Abeilles sauvages et pollinisation en milieux urbanisés : approches expérimentales et méta-analytiques à grande échelle

Today, biodiversity is declining globally and pollinating insects are not spared. Land use changes are among the main cause of pollinator loss, especially through urbanization, which is expected to increase in the future. In this context, it is particularly important to understand the effects of urbanization on pollinating insects and the pollination function they provide. However, most studies on the links between, urbanization and pollinators are based on either one or a small number of cities, making difficult to draw generalizable trends. In this work, we studied, at the western European scale, the relationship between urbanization and wild bee communities, as well as links between urbanization and pollination. We found that wild bee communities taxonomic and functional diversity was negatively affected by urbanization, through a reduction in species richness and an urban trait filtering, especially on nesting behavior and diet specialization. We also found that wild bee species richness in cities can be high, but is highly variable among cities. Wild bee species richness variation was largely driven by city size but not by urban green spaces area. We also show that cities may not be refugees for species with conservation status. Finally, we assessed pollination function in a range of 16 European cities, using a pollinometer species (Sinapis alba), and found it to be covered throughout the season, with a peak in early June. Hoverflies and small wild bees were important pollinators of Sinapis alba, with small wild bees having a direct effect on its reproductive success. Habitat fragmentation negatively affected small wild bee visitation rate, and a high morphogroup diversity of insects visiting the pollinometer plants enhanced their reproductive success. Overall, these results provide an overview of the relationships between pollinators, pollination and urbanization. They show that cities support a restricted range of pollinator species and traits, thus advocating for new management practices to safeguard urban pollinators and the pollination function they provide.Aujourd'hui, la biodiversité est menacée, et les pollinisateurs ne font pas exception. Les changements d'utilisation des terres font partie des principales causes du déclin des pollinisateurs, en particulier à cause de l'urbanisation croissante. Il est donc très important de comprendre l'effet de l'urbanisation sur les insectes pollinisateurs et la fonction de pollinisation. Cependant, la majorité des études sur la pollinisation en milieux urbains sont focalisées sur une ou quelques villes, ce qui empêche d’avoir une vision globale des processus en jeu. Au cours de cette thèse, nous avons étudié, à l'échelle de l’ouest de l’Europe, la relation entre l'urbanisation et les communautés d'abeilles sauvages, ainsi que l’état de la fonction de pollinisation en ville. Nous avons constaté que la diversité taxonomique et fonctionnelle des communautés d'abeilles sauvages était affectée négativement par l'urbanisation. En effet, nous avons mis en évidence une réduction de la richesse spécifique et un filtre urbain des traits fonctionnels chez les abeilles sauvages, notamment en ce qui concerne le comportement de nidification et la spécialisation alimentaire. Nous avons également constaté que la richesse spécifique des abeilles sauvages dans les villes peut être élevée, mais qu’elle est très variable d'une ville à l'autre. Nous avons montré que cette variation était largement influencée par la taille de la ville mais pas par celle des espaces verts urbains. Nous avons également mis en évidence que les villes ne semblent pas être des refuges pour les espèces à statut de conservation. Enfin, nous avons analysé l’état de la fonction de pollinisation dans 16 villes ouest-Européennes, via une espèce pollinomètre (Sinapis alba). Cette fonction était assurée tout au long de la saison, avec un pic début juin. Les syrphes et les petites abeilles sauvages étaient les principaux pollinisateurs de Sinapis alba, les petites abeilles sauvages ayant de plus un effet direct sur le succès reproducteur des plantes. La fragmentation de l'habitat urbain impactait négativement le taux de visite des petites abeilles sauvages et le succès reproducteur des plantes pollinomètres était directement relié au nombre de morphogroupes de pollinisateurs venant les visiter. L’ensemble de ces résultats fournit une vision générale des liens entre pollinisateurs, pollinisation et urbanisation. Ainsi, les villes hébergent une diversité spécifique et fonctionnelle réduite de pollinisateurs, et de nouvelles pratiques de gestion devront être mises en place pour préserver les pollinisateurs et la pollinisation en ville

Diversity of greenspace design and management impacts pollinator communities in a densely urbanized landscape: the city of Paris, France

International audienceThe response of insect pollinator communities to increasing urbanization is shaped by landscape and local factors. But what about habitats that are already highly artificial? We investigated the drivers of pollinator diversity in a dense urban matrix, the city of Paris. We monitored insect pollinator communities monthly (March-October) for two consecutive years in 12 green spaces that differed in their management practices, focusing on four insect orders (Hymenoptera, Diptera, Lepidoptera, Coleoptera). Pollinator abundance and species richness were both positively tied to green space size and flowering plant species richness, but negatively linked to surrounding impervious surfaces. In addition, environmental features at both the local and landscape scales influenced the composition and functional diversity of wild bee communities. Indeed, small and large bees responded differently, with the occurrence of large-bodied species being impaired by the proportion of impervious surfaces but strongly enhanced by plant species richness. Also, sites with a majority of spontaneous plant species had more functionally diverse bee communities, with oligolectic species more likely to be found. These results, consistent with the literature, can guide the design and management practices of urban green spaces to promote pollinator diversity and pollination function, even in dense urban environments

Pollination and the city: a collaborative study to measure pollination function in a range of European cities

International audienc

Pollination and the city: a collaborative study to measure pollination function in a range of European cities

International audienc

Pollination and the city: a collaborative study to measure pollination function in a range of European cities

International audienc

Pollination and the city: a collaborative study to measure pollination function in a range of European cities

International audienc

A large-scale dataset reveals taxonomic and functional specificities of wild bee communities in urban habitats of Western Europe

Wild bees are declining, mainly due to the expansion of urban habitats that have led to land-use changes. Effects of urbanization on wild bee communities are still unclear, as shown by contrasting reports on their species and functional diversities in urban habitats. To address this current controversy, we built a large dataset, merging 16 surveys carried out in 3 countries of Western Europe during the past decades, and tested whether urbanization influences local wild bee taxonomic and functional community composition. These surveys encompassed a range of urbanization levels, that were quantified using two complementary metrics: the proportion of impervious surfaces and the human population density. Urban expansion, when measured as a proportion of impervious surfaces, but not as human population density, was significantly and negatively correlated with wild bee community species richness. Taxonomic dissimilarity of the bee community was independent of both urbanization metrics. However, occurrence rates of functional traits revealed significant differences between lightly and highly urbanized communities, for both urbanization metrics. With higher human population density, probabilities of occurrence of above-ground nesters, generalist and small species increased. With higher soil sealing, probabilities of occurrence of above-ground nesters, generalists and social bees increased as well. Overall, these results, based on a large European dataset, suggest that urbanization can have negative impacts on wild bee diversity. They further identify some traits favored in urban environments, showing that several wild bee species can thrive in cities

A large-scale dataset reveals taxonomic and functional specificities of wild bee communities in urban habitats of Western Europe

International audienceWild bees are declining, mainly due to the expansion of urban habitats that have led to land-use changes. Effects of urbanization on wild bee communities are still unclear, as shown by contrasting reports on their species and functional diversities in urban habitats. To address this current controversy, we built a large dataset, merging 16 surveys carried out in 3 countries of Western Europe during the past decades, and tested whether urbanization influences local wild bee taxonomic and functional community composition. These surveys encompassed a range of urbanization levels, that were quantified using two complementary metrics: the proportion of impervious surfaces and the human population density. Urban expansion, when measured as a proportion of impervious surfaces, but not as human population density, was significantly and negatively correlated with wild bee community species richness. Taxonomic dissimilarity of the bee community was independent of both urbanization metrics. However, occurrence rates of functional traits revealed significant differences between lightly and highly urbanized communities, for both urbanization metrics. With higher human population density, probabilities of occurrence of above-ground nesters, generalist and small species increased. With higher soil sealing, probabilities of occurrence of above-ground nesters, generalists and social bees increased as well. Overall, these results, based on a large European dataset, suggest that urbanization can have negative impacts on wild bee diversity. They further identify some traits favored in urban environments, showing that several wild bee species can thrive in cities