positively related to agricultural surface area, a result that was corroborated by diverging blood δ15N values between habitat categories. Conversely, toad δ13C values did not vary according to the habitat. Our results suggest that isotopic values (especially δ15N) could be a powerful tool to assess agricultural habitat use in terrestrial taxa. Further studies should usefully investigate whether individual δ15N values can be used as a fingerprint of other constraints of agricultural habitats (e.g., contaminants) in agricultural landscapes.