Urbanization Shapes Dietary Composition and Snout Morphology of Salamanders

Urbanization rapidly alters landscapes, generating habitat mosaics characterized by increased impervious surface, reduced soil moisture and diminished vegetation cover. These recurring urban environmental features are expected to reduce community diversity, favouring urban-tolerant species, thereby promoting urban biotic homogenization. Although shifts in invertebrate community composition have been widely documented, less is known about how such changes propagate to higher trophic levels. In this study, we examined how urbanization relates to dietary and phenotypic variation in an amphibian species, the fire salamander (Salamandra salamandra), along an urban gradient in Porto, Portugal. Dietary diversity and composition were assessed through metabarcoding of faecal pellets, and geometric morphometric analyses of dorsal snout shape were conducted to determine whether dietary homogenization corresponds with morphological similarity and reduced phenotypic variation. Environmental indicators of urbanization were not correlated with a decline in overall dietary diversity; however, it was linked to an increased prevalence of isopods (Armadillidium spp.) in the diet. Morphological results showed a relationship between snout shape and dietary diversity, suggesting greater morphological similarity among urbanized sites. However, no significant relationship was detected between morphological differentiation and variation in dietary niche overlap or diversity. Overall, dietary and morphological traits were more similar among urban sites, suggesting emerging patterns consistent with urban biotic homogenization.