Abstract
Green roofs are potentially valuable habitats for plants and animals in urban areas. Wild bees are important pollinators for crops and wild plants and may be enhanced by anthropogenic structures, but little is known about wild bees on green roofs in cities. This study investigates the effects of green roof qualities (floral resources, substrate character and depth, roof height and age) on wild bee diversity, abundance and traits (nesting type, sociality, pollen specialisation, body size) on green roofs in Vienna. Nine green roofs were sampled monthly between March and September 2014 by a semi quantitative approach. Wild bees were collected in pre-defined sub-areas for the same amount of time and floral resources were recorded. Over all green roofs, 992 individuals belonging to 90 wild bee species were observed. Wild bee diversity and abundance was strongly positively affected by increasing forage availability and fine substrates. Wild bees on roofs were characteristically solitary, polylectic and 8.3–11.2 mm. Regarding nesting type, the percentage of above-ground nesting bees was higher compared to the common species composition in Middle Europe. Ground-nesting wild bees were mainly eusocial, smaller (6.4–9.6 mm) and positively affected by roofs with fine substrates. During June, when forage availability by wildflowers on roofs was “low” (5–15% flower coverage), flowering Sedum species were an important forage resource. We conclude that wild bee diversity and abundance on green roofs are enhanced by floral resources. Furthermore, the installations of areas with finer and deeper substrates benefit ground nesting and eusocial wild bees.
Highlights
The degradation and loss of habitats are seriously threatening wild bees (Winfree et al 2009; Le Féon et al 2010; Potts et al 2010; Kennedy et al 2013), leading to declines in pollination services for crops (Kremen et al 2002; Klein et al 2007) and wild plants (Biesmeijer et al 2006; Fontaine et al 2006)
Excluding A. mellifera (1470 individuals), which was present on all roofs, we identified 90 wild bee species belonging to 19 genera
Due to its geographical position, Vienna is influenced by the Pannonian climate, which could be one reason that the studied green roofs emerged with higher species numbers compared to the studies mentioned above
Summary
The degradation and loss of habitats are seriously threatening wild bees (Winfree et al 2009; Le Féon et al 2010; Potts et al 2010; Kennedy et al 2013), leading to declines in pollination services for crops (Kremen et al 2002; Klein et al 2007) and wild plants (Biesmeijer et al 2006; Fontaine et al 2006). As important pollinators in gardens, parks and other green spaces (Matteson et al 2008; Frankie et al 2009; Matteson and Langellotto 2010), wild bees play a crucial role for urban ecosystems. The availability of pollen and nectar as well as nesting sites within speciesspecific flying distances is important for wild bees to Institute for Integrative Nature Conservation Research, University of Natural Resources and Life Sciences, Vienna (BOKU), Gregor-Mendel-Straße 33, 1180 Vienna, Austria colonize habitats successfully (Westrich 1996; Gathmann and Tscharntke 2002; Zurbuchen et al 2010; Wojcik and McBride 2012). Research on urban sites likely to be populated by wild bees, provides important information for urban planning to enhance wild bee diversity (Hernandez et al 2009). From an ecological point of view, green roofs have high potential to restore
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