Trophic downgrading in the city: urban structural simplification erodes the snake component of reptile assemblages

Abstract

Abstract Urbanization is widely recognized as a major driver of biodiversity loss, yet its effects on the trophic structure of reptile communities remain poorly understood. This study re-analysed reptile presence–absence data from urban green-space fragments in Rome, Italy, to examine how urban habitat simplification affects the snake component and the trophic and functional structure of reptile assemblages. Fifteen reptile species were classified into functional and trophic groups, and several assemblage metrics were evaluated, including snake richness, lizard and gecko richness, trophic complexity, community trophic index, and dominance of urban-tolerant species. These metrics were related to fragment size, woodland cover, wetland presence, connectivity, and distance from the city centre. Results revealed a sharp, data-supported transition in snake occurrence across the woodland-cover gradient. Small fragments with limited woodland cover were dominated by urban-tolerant lizards and geckos, while snakes were largely absent. In contrast, larger and more structurally complex fragments retained diverse snake assemblages, particularly where wetlands were present. Woodland cover and wetlands were the strongest predictors of snake richness, whereas snake occurrence was best explained by total fragment size and wetland presence. Additional sensitivity analyses showed that the main pattern was robust to the exclusion of empty fragments and to alternative trophic-scoring schemes, although the strength of the trophic-complexity signal was reduced under the most conservative classification. Overall, our results suggest that urbanization-related habitat simplification is associated with taxon-linked trophic and functional simplification: small fragments retain some urban-tolerant lizards and geckos, but largely lose the snake component of the assemblage. These findings indicate that reptile assemblages can provide useful information on functional erosion in urban landscapes, while also emphasizing that trophic position, taxonomic affiliation and habitat-use traits may be difficult to disentangle in small vertebrate assemblages.