Salinization due to sea-level rise and marine submersions is expected to strongly impact coastal ecosystems. Exposure to salinity can negatively impact biodiversity especially in coastal wetlands. To understand comprehensively the consequences of environmental salinization on coastal biodiversity, it is essential to document how coastal species currently respond to exposure to salinity. In this study, we investigated how variations of environmental salinity relative to the distance to the ocean influence population structure (age ratio and sex ratio), and individual hydro-mineral balance (osmolality) and morphology (size, mass, condition) in the western spadefoot toad (Pelobates cultripes) in two populations from the French Atlantic coast. We show that distance to coastline strongly influences exposure to salt on a small spatial scale. Some variables (e.g., abundances, osmolality, morphology) responded similarly in both sites and may be related to salt deposition due to landward sea-spray. Interestingly, other variables (sex ratio and age ratio) displayed site-specific responses and appeared to be linked to the salinity of breeding sites. Distance to the shoreline appears to be a critical population structuring factor in this coastal salt-tolerant species. Future studies should investigate how distance to shoreline—and thus environmental salinity—can ultimately affect individual performances and fitness.
According to the Island rule, insular populations exhibit gigantism in small species and dwarfism in large species. These contexts offer opportunities to test for complementary aspects pertaining to the evolution of body size, and particularly sexual size dimorphism (SSD). ‘Rensch's rule' states that SSD should vary with increasing body size, depending on the larger sex. As a consequence, it is expected that dwarfism or gigantism occurring in insular populations should influence the magnitude of SSD. Using anuran amphibians as a study system (because most anuran species express a female‐biased SSD and a marked lability in size), we investigated our hypothesis both in a specific comparison of continental and insular populations of a widespread coastal amphibian, and using a large‐scale analysis across anurans (10 species across 62 sites). Both datasets yielded similar results, with increasing body size in insular populations reducing the magnitude of SSD through an increase of male body size, as expected by Rensch's rule. Detailed data on insular populations considering both sexes are scarce, and future studies are required to complement the existing literature in order to test for the validity of our hypothesis at a wider scale and to infer the mechanistic causes of size variations, which remain unknown. In addition, further investigations are required to explore the consequences of insularity on the magnitude of SSD including various taxa (e.g. mammals, birds, lizards and snakes) in which the Island rule has found support, and considering taxa in which males are the larger sex. Keywords: amphibians, body size, insularity, Island rule, Rensch's rule, sexual dimorphism
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