Can stable isotopes assess habitat use in complex coastal wetlands? A case study in an amphibian species

Coastal ecosystems are among the most diverse and productive systems on earth, but they are threatened by various anthropogenic pressures. To understand the magnitude of these constraints on biodiversity, it is essential to assess habitat use of coastal species, which can be challenging in the complex matrix of habitats in coastal environments. The stable isotope method is a powerful tool to assess foraging habitat, as marine influences on the trophic web should decrease with increasing distance from the ocean. In this study, we evaluated whether isotopic values (delta N-15, delta C-13 and delta S-34) can be useful to assess habitat use in a coastal amphibian, the western spadefoot toad (Pelobates cultripes). In two coastal wetlands of the French Atlantic coast, we captured individuals from 20 to 1300 m from the seashore in order to assess whether isotopic values were related to the distance to the coastline, and we combined this dataset with data on osmolality that correlate to distance to the ocean in this species. We found divergent site-specific responses, presumably linked to the site-specific extent of a recent marine submersion, highlighting the long-lasting effects of such extreme weather event on trophic webs. In both sites however, we found a larger isotopic niche closer to the seashore suggesting wider diet or prey depending on wider resources near the sea. Overall, our results suggest that isotopic values can provide insights on habitat use depending on site-specific characteristics. Combining isotopic values with other metrics (e.g., osmolality) allows to better understand isoscapes in a site-specific context.