Roads, buildings and other infrastructure can restrict the movements of wildlife in cities. As a result, urban wildlife from different areas encounter one another less frequently, potentially resulting in reduced genetic diversity. Scientists from the Leibniz Institute for Zoo and Wildlife Research (Leibniz-IZW) and the Technische Universität Berlin (TUB) investigated whether these effects are also evident in squirrels in Berlin. While genetic differences between the populations studied were generally small, subtle differences between locations were already evident. These suggest that the exchange of genetic variations (alleles) within the city may be limited. However, this does not yet have a significant impact on survival probability. The study has been published in the journal Ecology and Evolution.

Eurasian red squirrels (Sciurus vulgaris) are well adapted to life in cities. They adjust their diurnal activities very flexibly to humans, pets and wildlife in the city, actively avoiding predators such as cats, martens and dogs. They readily make use of human food sources and generally thrive in gardens and parks. Nevertheless, frequent sightings should not obscure the fact that their habitats are small and fragmented, and thus are more reminiscent of a patchwork quilt. A new study by the Leibniz-IZW has now investigated whether this habitat fragmentation is already detectable in the genomes of red squirrels from different locations in Berlin, and whether these differences have an impact on the animals’ probability of survival.

141 squirrels, three locations, two genetic groups
To this end, the team studied squirrels at three locations in Berlin from autumn 2021 to spring 2025: an inner-city cemetery, a large city park and a woodland area on the outskirts of the city. The areas differ significantly in terms of development, infrastructure and human activity. The three sites were specifically selected because, despite generally providing suitable habitats for squirrels, they represent varying degrees of urbanisation and different levels of integration into the urban landscape: ranging from a contiguous woodland area to a city park heavily surrounded by urban infrastructure. A total of 141 squirrels were captured and individually marked. For the genetic analyses, 135 individual genetic profiles were generated. This enabled the scientists to investigate how genetically diverse the populations are and to what extent they differ from one another. In a well-connected urban landscape, it is to be expected that populations are more genetically intermixed and therefore differ less from one another.
“Genetic diversity was high at all three sites. If one considered only the geographical location, one would have expected the populations in the city park and the cemetery – which are just a few kilometres apart – to be genetically similar, whilst the forest population, which is significantly further away, would stand out more,” explains lead author Sinah Drenske from the Leibniz-IZW. However, the genetic analyses revealed a different pattern: Whilst the squirrels from the woodland and the cemetery were predominantly assigned to a single genetic group, the animals from the city park differed more markedly from the other two. “We do not see any highly isolated populations,” says Drenske. “But we are already detecting subtle genetic differences that suggest that squirrels cannot move completely freely through the city.”

Subtle genetic differences, but no clear differences in survival
The team also investigated whether these genetic differences were already having an impact on the animals, using catch-recapture data collected over several years. The estimated probability of survival over a six-month period was similar across the three sites. Although it was slightly lower in the heavily urbanised city park, the statistical uncertainty was too great to conclude that urbanisation had a clear negative effect.
Stephanie Kramer-Schadt from the Leibniz Institute for Zoo and Wildlife Research (IZW) and the Technische Universität Berlin (TUB) explains: “The results show that Berlin’s squirrels exhibit a high level of genetic diversity and that there is currently no evidence of significant inbreeding or advanced isolation. Should connectivity between urban habitats continue to decline in future, this could also lead to a further reduction in genetic exchange.” For squirrels and other species that depend on tree-rich habitats, green spaces and tree-lined corridors could therefore play an important role in maintaining exchange between populations. Wildlife-friendly urban planning should take this into account.