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Could urbanisation and biodiversity be compatible?
More than 900 species of wild bees are found in France, but many of them - such as bumblebees - are in decline. INRA scientists, working in collaboration with the naturalist association Arthropologia, have carried out the first exhaustive study in Europe to evaluate the impact of urbanisation on the wild bee community. They studied 24 more or less urbanised sites in and around Lyon and recorded 291 different bee species. Although bee abundance decreased with an increasing level of urbanisation, the number of species present was at its peak in periurban areas, and 60 species - a considerable number - were found at the most urban site. These findings are published in the open-access journal PLOS ONE on 13 August 2014.
For a bee species to be present in an urban environment, it must be able to find sufficient food resources and appropriate nesting sites. Some bee species are ground-nesting, such as mining bees, while others (such as Osmia mason bees) nest in pre-existing cavities. Aside from its nesting behaviour, each bee species has its own biological characteristics and may therefore respond differently to urbanisation. The different species and their characteristics then become essential elements to study the impact of urbanisation on the structure of wild bee communities, defined as all the species found in a given environment.
In the context of the European LIFE Urbanbees1 programme, wild bee communities were studied at 24 sites along an urbanisation gradient in the Urban Community of Lyon. This programme, launched in 2010 for a 5-year period, is operated by INRA in Avignon, in partnership with the association Arthropologia, the cities of Lyon and Villeurbanne, the University of Lyon and the Natural History Museum in London. It is funded jointly by the European Union, the French Ministry for Ecology, the Rhône-Alpes Regional Council, the Urban Community of Lyon and Botanic®.
Every month for two years, bees were sampled using coloured pan traps and insect nets at the 24 sites around Lyon. The pan traps were left active for 24 hours. The nets were used to capture insects on all flowering plants within a radius of 100 m from the traps. In order to study changes in the composition of the community, the scientists took into account different bee characteristics: host/parasite status and nesting mode.
In this study, 291 species of wild bee were captured, or almost a third of the more than 900 wild bee species known in France. Bee abundance was negatively correlated with urbanisation, but species richness reached its maximum in sites at an intermediate proportion of urbanisation (50% of impervious surface within 500 metres, that is periurban environment). Nevertheless, scientists found considerable richness in the most urbanised areas (60 species in Villeurbanne at a site with over 98% of impervious surface). The structure of the community changed along the urbanisation gradient, with more parasitic species in periurban environments. Cavity-nesting bees were more diversified in urban environments than ground-nesting bees.
The considerable diversity of wild bees recorded in the city centre showed that through appropriate management, even the most urban areas could be interesting environments in terms of ecology and conservation in order to safeguard these pollinators and the mutual relationship they maintain with wild and cultivated plants. The diversity of wild bees in cities also means they constitute a flagship group to raise the awareness of urban populations to ecology and ecosystem services, showing them that biodiversity can be encountered everywhere and on an everyday basis.
Fortel, L., Henry, M., Guilbaud, L., Guirao, A.L., Kuhlmann, M., Mouret, H., Rollin, O. & Vaissière, B.E. (2014) Decreasing abundance, increasing diversity and changing structure of the wild bee community (Hymenoptera: Anthophila) along an urbanization gradient. PLoS ONE 9(8): e104679. doi:10.1371/journal.pone.0104679
For more information on wild bees and urbanisation
Wild bees such as bumblebees or leaf-cutting bees are poorly known even though they are pollinating insects with a key role that is now increasingly recognized.
With more than 900 species in France and 2500 in Europe, bees are a highly diversified group because they exceed the total of all amphibian, reptile, bird and mammalian species combined. However, the decline of bees in Europe is now well established, with bumblebees a good example of affected species.
Urbanisation is considered as a major cause of the loss of biodiversity, because it results in drastic and irreversible changes in habitats. An urban landscape is defined as being a mosaic of impervious surfaces (buildings, parking lots and roads, for example) and permeable surfaces that are regularly disturbed. An urban environment can be characterised by its proportion of impervious surface and the connectivity of its habitats, which are two key elements for the wild fauna.
No exhaustive study of the situation of wild bees in European cities had previously been conducted. Even though urbanisation has a negative effect on insect fauna, wild bees are found in urban environments. Indeed, flowers are present there throughout the year, the temperature is a few degrees higher than in surrounding environments, and pesticide use is increasingly reasoned to reduce it.