Urbanization increases seasonal differences in plant-pollinator networks

Press release: Urbanization amplifies seasonal differences in plant-pollinator networks

Research team investigates importance of season and environment in tropical megacity

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Increasing urbanization worldwide is a growing threat to biodiversity. At the same time, flowering plants are often more diverse in cities than in the countryside. This is due to ornamental plants and crops that are increasingly being grown in cities. A recent study shows that the interactions between plants and pollinators, which are important for agricultural production, are surprisingly dynamic. For example, the plant and bee species involved in pollination vary greatly between the seasons. This was determined by an international research team led by the University of Göttingen through studies in vegetable farms in the southern Indian metropolis of Bengaluru - a prime example of rapidly growing cities in the tropics. Urbanization intensifies the seasonal differences in plant-pollinator networks, as a comparison of urban and rural cultivation areas revealed. The results have been published in the journal Ecology Letters .

In order to identify influences on the interactions between pollinators and plants, the researchers studied 36 vegetable farms in Bengaluru every month for a year. In this way, they covered the seasons that characterize the local climate: the mild-dry winter, the hot-dry summer and the rainy monsoon. The farms were distributed along two routes that ran from the city center to the rural villages. At each site, the researchers recorded the bee species present, the plant species visited by bees and the frequency of these interactions. From the data, they determined plant-pollinator networks for each location and each season. They analyzed which factors explain differences in the interactions: the time of year, the degree of urbanity of the environment (with the proportion of sealed surfaces as an indicator) or the spatial distance from the city center.

Differences in plant diversity, not bee diversity, appeared to significantly drive differences in interactions (i.e. interaction turnover or beta diversity of interactions) between plots. Compared to rural areas, urban areas were characterized by higher temperatures and a large dynamic in the composition of plant communities. In contrast, the bee species reacted very flexibly to the temporal and spatial changes in their nectar and pollen sources and thus showed fewer differences along the urban-rural gradient.

-Our study provides new insights into the role of urbanization in the dynamics of plant-pollinator networks in the understudied tropics. This is particularly important as current and future urban expansion is largely occurring in tropical regions, where it is subject to different ecological, climatic and social constraints than in temperate zones," explains first author Dr. Gabriel Marcacci, former PhD student in the Functional Agrobiodiversity group at the University of Göttingen and now a postdoc at the Swiss Ornithological Institute Sempach and the University of Neuchâtel. -Our results point to the major changes in plant-pollinator networks over the course of the year and to the often neglected importance of seasonality for the interactions between plants and their pollinators, especially in rapidly growing tropical megacities," emphasize co-authors Catrin Westphal and Teja Tscharntke from the University of Göttingen and Ingo Grass from the University of Hohenheim.

The research was carried out as part of an interdisciplinary DFG research group on changes in social-ecological systems along Indian urban-rural gradients. Further information on the research group can be found here: https://www.uni-kassel.de/fb11agrar/fachgebiete-/-einrichtungen/opats/for2432-1 .

Original publication: Gabriel Marcacci et al. (2023). Urbanization alters the spatiotemporal dynamics of plant-pollinator networks in a tropical megacity. Ecology Letters. DOI: 10.1111/ele.14324