> Little is known in North America about the habitat requirements for each species, and therefore designing species specific conservation programs is problematic. It may not be fruitful to search for a single reason for the decline of numerous species of bumblebees worldwide: reasons for the decline likely differ among species within a habitat and among bumblebee communities across the landscape. Nonetheless, given the importance of bumblebees as pollinators and as indicators of ecosystem health, it is essential that we obtain a better understanding of this subject. 

Evidence for decline in eastern North American bumblebees
Sheila R. Colla, Laurence Packer
Biodivers Conserv (2008) 17:1379–1391

[The subject of pollinator decline is too broad and complex to reduce to a few back and forth exchanges -- PLB]

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