19.05.2020 | Beate Kittl | News WSL
When it comes to the pollination of wild plants and crops, wild bees are just as important as honeybees, which are being celebrated on World Bee Day on 20 May. A study by WSL researchers has found that a quarter of all wild bee species present in Switzerland are found in gardens, parks and other green spaces in the city of Zurich.
At least as far as wild bees are concerned, Zurich is neither a concrete desert nor an ecological wasteland, with 164 of the approximately 600 wild bee species native to Switzerland living in the city. Researchers David Frey and Marco Moretti of the Swiss Federal Institute for Forest, Snow and Landscape Research (WSL) and Bertrand Fournier of Concordia University in Montreal, Canada, came to this conclusion using data from three large-scale WSL studies on biodiversity in the city of Zurich. Gardens, parks, urban wastelands and green roofs provide habitats for very different groups of species.
The researchers have therefore shown that, at least as far as bees are concerned, urbanisation does not inevitably have to result in the homogenisation of fauna down to just a few species, as is often feared. Indeed, between 25 and 30 species of wild bee can be heard buzzing in the average Zurich garden or allotment. These bees do not sting and are harmless to humans. "The city provides a diverse set of habitats and the wild bees that live here seem to be well adapted to the conditions," David Frey explains.
Mason bees lay their eggs in cavities and store pollen in the cavities as food for the growing larvae. (Photo: Sofia Mangili)
Many wild bee species that are well adapted to urban life build their nests in existing cavities such as bee hotels, but species with which use other breeding sites are also doing well. (Photo: Sofia Mangili)
Early flowering plants such as grape hyacinths are a good source of food for city bees. Pictured: a red mason bee (Osmia bicornis) (Photo: Sofia Mangili)
Common mourning bee (Melecta albifrons) on an apple blossom (Photo: Sofia Mangili)
No substitution of green spaces ¶
The scientists examined the ecological characteristics of Zurich's bees, and found that most belong to species that can use the city's great diversity of plants as food. They are active earlier in the year and for a longer period and often nest in existing cavities such as 'bee hotels'.
The types of urban green space investigated – gardens, parks, wasteland and green roofs – are home to very different communities of bees. While around 25 wild bee species on average live in gardens and wasteland, that number falls to 15 in parks and on green roofs. Moreover, garden bees are more 'specialised', meaning that they are more particular about the quality of their habitat. Such specialists tend to be at greater risk from habitat loss than generalists and are therefore more often the target of conservation measures.
"A variety of bee communities live in the city," Frey concludes. There is no evidence for a homogenisation of these communities. Different ecological strategies appear to be successful, for example when it comes to the choice of nesting sites or fodder plants. "Cuckoo bees" – i.e. bees that lay their eggs in the nests of other species – are an exception, with such brood parasites being much rarer in the city.
On the whole, though, it is far from the case that just a few dominant species are doing well. Another implication of their findings, according to Frey, is that one kind of green space cannot simply be substituted for another – flat roofs instead of urban gardens, for example. The researchers report on their results in the Journal of Biogeography.
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