Pollinator conservation with an island-wide approach: from community characterisation to demonstrating competition with honey bees

The global decline in biodiversity poses a significant challenge for conservation biologists, depicting an alarming picture for wild pollinators. Multiple anthropogenic pressures—including climate change, habitat fragmentation, pesticide use, and competition from invasive or managed species—have contributed to the worldwide decline of bee populations. However, knowledge gaps remain substantial, especially for wild bee species in under-studied regions. Expanding our understanding of pollinator diversity in these areas is crucial to guide effective conservation policies. Islands, due to their isolation, endemic species, and ecological vulnerability, offer unique opportunities to study biodiversity and conservation. However, their small, isolated populations are particularly susceptible to extinction, especially when faced with external stressors such as introduced species or human activities. Local extinctions on islands are virtually impossible to reverse, emphasizing the need for proactive conservation measures that account for species diversity and local features. Giannutri Island in the Tuscan Archipelago exemplifies such a fragile system, yet until recently, little was known about its pollinator communities. This research project filled these knowledge gaps by providing a comprehensive overview of the island's pollinators, evaluating population sizes of key species, and examining a pressing threats such as competition from managed honey bees. We characterised the island pollinator community providing a detailed checklist of three major pollinator groups: bees, hoverflies, and butterflies. We found six bee species new to Giannutri, one of which was previously unrecorded in the archipelago, 14 hoverfly species (three new to the archipelago and one new to Italy), and ten previously undocumented butterfly species significantly expanding the understanding of the island's biodiversity. A rare and valuable element of the island biodiversity was studied: Bombus xanthopus, a bumblebee species endemic to the Tuscan Archipelago and Corsica. We combined extensive samplings on all the island of the Archipelago, morphological analyses and DNA barcoding to assess its distribution and potential hybridization with Bombus terrestris. We highlighted the need to preserve the genetic integrity of B. xanthopus. Estimating the population sizes of wild bees was another crucial step. We achieved that by applying the Distance Sampling method, which accounts for varying detection probabilities. This method was validated against known honey bee hive populations, proving its accuracy and effectiveness. Population sizes for Anthophora dispar and Bombus terrestris were estimated, providing valuable data for long-term monitoring. Additionally, we evaluated the impact of managed honey bee colonies on wild bee populations. Honey bee hives were temporarily closed over three years, allowing researchers to observe changes in nectar availability, pollinator visitation rates, and foraging efficiency. Results showed that in the absence of honey bees, nectar availability increased, and wild bee foraging efficiency improved. Long-term monitoring revealed a declining trend in wild bee populations, suggesting that competition with honey bees may be a key factor. This integrated research underscores the importance of combining species surveys, genetic studies, quantitative methods, and experimental approaches to understand and preserve pollinator communities, particularly in vulnerable island ecosystems. The findings support the need for careful management of honey bee populations and provide transferable methodologies for pollinator conservation worldwide.