Citizen science provides insights on pollination services in urban community gardens

Urban community gardens are collectively managed (agro)ecosystems shaped by diverse gardening practices that influence both planned and associated biodiversity. Yet, biodiversity-mediated ecosystem services in these gardens remain underexplored, and gardeners are rarely engaged as collaborators in scientific research. In this study, we investigated how bee diversity and urbanization factors affect pollination services, measured by the fruit set of common garden crops under real-world gardening conditions. Additionally, we examined participation dynamics within our citizen science approach and explored their relationship with the collected data as well as participants-reported challenges such as poor plant health. We collaborated with 73 gardeners in 22 gardens in Berlin and Munich, Germany, to measure the fruit set of 150 crops, mainly from the families Cucurbitaceae and Solanaceae. In parallel, researchers conducted systematic bee observations and quantified urbanization factors such as air temperature and surrounding imperviousness. We found a significant positive interaction between bee species richness and landscape imperviousness for the fruit set of Cucurbitaceae, emphasizing the importance of species-rich bee communities for pollination services in urban gardens. In contrast, no significant environmental predictors explained the fruit set of the Solanaceae group. Instead, we found that differences in sampling frequency led to significant differences in fruit set measurements, suggesting that self-pollination or an observer bias may contribute to variability. Participation types (differing in duration and frequency of data collection) were not affected by plant health issues or difficulties with the protocol, indicating that participant engagement in this real-world citizen science project was robust to practical challenges. Our findings highlight the potential of diverse bee communities to mitigate negative effects of urbanization on crop productivity in urban community gardens and demonstrate the value of citizen science for pollination research. Future citizen science approaches should balance scientific accuracy with participant autonomy, for instance, by coordinating crop selection or fostering a systematic recording of management practices. Key lessons learned include involving target groups early in project design and ensuring clear communication of data quality standards.