This study offers insights into the composition, species diversity and evenness of a small mammal community across a gradient of urbanization. The community composition varied between study sites, yet no discernible pattern emerged regarding an increase or decrease in indices of diversity and evenness with impervious surface cover.
Our camera trap study revealed the presence of eight small ground-dwelling mammal species/taxa within the study area, with Apodemus spp. appearing in the majority (74%) of all captured photos. The predominance of Apodemus spp. aligns with their generalist behavior and ability to thrive in diverse habitats, especially A. sylvaticus [35, 53]. Similarly to our results, the probability of occurrence of different Apodemus species (A. agrarius, A. flavicollis, A. sylvaticus) has been found to be unaffected by human population density in Poland, while that of bank voles (M. glareolus) decreased with increasing human population density [35]. Contrary to our expectations and in contrast to other studies [29, 35, 53], we found no decrease in species richness, Shannon diversity index or Pielou’s evenness index with increasing impervious surface cover in any of the three different buffer zones of 100 m, 500 m and 1000 m. This result cannot be explained by a lack of either very urban or rural areas because impervious surface cover of the study sites covered large ranges at all buffer sizes (1000 m buffer: 8.2–71.7%; 500 m buffer: 2.9–76.6%; 100 m buffer: 4.2–89.3%). Moreover, the asymptotic trend observed in the species accumulation curve suggests that the majority of mammal species in our study area have been adequately sampled and additional sampling efforts at additional sites would have resulted in the discovery of very few additional species, if any at all. These results suggest that the small mammal community composition of Münster is relatively stable across the urbanization gradient examined.
The development of Münster into a city commenced in 1850 [54]. The extant small mammal community comprises species that managed to survive the drastic change of a more natural habitat into a high-density human settlement. These species were able to pass the urban filters, potentially via the expression of high phenotypic plasticity, which may have allowed them to adjust to urban conditions [55–57]. Urban areas can offer a greater continuity of natural food resources because urban areas can harbor a high plant diversity. In addition, the ‘urban heat island effect’, where temperatures in urban areas are elevated in comparison to their surrounding rural areas [58], can result in earlier flowering and extended growing seasons [4, 59, 60]. Moreover, in addition to natural resources (e.g., fruit and nut trees), urban habitats provide predictable food resources for wildlife, either directly (i.e. through bird feeders) or unintentionally via accidental feeding of wildlife (i.e. garbage or composts) [61–64]. Cities provide not only predictable food sources, but also reliable water resources and shelter in anthropogenic structures, which might contribute to the maintenance of small mammal populations and could be one explanation for the absence of patterns in species diversity along the urbanization gradient. Furthermore, Münster's long history may have provided wildlife with sufficient time to adapt to the novel urban conditions (e.g., altered dietary quality, human disturbance, and noise, light and chemical pollution), either via phenotypic plasticity or evolutionary change. Observable evolutionary change can occur in as little as two generations [65, 66]. It is also conceivable that differences in species richness and diversity may have existed in the past along the rural-urban gradient in Münster, and that these differences may have disappeared over time due to species adaptation or the displacement of non-adapted species.
The magnitude and the direction of species’ responses to urbanization can vary between cities and can be affected, for example, by housing density and green space availability. For example, a multi-city study showed that consistent decreases in mammal diversity occurred only above average housing densities [11], while another study reported similar or higher mammalian species richness and diversity in developed areas of two large cities compared to wild areas [32]. Here, we focused our study on small mammals, which are often under-represented or not detected at all in camera trapping studies [37]. Across North America, larger-bodied mammal species are more negatively affected by urbanization compared to smaller-bodied species [30]. It is unclear, if the same patterns are present in European mammals. If European smaller-bodied species are also less affected by urbanization than larger-bodied species, this may offer one explanation for the absence of the expected patterns in our data. Moreover, the extent to which wildlife communities use human-modified habitats can vary across and between years, depending on fluctuations in human land use, and changes in resource availability and human activity [67–69]. For example, anthropogenic food sources can reduce starvation risk and positively influence survival of wildlife, especially during the winter [61, 70]. Moreover, the ‘urban heat island effect’, where air temperatures in urban areas are elevated compared to rural surroundings [58], may alleviate some of the thermal challenges [71] and reduce energy expenditure during winter, allowing especially small bodied species, to remain active for prolonged periods in winter [72]. Consequently, studies encompassing multiple seasons and/or years might yield different results.
Species co-occurrence patterns
Our analysis of species co-occurrence patterns revealed predominantly neutral associations among the species. The results showed a trend for a positive association between Crocidura spp. and Myodes glareolus. Both taxa use the same aboveground runways and underground tunnel systems and favor habitats with dense ground cover, thus our finding may be a result of shared habitat requirements [73–75]. Otherwise, positive associations were notably absent, indicating a lack of preferential co-occurrence patterns among the studied species pairs. We observed consistent negative associations between two pairs: Apodemus spp. and Crocidura spp., and Rattus norvegicus and Sciurus vulgaris. These negative associations suggest potential competition or avoidance behaviors. However, the negative associations between both species’ pairs were present together in only 5 out of 30 iterations (17%) when looping through the data. Therefore, it is possible that these results were influenced strongly by patterns recorded in specific gardens and do not reflect general co-occurrence patterns of these species.
One limitation of our study is that we focused on a single urban area, the city of Münster, and a limited set of mammal species (e.g. bats were not included). Future research could expand the geographic scope by studying multiple cities to provide a more comprehensive understanding of urban mammal ecology. Long-term monitoring could further elucidate the mechanisms driving species interactions and community dynamics in response to ongoing urbanization and environmental change.
In conclusion, our study contributes to the growing body of literature on urban ecology by providing empirical insights into the composition, structure, and interactions of small mammal communities in an urban environment.