Key drivers of flowering phenology of alpine plant communities: exploring the contributions of climatic restriction and flower-visitor composition across geographic regions

Flowering phenology of alpine plant communities is determined by the interaction between abiotic and biological factors. Bees and flies are major pollinators in alpine ecosystems. The abundance of bumble bees consistently increases with seasonal progress reflecting the colony development cycle, while fly abundance fluctuates unpredictably. Responding to the seasonal dynamics of pollinators, flowering phenology of alpine communities is expected to vary between bee-visited and fly-visited plants within and among regions. We compared the relationship between flower-visitor composition and flowering phenology across geographic regions: fly-dominated New Zealand alpine, subtropical Taiwan alpine, mid-latitudinal alpines in central and northern Japan, and high-elevation Mongolian grassland. Thermal gradient was a fundamental factor regulating flower patterns across regions, and clear seasonality at higher latitudes created diverse flower patterns within communities. Floral abundance of fly-visited plants was less predictable with large variation, whereas that of bee-visited plants showed consistent patterns across regions reflecting the seasonality of bee activity. In New Zealand, most plants were linked to syrphid and/or non-syrphid flies. The network structures of the East Asian alpines were commonly constituted by syrphid flies, non-syrphid flies, and bumble bees, and these groups had specific niche width. In the Mongolian grassland, many insect groups formed diverse networks with small niche overlap. Overall, bumble bees are suggested to be a driver of diverse flowering phenology in alpine ecosystems. In contrast, flies may not be a powerful driver of flowering phenology. Pollination networks between bumble bees and alpine plants are expected to be sensitive to climate change.