Resource partitioning in bumblebees: the significance of differences in proboscis length.

The resource utilization by nine species of bumblebees in a subarctic community was studied by analysing the distribution of the depths of the corolla tubes of the visited flower species. When measured in this way, the food niches of the bumblebee species showed a wide overlap. Species with a long proboscis could use flowers with short corolla tubes for feeding, whereas the reverse was rare. We postulate that foraging efficiency is maximal when the proboscis length of a species corresponds to the corolla tube depth of the flower visited. An analysis of published data supports this hypothesis. The paucity of plant species with deep corolla tubes thus explains the small number and low abundance of bumblebee species with long proboscis. The continuously changing pattern of corolla tube depths due to the seasonal succession of plant species facilitates the coexistence of many bumblebee species in a community in spite of small differences in proboscis length.