Tawny Owl and Urbanization Structure: History
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Subjects: Ecology | Ornithology
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Nerea Pagaldai

Tawny owl (Strix aluco) is a ubiquitous and resilient raptor that successfully exploits heterogeneous patchy habitats. Due to its abundance and broad geographic range, it is an appropriate bird species model with which to test how raptors colonize urban environments and which drivers from these urban habitats play a role in explaining the occurrence of these species. It is necessary to understand how urbanization affects the tawny owl at the landscape scale. The effect of habitat fragmentation (i.e., forest fragmentation) on tawny owl populations has been studied previously ; however, to our knowledge, the effect of urban habitat fragmentation remains unknown. In addition, it is not common to study habitat selection through a multiscale approach, even though different variables can be important to a species at different spatial scales. As such, studies conducted at arbitrarily defined scales may suffer from serious limitations .

  • binomial N-mixture models
  • landscape metrics
  • patchy habitat
  • spatial scales
  • urban environments
  • urban raptors

1. Introduction

The process of urbanization has led to the alteration and destruction of the natural habitat and a decrease in species abundance and richness [1]. As a result [2][3], only a few species are able to successfully adapt to this new urban environment. Although habitat loss is a major consequence of the conversion from native to non-native habitats, habitat fragmentation does have distinct and sometimes influential effects (e.g., [4]). The fragmentation of habitats due to the expansion of cities into their urban surroundings affects the habitats available for birds in all cities across the world [5]. However, the ability to cope with the challenges of living in urban areas depends not only on the characteristics of each species but also on local conditions [6]. Despite a large number of negative influences on urban birds [7], cities still allow ecological niches to occur, meaning that those species able to exploit them perform well [8]. While this leads to an increase in avian biomass, the downside is a reduction in richness [9].
Differences have been observed in bird species along rural–urban gradients, with sensitive species generally tending to locate at the outskirts of the city which still have a relatively high proportion of “natural” habitat. However, with increasing expanses of urbanization within the “core” area of the city, the avian community tends to be dominated by four or five “urban” bird species [10]. Raptors are considered “sentinels” of different local and large-scale environmental change, being sensitive to changes in land use and highly susceptible to local extinctions [11]. For instance, raptor richness tends to be negatively affected by urban development [12]. Indeed, urbanization is leading to systematic raptor population declines due to the reduction in the area of suitable habitat available [13][14]. Nevertheless, some studies have revealed that this group of birds has a certain capacity to colonize urban areas [15][16]. This could be due to the fact that raptors usually have large home ranges, which can extend well beyond the urban boundary and, therefore, do not need to meet all their ecological requirements within urban spaces [9]. In addition, moderate levels of landscape alteration may also permit the co-occurrence of many species, mostly due to edge effects [17]. In such conditions, community structure changes along the transition zone, or ecotone, between different land uses [18]. Some generalist species may in fact even show higher densities and reproduction and survival levels in urban areas [19].

2. Colonization of Urban Habitats: Tawny Owl Abundance Is Conditioned by Urbanization Structure

In a previous study, we analyzed the effects of survey related factors (i.e., the observer’s experience and wind speed) and the principal site-related habitat factors on tawny owl detection and abundance in the study area [20]. We demonstrated that the presence of forested habitats positively affected its abundance, whereas urban habitats had a negative effect. In this study, which complements the previous one, we focused on analyzing the effects the distribution and structure of urban habitats have on the species abundance. Our results show that the abundance of tawny owls in urban habitats strongly depends on the forested areas that surround them, as well as on the structure of the urban area itself. As such, the best sites for tawny owls in cities are periurban areas or areas where an urban–forested matrix is present. Therefore, our results also show new biodiversity potential associated with urban areas as an urban ecosystem service [21] through increasing numbers of tawny owls.
When a local-scale approach was used, the species was found to be more abundant in places with a higher proportion of forests, such as the Cantabric region. This confirms results from our previous study which analyzed the main distribution pattern of this owl in the region [22]. One novel finding, however, is that we detected a quadratic effect, indicating that both extensive forest areas and small patches of forest negatively affected tawny owl abundance (for similar results, see also [23]), this being different to results obtained in another work [24]. It seems, therefore, that the species may benefit from using woodland areas with some degree of forest fragmentation, with the non-forested areas comprising small patches of open habitats such as grasslands, meadows, or pastures [25]. Urbanization showed a forest-dependent effect in addition to its overall negative effect. When the forested area was low (<30%), the percentage of urban area had a slightly negative effect on tawny owl abundance. At intermediate forest percentages (30–60%), urbanization had almost no effect, with abundance being more or less constant throughout the urban range. However, when forest covered extensive areas (>60%), the percentage of urbanized area showed the worst effect. This is probably due to the permanent food supplies that urban environments provide [26], as well as the availability of suitable new nesting and roosting sites [27]. In addition, it has been demonstrated that home range size decreases at the intermediate level of forested area [25], with higher densities being found in urban areas mixed with patchily distributed forests than in purely forested areas [28][29], but always depending on the proportion of suitable habitats available [19]. Regarding the aggregation level of urban patches (CLU), something similar happens in areas of low urbanization (<10%), where aggregation level has a slight, nearly constant, effect on tawny owl abundance. Nevertheless, the negative effect becomes stronger as urbanization level increases, probably due to the lack of suitable habitats since tawny owl is a forest-dwelling raptor and, therefore, needs trees to be able to exploit urban areas. Moreover, in highly urbanized areas (>20%), where urban patches tend to be more aggregated, there are not so many trophic resources to exploit due to the reduced availability of green areas. Tawny owls have been shown to prefer heterogeneous patchy habitats [30], although urban habitat was not taken into account in this study. Our results suggest that this habitat could in fact be included as yet another habitat type that the species can exploit. At this point, we should note that the degree of urbanization within our region ranged from moderate to low since our SUs and SPs were never surrounded by more than 20% of urban land use. This is a good reflection of our study area, which mainly comprises spread out rural villages with few cities.
At the landscape scale, our results also showed that the highest abundance was at intermediate levels of forest cover, again contrasting with previous results [24]. Nevertheless, the amount of urban habitat did not appear to affect abundance, which could be due to the habitat variability encompassed at this scale. However, two urban indices showed significant effects, which means that urbanization affects tawny owl abundance differently when considered at different scales, showing that multiple scales need to be taken into account in future studies [31]. The distance to the nearest neighboring patch of the same class (ENN) resulted in a quadratic effect. That is, when urban patches were very close to each other, the space for forest or crop patches was not big enough or did not contain sufficient diversity to ensure high tawny owl abundances. In contrast, when urban patches were very far apart, the forest or crop habitats between them were larger. As a consequence of tawny owl preference for an urban–rural mix, with high levels of forest or cropland, the abundance of the species will decrease. It is, thus, clear that there is an optimal point where the gap between urban areas is sufficient to maintain a mixture of different crops and forested areas capable of ensuring a high trophic resource, which likely results in an increase in tawny owl abundance. Regarding the urban patch shape index (SHAPE), our results do not match with other researchers [32][33], who observed that the shape index was not relevant to species abundance. In the present study, however, an increase in shape complexity, which is related to a lower urban core area, showed a negative effect on species abundance. This could be attributable to the low urbanization rate in the study area, as well as the narrow range of the variable. The shape index starts at 0 and can increase indefinitely, but our shape index range was only from 1.2 to 1.7. As our shape complexity ratio was so small, we probably did not observe the real effect of this variable. Thus, a wider range of shape complexity should be considered in future studies. The number of urban patches showed a positive effect on species abundance here, which may have been a consequence of the previously mentioned suitable habitats [19] and resource availability, as towns and villages offer high levels of trophic resources and nesting niches.

3. Conclusions

Tawny owls have the capacity to colonize new environments [34][35] depending on the surrounding habitat and urban habitat structure [18][28]. This ability is known to be associated with success in the colonization of urban habitats, as well as with ecological and evolutionary consequences in terms of consistent changes in distribution, abundance, behavior, and life history [36]. At the local scale, when minimum forest structure is available in urbanized areas or its surroundings, and when urbanization is patchier, the species can easily exploit it, which has a positive effect on its abundance [18][28]. At the landscape scale, we observed that this species prefers smaller urban villages split in more patches than a large, urbanized city. In this sense, we think that the current tendency toward “green capitals” (the Green Capital Award is a European award for cities based on their environmental records) and the environmental changes involved [37][38] may benefit tawny owl populations, at least in terms of abundance in urban areas. From a biodiversity [39] and pollution [40] point of view, it is valuable to have an urban bioindicator such as an urban tawny owl population, as well as the possibility of monitoring it over time. Nevertheless, we observed that tawny owls do not exploit highly urbanized environments. Our study contributes to the building of friendly cities through adding to our knowledge of how new urban areas should be constructed. It is recommended to maintain small forest patches, transforming the urban matrix into heterogeneous areas, where tawny owls can exploit them as foraging areas in this new urban ecosystem [28]. In the context of global change where landscapes are increasingly anthropized, we want to highlight the importance of assessing multiscale relationships between urban biodiversity and the generation of new urban ecosystem services [21].

This entry is adapted from the peer-reviewed paper 10.3390/ani11102954

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