In the alpine zone, bumblebees (genus Bombus) provide essential and effective pollination service to angiosperms due to their floral constancy, high visitation frequency, and cold tolerance. As the flowering plant community changes with the growing season, bumblebees respond by foraging on available floral resources. The foraging behavior of bumblebees is influenced by morphological matching between the bumblebee and the flowers it visits, such as tongue length and corolla length. Intra- and interspecific competition for limited floral resources also occur among bumblebees. In this study, we investigate floral resource sharing between alpine bumblebee species with respect to morphological matching, and seasonal shifts in floral resource and bumblebee abundance.

This study was conducted at around 10-day intervals throughout the flowering season in 2020–2024, focusing on three dominant bumblebee species: B. beaticola, B. hypocrita, and B. yezoensis. We surveyed bumblebee visitations to alpine plants along the hiking trails at two sites (Akadake and Kurodake) in Daisetsuzan National Park. Flowering alpine species diversity and floral abundance were measured by setting 1 m² quadrats in flowering patches and counting the number of flowers per plant species within the quadrat.

Alpine plants displayed sequential flowering at both sites, with floral abundance and species diversity peaking mid-season. Proportions of bumblebee abundance varied between sites and across years, which suggests that bumblebee abundance is influenced by species-specific factors. B. hypocrita increased its foraging diversity when conspecific bee abundance is high, reflecting the generalist nature of short-tongued bumblebees. Increases in floral diversity is correlated with increased B. hypocrita foraging diversity at Akadake but not at Kurodake, which may be explained by the lower plant species diversity and the abundant Cirsium kamtschaticum at Kurodake. In contrast, B. beaticola and B. yezoensis did not change their foraging diversity in response to increasing floral diversity, suggesting that the two species exhibit specialist foraging behavior.

This study shows that short-tongued B. hypocrita is a generalist which flexibly changes its foraging habits in response to the seasonal progress of floral diversity, while long-tongued B. beaticola and B. yezoensis are more specialized floral visitors that focus their foraging efforts on Cirsium and Phyllodoce species.