Squirrel: Tricky island life in the city

Researchers examine the squirrels of Berlin. Leibniz-IZW/Stephanie Kramer-Schadt

Even in big cities one can occasionally enjoy the sight of a squirrel. But these sightings belie just how problematic city life can be for squirrels, researchers report. Using Berlin as an example, her study, which involved citizen scientists, shows that the urban habitats of squirrels are often critically small and fragmented. The information can help protect populations of these iconic wildlife ambassadors from disruptions caused by urban development, the researchers say.

Apparently, the cute rodents basically get along with the bit of nature that some big cities like Berlin have to offer: In contrast to many other animal species, the red Eurasian squirrel (Sciurus vulgaris) seems to be extremely adaptable. "However, this can lead to the misconception that they find very good living conditions in the big city and that we know quite a lot about their lifestyle and health," says Sinah Drenske from the Leibniz Institute for Zoo and Wildlife Research in Berlin. "A lot of supposed knowledge about their movement patterns, their diet or their state of health is actually only anecdotal knowledge," emphasizes the researcher. With their research work, they and their colleagues want to provide well-founded information about the animal city dwellers.

Support from citizen scientists

The current study is based on two principles: The scientists use data from citizen science projects in which Berlin citizens report sightings of squirrels. In addition, wildlife cameras set up at certain locations provided information about the occurrence of the animals. “These data vary in quality and degree of structuring: for example, wildlife cameras were evenly distributed across Berlin using a two-by-two kilometer grid, while sightings happened randomly where and then people saw the animals.” , explains lead author Marius Grabow.

For their study, the researchers combined the sighting data with information about the local living environment: They developed mathematical computer models that used environmental variables to best predict the occurrence of squirrels. These factors included, for example, the distance to the nearest green space, the nearest street, the number and age of trees, temperature values ​​and the degree of sealing of the urban structures. "Our goal was to improve spatial models in such a way that we can use existing environmental data to make predictions on the actual occurrence of the animals that are as accurate as possible," says Grabow.

"Islands" in the metropolis

As the team reports, their results show that the distribution of the habitats of the squirrels in Berlin is like a patchwork quilt. It becomes clear that the big city is a challenging place for the rodents and suitable habitats are small and fragmented. These islands are often critically isolated and it appears that roads, among other things, often pose major challenges for the animals. There is also evidence of how densification of the city's building stock further degrades the connectivity of the fragmented habitats, bringing individual populations closer to the edge of a "living wage". The researchers say that the increasing formation of heat islands in cities due to climate change could also put increasing strain on the animals.

The team also used the models to identify critical hotspots in Berlin, where the connections between habitat islands are particularly important. Among other things, an important and long corridor for the city's squirrels, which is formed by several green spaces on the Spree, attracted attention. "This belt has the potential to connect districts in East and West and is only interrupted by individual, massive structural barriers," says Grabow. The study was thus able to identify areas where corridors connecting fragmented habitats could be protected or created.

The researchers now want to take a closer look at Berlin's squirrels: in the coming years, there are plans to examine the animals physically and equip them with chips and transmitters. In this way, the team wants to gain insights into the genetic structure, state of health and physical activity behavior. "Before there is a population decline, we want to generate the knowledge that can help to secure the squirrel population in the city in the long term," Drenske concludes.

Source: Leibniz Institute for Zoo and Wildlife Research, specialist article: Front. Ecol. Evol., doi:10.3389/fevo.2022.881247

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