Competition for flower resources and nest sites between Bombus terrestris (L.) and Japanese native bumblebees

The non-native bumblebee, Bombus terrestris, has recently become naturalized in Japan and is now dominant in some local communities. Here, we review previous studies reporting the effects of B. terrestris on native bees in both Japan and other countries. We also introduce our work to assess possible competitive exclusion by examining potential niche overlap, resource limitation for bumblebees, and the reproductive capacity of B. terrestris. Previous studies have reported the high niche overlap and some declines of native bees, indicating that competitive exclusion may be occurring. However, there is no indisputable evidence for this invader's ecological impacts on native species through competitive exclusion. Our studies showed that in terms of niche overlap for limited nest sites, two native species, B. hypocrita supporoensis and B. diversus tersatus, were most likely to be affected through competitive exclusion. Feral colonies of B. terrestris produced a mean of 90.2 gynes and showed high reproductive capacity, which would presumably allow the species to outcompete native bumblebees. Field surveys demonstrated that these two native species have declined accompanied by an increase in B. terrestris. The present study provides strong circumstantial evidence for competitive exclusion of native bumblebees owing to the establishment of B. terrestris.