Changes in Community Composition and Functional Traits of Bumblebees in an Alpine Ecosystem Relate to Climate Warming

SIMPLE SUMMARY: Climate warming affects diversity, community composition, and spatial distribution of several plant and invertebrate species. Bumblebees in alpine ecosystems are particularly exposed to climate change due to even stronger warming compared to the global mean. To investigate the effects of climate warming, we sampled bumblebees along mountain slopes, compared the records to historical data from 1935 and 1936 and related our findings to climate models. We found that bumblebee species communities differed significantly between the two sampling periods. Our analyses showed that rising temperatures in the spring were the most plausible factor explaining this shift in community composition. Moreover, the recent bumblebee data showed significantly lower species diversity compared to the historical records. For example, the number of cuckoo bee species (socio-parasitic bumblebees that use the nests of other species for reproduction) was significantly lower compared to the historical data. Even though we did not detect more warmth-loving species, recent communities showed an increase in species that can deal well with variable climatic conditions. We conclude that the composition and functionality of bumblebee communities in the study area have been significantly affected by climate warming, with land use and vegetation changes likely playing an additional important role. ABSTRACT: Climate warming has been observed as the main cause of changes in diversity, community composition, and spatial distribution of different plant and invertebrate species. Due to even stronger warming compared to the global mean, bumblebees in alpine ecosystems are particularly exposed to these changes. To investigate the effects of climate warming, we sampled bumblebees along an elevational gradient, compared the records with data from 1935 and 1936, and related our results to climate models. We found that bumblebee community composition differed significantly between sampling periods and that increasing temperatures in spring were the most plausible factor explaining these range shifts. In addition, species diversity estimates were significantly lower compared to historical records. The number of socio-parasitic species was significantly higher in the historical communities, while recent communities showed increases in climate generalists and forest species at lower elevations. Nevertheless, no significant changes in community-weighted means of a species temperature index (STI) or the number of cold-adapted species were detected, likely due to the historical data resolution. We conclude that the composition and functionality of bumblebee communities in the study area have been significantly affected by climate warming, with changes in land use and vegetation cover likely playing an additional important role.