Phenological mismatch between alpine flowers and bumble bees: its mechanism and impacts on the population dynamics of bumble bees.

To determine the impacts of global warming on pollinator-plant interactions, we recorded phenological variations in alpine flowers and bumble bees during 10-12 years in northern Japan, and analyzed the effects of weather conditions and phenological shift on worker population dynamics of four Bombus species. Flowering patterns of alpine plants were formed by the combination of early-flowering fellfield and late-flowering snowbed communities, where snowbed flowers were important resources for worker bees. The flowering phenology of the fellfield communities was correlated with early season air temperature, whereas that of the snowbed communities was clearly predicted by snowmelt time. It was predicted that 1 °C warming with 10 days earlier snowmelt would advance the peak flowering time of the fellfield and snowbed communities by 3.6 and 9.5 days, respectively, resulting in a 9.2 day shorter flowering period. In contrast, the peak time of worker abundance was consistent between years, independent of temperature and snowmelt time. As a result, the time lag between the peak flowering of snowbed plants and the peak abundance of worker bees, i.e., the phenological mismatch, increased with earlier snowmelt. Mid-summer temperature was negatively correlated with worker abundance for three Bombus species, and the effect of phenological mismatch varied between Bombus species. The abundance of B. hypocrita decreased with increasing phenological mismatch in the previous year, the abundance of B. beaticola and B. yezoensis increased, and B. hypnorum showed no clear response. Therefore, changes in the phenology of alpine plants due to global warming affect pollinators in highly species-specific ways.