Pollinator decline is significant problem we are currently facing. Many studies have shown a drastic decrease of insects, including pollinators (Hallmann et al., 2017). Not only is this the loss of unique species and detrimental to ecosystem functioning, but pollination is also an ecosystem service important for production of many crops (Vanbergen et al., 2013). These insect declines are caused by a combination of factors such as use of pesticides, loss of food sources, climate change and habitat loss through land use change and urbanization.
Although urbanization contributes to the decline of insects, it cannot be seen as having only a negative impact on pollinator population. At first sight, cities may have little suitable habitat for pollinators, but an urban area contains a large diversity of habitats, some of which do allow for healthy pollinator populations. Habitats such as gardens, parks, road verges and other green spaces can supply pollinators with the necessary nectar and pollen sources needed to survive (Theodorou et al., 2020).
While a landscape with many floral resources is vital for pollinators to survive, it is not sufficient by itself. Places to nest are just as important. In many urban green spaces such as gardens and parks, so called or ‘bee hotels’ can be found, collections of hollow stems and holes drilled in wood for solitary bees and other insects to nest in. Cavity nesting bee species can use these structures to nest, however, only a minority of bee species actually nest this way. The majority of bee species are ground nesting species and need bare ground in which to dig their nests. The opening at the soil surface leads to a straight or branching tunnel connected to brood cells where eggs are laid with a storage of pollen for the developing larvae. Urban areas contain many impervious substrates like pavement, which can be problem for ground nesting species, including bees. (Antoine & Forrest, 2021)
Although impervious surfaces like roads, streets, parking spaces and other paved areas seem to be an impenetrable substrate for nesting bees, several ground nesting species of bees and wasps have been documented to be able to use these substrates for nesting. They burrow in the joints between pavement stones to find the loose soil underneath it. The sun heating up the pavement creates a warm microclimate underneath it, similar to natural breeding sites such as sun-lit bare soil.
For many natural systems, the bee communities are well researched. However, information on which bee species are nesting in the between pavement is limited, and only in the last few years attention has been given to this habitat. A study in Brussels (Noël et al., 2024) found 22 species of ground-nesting insects that use pavement substrates, half of which were bee species. Similarly, a study in Berlin (Weber et al., 2024) found 66 species, 28 of which were bees. While these nests are first used by the diggers of the nests, many other kleptoparasitic species make use of the pollen stored in the nests to feed their own larvae and parasitoids feed on the larvae themselves, resulting in a network of species interacting underneath our feet.
These studies show that pavement stones can be used as nest site, but not all pavement works as well as nesting site. Many factors affect whether a pavement joint is suitable, such as the width of the joint, whether the joint is bound and the quality of this bond, the soil underneath the pavement and the material of the pavement itself. With the limited amount of research on pavement nesting it is hard to say what an optimal habitat looks like, but research suggests that an unbound joint of at least 10mm on an unbound foundation works well. In the studied sites most foundations consisted of sand, which favored species specialized in sandy soils.
Pavement with open sandy joints is unfortunately not the only impervious surface used in an urban habitat. Increasingly, other types of surface hardening are used such as asphalt and concrete. Without the accessibility to the soil underneath the pavement, the use for ground nesting insects disappears. To ensure these urban pollinators persist in our cities, it is therefore vital that suitable pavement is used. Further study on the species using pavement nesting sites and highlighting the importance of insects to urban planners is necessary to conserve the buzzing life right beneath our feet.
Literature
Antoine, C. M., & Forrest, J. R. K. (2021). Nesting habitat of ground-nesting bees: a review. In Ecological Entomology (Vol. 46, Issue 2, pp. 143–159). Blackwell Publishing Ltd. https://doi.org/10.1111/een.12986
Hallmann, C. A., Sorg, M., Jongejans, E., Siepel, H., Hofland, N., Schwan, H., Stenmans, W., Müller, A., Sumser, H., Hörren, T., Goulson, D., & De Kroon, H. (2017). More than 75 percent decline over 27 years in total flying insect biomass in protected areas. PLoS ONE, 12(10). https://doi.org/10.1371/journal.pone.0185809
Noël, G., Van Keymeulen, V., Barbier, Y., Smets, S., Van Damme, O., Colinet, G., Lokatis, S., Ruelle, J., & Francis, F. (2024). Nest aggregations of wild bees and apoid wasps in urban pavements: A ‘street life’ to be promoted in urban planning. Insect Conservation and Diversity, 17(2), 396–408. https://doi.org/10.1111/icad.12689
Theodorou, P., Radzevičiūtė, R., Lentendu, G., Kahnt, B., Husemann, M., Bleidorn, C., Settele, J., Schweiger, O., Grosse, I., Wubet, T., Murray, T. E., & Paxton, R. J. (2020). Urban areas as hotspots for bees and pollination but not a panacea for all insects. Nature Communications, 11(1), 1–13. https://doi.org/10.1038/s41467-020-14496-6
Vanbergen, A. J., Garratt, M. P., Vanbergen, A. J., Baude, M., Biesmeijer, J. C., Britton, N. F., Brown, M. J. F., Brown, M., Bryden, J., Budge, G. E., Bull, J. C., Carvell, C., Challinor, A. J., Connolly, C. N., Evans, D. J., Feil, E. J., Garratt, M. P., Greco, M. K., Heard, M. S., … Wright, G. A. (2013). Threats to an ecosystem service: Pressures on pollinators. In Frontiers in Ecology and the Environment (Vol. 11, Issue 5, pp. 251–259). Wiley Blackwell. https://doi.org/10.1890/120126
Weber, C., Noël, G., Sickel, W., Monaghan, M. T., Bonn, A., & Lokatis, S. (2024). Urban pavements as a novel habitat for wild bees and other ground-nesting insects. Urban Ecosystems. https://doi.org/10.1007/s11252-024-01569-3
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