| Summary: | Urbanisation is one of the major drivers of land use change, habitat fragmentation and biodiversity loss. However, urban areas should not be considered as “ecological deserts”. Instead, they have the potential to support diverse communities, on condition that suitable habitat patches occur. To reach adequate management decisions and design actions mitigating biodiversity loss in urban areas, it is necessary to identify the major factors that shape up biotic communities.
In this context, the current study investigated how biodiversity responds to urbanisation in Patras (Greece), a medium-sized densely built Mediterranean city. In order to comprehend species responses among taxa that manifest diverse ways of using their environment, a multi-taxon approach was used: the groups of birds, butterflies and bats were studied, as they are considered good indicators of environmental change. Their diversity patterns and community structure were investigated, in order to determine the way they are influenced by landscape composition (land cover) and specific habitat characteristics.
For this, the urban landscape was described in terms of an urbanisation gradient: the study area was stratified into three zones of decreasing urbanisation, namely the urban (> 50% built cover), the suburban (30-50% b.c.) and the peri-urban zone (< 30% b.c.). Built cover was calculated using Urban Atlas as a baselayer.
Sampling sites were randomly selected and equally distributed along the urbanisation gradient. Bird surveys were conducted both in spring and winter in 90 sites, using 10-min point counts. Butterflies were sampled during spring in 45 sites with 300-m transects. Bat surveys were also conducted with 300-m transects in the same sites as for butterflies, during bat post-breeding season for two consecutive years.
Landscape composition (land cover) was assessed within a buffer zone around each sampling site, using Google Earth imagery as a baselayer. The main land cover types in the study area were buildings, impervious surfaces, woody vegetation, open green spaces with grass and herbs, and water bodies. In addition, local habitat was assessed for butterflies, as the latter are known to be associated with plant species richness, flower and larval host-plant abundance, and vegetation height. Likewise, temperature, relative humidity, the number of streetlamps and distance to the closest water body were measured for each bat sampling site,
as these factors were expected to influence bat activity.
Diversity comparisons among the urbanisation zones were performed with non-parametric tests. The effect of land cover and habitat characteristics on diversity was assessed with Generalized Linear Models (GLMs) in the case of birds and butterflies, whereas Generalized Linear Mixed Models (GLMMs) were used for bats. The relationship between community
structure and land cover and habitat was investigated with Redundancy Discriminant Analysis (RDA).
Overall, a negative effect of urbanisation was found for birds, butterflies and bats, in increasing order. Bird diversity increased along the decreasing urbanisation gradient and peaked in the peri-urban zone. The urbanised areas were documented to support a more diverse wintering bird community compared to the breeding one. Open green spaces were found to be very important for birds, especially for breeding ones, possibly because of the large number of farmland species occurring in the study area during the breeding season. In addition, woody vegetation cover was positively associated with the wintering community,
apparently because the latter consists of several forest species. Impervious surface cover had also a positive effect on wintering birds, perhaps because birds in winter often form larger, more mobile groups in order to forage.
Butterfly diversity was generally low in the urban and suburban zones but increased significantly in the peri-urban zone. It was strongly affected by landscape composition; however, surprisingly, the effect of local habitat characteristics was negligible. Specifically, diversity and community structure were negatively associated with built-up areas, while they were positively influenced by woody vegetation cover. However, the increase of vegetation cover along the urbanisation gradient did not result in a corresponding increase in butterfly diversity. These findings imply that there might be other limiting factors for butterfly
diversity, possibly related to the size and isolation of green patches in the urban and suburban zones, or alternatively, the existing habitats might not be suitable.
Bats were found to be largely affected by urbanisation, as their diversity in the entire study area was generally low. The community was dominated by the synurbic species Pipistrellus kuhlii, which comprised more than 70% of the total bat activity. A positive relationship between built-up areas and bat activity was found, probably because P. kuhlii usually forages
around streetlamps in urban areas. Other species were generally rare and were mostly recorded close to water bodies, highlighting their value for foraging bats. Moreover, bat activity was significantly higher in the second year of the study. However, as bat activity patterns may be subject to spatio-temporal variation of environmental conditions and prey availability, further research would provide more insights into species occurrences and the factors affecting them.
The findings of the current study underline the negative effect of the densely built areas and the need for efficient management of the few remaining urban green spaces. A clear updated city plan needs to be urgently designed; this should be able to strictly control urban and suburban sprawl. Management actions should principally aim at increasing green patch
size, connectivity and habitat suitability, as well as at restoring water bodies and preserving their natural characteristics.
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