We are witnessing the accelerated transformation of natural areas into cities. According to the United Nations, 55% of the world’s population lived in cities in 2018, a figure projected to rise to 68% by 2050. This ongoing transformation of our planet has led researchers from different fields to look at cities and ask how these environments affect ecosystem dynamics. This question, which might seem rather simple, is far from having a straightforward answer as creating a city implies too many changes, from replacing native vegetation with hard, impervious surfaces to building an infrastructure that can disrupt how wildlife moves and interacts. Moreover, these shifts pose a crucial question for us: how might our societies be affected by these changes in natural ecosystems?
In the last decades, researchers have made a tremendous effort to survey biodiversity in cities, especially groups directly related to human welfare, such as pollinators. For instance, a recent analysis of 446 studies from 255 cities worldwide showed that urban green areas host more than 3,100 pollinator species, suggesting that cities can host diverse pollinator communities. What remains unclear, however, is how these pollinator species interact with urban plants and how their pollination may be affected.
To contribute to this gap, a research team led by Prof. Pietro K. Maruyama aimed to investigate how urbanisation changed the interactions between plants and an emblematic group of birds from the American tropics: hummingbirds. This group of birds is known to interact with around 7000 species of plants, on which they are highly dependent for the nectar they need to sustain their fast and steady flight style. To understand how urban life affects hummingbirds, the team compiled data from 103 hummingbird-plant interaction networks across the Americas—spanning from Mexico to Southern Brazil. This massive dataset included 176 hummingbirds and 1,180 plant species, providing an unprecedented look at how these birds adapt to city environments.
Each interaction network consolidates information on the different plant species visited by each hummingbird (and vice versa) and how frequently these visits occur at a given site. Since both plants and hummingbirds typically interact with multiple species, each network forms a web of interactions whose structure helps researchers understand the dynamics of plant-pollinator relationships in that environment.
One of the key findings of this study is that urban interaction networks tended to be more nested but less specialised and modular. This means that urban hummingbirds fed from a broader range of plant species and shared more food sources, making their networks more cohesive and interconnected than those in natural environments. According to Maruyama and his colleagues, one possible explanation for these shifts is the high abundance of non-native plants in cities, which hummingbirds visited far more frequently than in natural habitats. For instance, non-native plants commonly found in urban environments tend to have flower characteristics that allow them to be visited by a broader range of hummingbird species. Moreover, non-native plants in cities often flower outside the peak blooming periods of native species and for extended periods, ensuring a continuous nectar supply. By providing easily accessible and abundant floral resources, these non-native plants may reduce the reliance of hummingbirds on specific native plants, ultimately leading to more generalised interactions.
A large hummingbird with short bill (Eupetonema macroura) visiting the flowers of Bauhinia blakeana, a non-native tree species widely planted in Brazil. Photo by Diogo Bueno Kanouté (Wikicommons).
Despite all these changes in network structure, robustness remained similar between natural and urban habitats, implying that networks in both areas are similarly resilient to losing a plant or pollinator species. One reason for this is the increase in network nestedness in urban areas because even the plants visited by specialist hummingbirds are also visited by generalists; the loss of one species does not dramatically alter the network structure. Another alternative is that non-native plants, disproportionately visited in urban networks, may act as redundant resources, buffering against native species loss.
However, a similar robustness between the networks of urban and natural areas cannot be taken as a sign that hummingbird communities are the same. For instance, urban areas host fewer hummingbird species, most of them with larger bodies, shorter bills and broader dietary breadth. These differences imply that cities filter out tiny specialist hummingbirds with long bills, making the plants with long, tubular flowers particularly vulnerable to reduced pollination visits. Notably, these groups of hummingbirds are typical of more closed vegetation, implying they do not adapt well to the simpler and open vegetation found in most urban areas.
Phaethornis striigularis —a small hummingbird with long bill— visiting a Golden Shrimp Plant (Pachystachys lutea). Photo by Bernard Dupont (Wikicommons).
As a result, the research by Maruyama and colleagues provides a detailed perspective into the effect of urbanisation on the interaction between hummingbirds and plants and the pollination carried by these birds. Notably, the study underscores the role of non-native plants in such interactions as hummingbirds become increasingly reliant on them. Regarding this, in an interview with Botany One, Maruyama said that this is indeed a delicate matter and that there is no “one-fits-all” answer. He explains that “we should carefully plan to replace these species with native plants as much as possible, but without neglecting their role in maintaining pollinators in urban areas. While it would be great if we could fill all the needs for urban greening by only using native flora, this is not always possible, and we cannot forget that non-native plants still offer other resources to wildlife, such as fruits and nidifcation places”.
The authors also highlight that plants with longer, tubular flowers might be more affected by urbanisation, as the hummingbird species capable of pollinating them are scarce in cities. According to Maruyama, “one strategy would be to promote structurally more complex vegetation in urban parks that can host specialist hummingbirds and to include more of the plants they usually visit”. This research highlights how cities are reshaping hummingbird-plant interactions, with non-native plants playing an outsized role. As urban areas expand, understanding these changes could help design greener, more pollinator-friendly cities—ensuring hummingbirds and the plants they depend on continue to thrive.
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Maruyama, P.K., Bosenbecker, C., Cardoso, J.C.F., Sonne, J., Ballarin, C.S., Souza, C.S., Leguizamón, J., Lopes, A.V., Maglianesi, M.A., Fernández Otárola, M. and Parra, J.L., 2024. Urban environments increase generalization of hummingbird–plant networks across climate gradients. Proceedings of the National Academy of Sciences, 121(48), p.e2322347121.
Carlos A. Ordóñez-Parra
Carlos (he/him) is a Colombian seed ecologist currently doing his PhD at Universidade Federal de Minas Gerais (Belo Horizonte, Brazil) and working as a Science Editor at Botany One and a Communications Officer at the International Society for Seed Science. You can follow him on Bluesky at @caordonezparra.
Cover picture: Chionomesa lactea. Photo by Dominic Sherony (Wikicommons).
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