City bees under stress? Interactive effects of urbanization factors on bees and pollination

Cities represent a new frontier for biodiversity conservation and fundamental research on biodiversity–ecosystem function relationships. Urban habitats can maintain species-rich insect pollinator communities through the provision of diverse floral resources, but their interactions with flowers and resulting pollination services are at the same time threatened by urban stressors not encountered in their natural habitats, e.g. urban heat, high air pollution or heavy metals. Ecological theory predicts that pollinator communities with higher functional diversity and balanced distributions of traits should be more resilient to such stressors, thus maintaining pollination function even where species are lost. Yet, studies on the effect of urban stressors on pollinator communities, pollination networks and resulting service provisioning are still scarce. Likewise, little is known on how local habitat management to provision resources may mitigate larger-scale stressors. The objective of the CityBees project is to understand how urban environmental stressors impact wild bees, their interactions with plants, and pollination services in urban gardens. We examine how bee species functional traits, floral traits, bee–plant networks, bee provisioning and pollination services are impacted by urban stressors, including landscape imperviousness, extreme high temperatures, and air pollution.
Further, we ask how stressors are potentially mitigated by flora habitat management. To do so, we approach research questions from both the perspective of bees and the perspective of plants. We tackle our questions within 40 urban gardens in Berlin and Munich metropolitan regions using (i) empirical work to investigate the distribution of traits and bee–flower interactions, and (ii) experimental work to measure bee provision and crop pollination, both in relation to interacting urban stressors and management. We also harness (iii) social science methods to investigate drivers of human management decisions that may mitigate or exacerbate stressors on wild bees to either maintain or diminish services.

Responsible team member: Gaya ten Kate

In collaboration: with Prof. Dr. Monika Egerer (UPE).