Urbanization can impact wildlife in different ways, depending on the organism and how it interacts with humans and the built environment. Geographical barriers may limit or prevent an organism from moving through a city and an organism’s behavioral variability (particularly its ability to tolerate humans) may also influence if and how it disperses throughout a city. Understanding how connected the landscape is from the species perspective can help clarify the importance of the different physical and behavioral barriers.
With this in mind, a team led by Sophia Kimmig recently used microsatellites to investigate 374 red foxes (Vulpes vulpes) in Berlin and the surrounding rural region of Brandenburg. The study area spanned a rural to urban gradient allowing the researchers to investigate how movement differed across landscapes (figure 1). The authors tested three hypotheses:
- Foxes disperse throughout the city due to their high mobility, leading to a panmictic population across the urban to rural gradient.
- Physical barriers such as highways and built-up areas prevent dispersal within the city.
- Dispersal is affected by behavioral barriers which likely occur where the rural environment changes into the urban environment.
First, the authors used two programs to assess the number of genetic clusters in their dataset: STRUCTURE and GENELAND. Both programs found two clusters (figure 2) which were primarily assigned to inside the city of Berlin and outside the urban area.
Kimmig et al. (2020) also conducted landscape resistance modeling to determine the most likely features that facilitate the movement of foxes (figure 3). When analyzing the different features separately, the authors found that motorways were always the most significant feature that facilitated gene flow and that railways were the second-highest ranked model. They also found that waterways were a strong barrier to gene flow. When combining variables Kimmig et al. (2020) found that their city border models were supported indicating that the city border was a barrier to gene flow.
Overall, this study supports the urban island hypothesis, with genetic differentiation between urban and rural populations of red foxes. However, the genetic structure was weak indicating that dispersal from the surrounding rural area into Berlin still occurs. The authors point out that high-quality food and lack of predators (specifically hunting pressure from humans) in urban areas like Berlin may lead to better habitat within the city compared with the surrounding countryside. The authors also note that rural foxes, unlike urban foxes, do not use artificial structures for dispersal suggesting that fear may hinder rural foxes from entering the urban island.
Additionally, the authors consider the implications of historical landscape features that may have contributed to the current genetic structure. From 1961 to 1989 West Berlin was surrounded by the Berlin Wall. The authors suggest that while a founder effect may have led to an initial reduction in the genetic diversity of urban foxes, the Berlin Wall may have contributed to maintaining this genetic differentiation. Since the wall came down 30 years ago, the urban island may have persisted due to the behavioral limitations of foxes. Thus, human presence may be a key factor that influences the genetic difference between urban and rural foxes while also influencing the ways in which foxes disperse through the urban matrix.
Read the full paper here:
Kimmig, S. E., J. Beninde, M. Brandt, A. Schleimer, S. Kramer-Schadt, H. Hofer, K. Börner, C. Schulze, U. Wittstatt, M. Heddergott, T. Halczok, X. Staubach, and A. C. Frantz. 2020. Beyond the landscape: Resistance modelling infers physical and behavioural gene flow barriers to a mobile carnivore across a metropolitan area. Mol. Ecol. 1–19.
Featured image: “Red Fox” by kckellner is licensed under CC BY 2.0.
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