This study demonstrates that stingless bees (Apidae, Hymenoptera) amount to approximately 70% of all bees foraging on flowers in the Brazilian Tropical Atlantic Rainforest. They also are the unique bee group concentrated in the upper stratum. It is hypothesized that this vertical distribution is a result of an uneven distribution of preferred floral resources within the forest strata. In the upper stratum, most of the trees that are highly visited by stingless bees produce small, inconspicuous, generalized flowers, clustered in dense inflorescences (mass flowering). These trees represent only 15% of the total melittophilous flora in the study area (96 plant species). In contrast, they attracted all 17 stingless bee species and more than 70% of all stingless bee individuals. Almost all these mass-flowering trees are hermaphroditic or monoecious, therefore the hypothesis that the stingless bees would be related to the abundance of dioecious trees in tropical rainforests was reexamined. It is proposed that small generalist stingless bees facilitate self-pollination and occasionally cross-pollinate these mass flowering trees. The tight relationship between stingless bees and mass-flowering trees is more properly in the center of a diffuse coevolutionary process, with the gradual replacement of other unpredictable, generalist and poor pollinators (e.g. small beetles) at the forest canopy.
stingless bees; forest canopy; foraging stratification; mass flowering; Atlantic forest
