Scientists Investigate What Makes Urban Gardens Buzz With Bees

A common question is what is the best plant for pollinators, but that might be the wrong approach. A new study published in Urban Ecosystems reveals how the diversity and abundance of flowers in urban gardens can shape wild bee communities. Bruno Rossi Rotondi and colleagues surveyed bees and blooms in 13 gardens across the city of Córdoba, Argentina, to uncover what garden features are most important for supporting bee biodiversity. Their findings highlight how urban green spaces like gardens can aid pollinator conservation, even in densely populated cities.

The scientists found that floral evenness is crucial for supporting bees in urban gardens. Floral evenness refers to how evenly distributed the abundances of different flower species are within a plant community. Gardens with a more evenly distributed mix of blooming plants, rather than simply having more species or a bigger area, hosted more diverse and abundant bee communities. They attracted more bee species and a wider range of bee traits related to pollination. Specifically, the evenness of flower species was the strongest driver of bee abundance, diversity, and functional traits compared to other garden features.

Surprisingly, simply having more flower species or a bigger garden area didn’t boost bee diversity. The team also found no major impact from nearby green cover or from competition with introduced honeybees.

Wild bees play a vital role as pollinators, but face declines globally from urbanisation and other threats. Urban gardens present a refuge, but how garden features influence bees is understudied in Latin America. Using field surveys, the researchers recorded bee-flower interactions across one year in Córdoba while measuring garden traits like area and plant diversity. They also characterised key bee and flower traits related to foraging and accessibility.

The results showed the evenness of flower species was the strongest driver of bee abundance, species diversity, and functional trait diversity. Bees benefited most from gardens with flower abundances evenly spread among species, indicating a diversity of pollen sources and fewer ultra-dominant plants. These findings provide practical steps gardeners can take to support wild pollinators through strategic plant choices. The study demonstrates that thoughtfully designed greenspaces like gardens can sustain diverse bee communities, even in bustling cities. Rossi Rotondi and colleagues write:

We showed that urban gardens can support taxonomic and functionally diverse bee communities, especially those gardens with plant assemblages with evenly distributed flower species. These results highlight the impact of the owner´s decisions on bee communities when selecting plant species and their relative abundance to be cultivated in their gardens. Thus, our study indicates that planting several flowering plant species with even abundances will have a higher impact on attracting a diverse bee community than planting the same individual number of just one species, as seen previously (Braatz et al. 2021), or the same number of species with few dominant ones.

Rotondi et al. 2023

It would seem that if you want to help support biodiversity in your garden, the best place to start is to have biodiversity in your planting.

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Rotondi, B.A.R., Casanelles-Abella, J., Fontana, S., Moretti, M., Videla, M. and Fenoglio, M.S. (2023) “Floral species evenness is the major driver of wild bee communities in urban gardens,” Urban Ecosystems. Available at: https://doi.org/10.1007/s11252-023-01440-x.

Cover: Sarmiento Park Stairs viewpoint (Escaleras) – Cordoba, Argentina. Image, Canva.