Urban bees: pollinator diversity and plant interactions in city green spaces

With the right mix of plants, urban green spaces can be a rich habitat to support diverse pollinators, according to a study published July 15, 2020 in the open-access journal PLOS ONE by Benjamin Daniels from RWTH Aachen University, Germany, and colleagues. 

With cities expanding and insect populations rapidly declining across the globe, incorporating insect-friendly green spaces into urban design is a critical step to bolster pollinator populations--intrinsically valuable in their own right as well as necessary for more than 75 percent of the world's food supply. In this study, Daniels and colleagues surveyed diverse green spaces across the German city of Aachen to assess plant-pollinator interactions within an urban environment. 

The authors categorized Aachen's green spaces into four categories: recreational parks; community gardens; the cemetery; and a "representative" park in the city center, with a rural reference site at Aachen's outskirts as a control--focusing on two different park elements within each study site, flower beds and insect-pollinating trees (specifically Tilia, or linden trees). Daniels and colleagues then surveyed these sites to observe visiting pollinators in thirty-minute intervals from 10AM-5PM in sunny, non-windy conditions from May to August 2016 (with the linden trees observed during their bloom period, from June 21-July 15)--recording 7723 total interactions between pollinators and plants. 

At the rural control site, the average number of observed pollinator visits was 46.2±14.6 within 30 minutes. Community gardens showed similar levels of visitation (43.7±23.0 per 30 minutes), but average insect visits for the representative park, recreational parks, and cemetery were significantly lower. The authors note that the composition of pollinators varied across park types as well, with community gardens and the rural site showing similar--and significantly higher--frequencies of wild bee and hoverfly visits. In contrast, the visitation rates of flowering linden trees showed similar frequencies for all tested park types (with an average ranging from 12.9 visits/30 minutes (cemetery site) to 16.4 visits/30 minutes (recreational parks). The linden trees also showed higher rates of bumblebee and honeybee visitors as compared to other pollinators.