Urbanisation Leaves a Genetic Signature but No Detectable Demographic Effects in a Small Mammal Population

Urbanisation influences ecological and evolutionary processes, including gene flow and survival. Understanding how genetic and demographic rates respond to urbanisation is therefore essential for predicting and managing wildlife viability in human-dominated landscapes. We tested whether population genetic diversity, genetic structure, and apparent survival of Eurasian red squirrels (Sciurus vulgaris) vary in concert along an urbanisation gradient. We combined a multi-year capture-mark-recapture study with microsatellite-based population genetic analyses at three sites in Berlin, Germany, representing increasing levels of urbanisation. Genetic diversity was broadly comparable among sites, indicating no strong genetic erosion in urban populations. However, weak heterozygote deficits and subtle differences in individual heterozygosity suggest mild restrictions to gene flow along the urban gradient. Bayesian clustering analyses identified two genetic clusters, broadly corresponding to study sites, revealing emerging fine-scale population structure despite the small spatial extent of the study area. Apparent survival did not differ clearly among sites. Several predictors received similar support, highlighting substantial uncertainty in the drivers of survival in this system. By integrating genetic and demographic data, our study suggests that fine-scale genetic structuring may emerge before detectable effects on apparent survival become evident. These findings highlight that genetic and demographic responses to urbanisation may occur on different timescales and emphasise the importance of integrated approaches for understanding population responses in human-modified environments.