What is odd is that only certain species of bumble bee seem to be in decline while others are thriving

We have quantified dramatic rangewide
population declines in B. occidentalis, B. pensylvanicus,
B. affinis, and B. terricola that have occurred over the last few
decades. 

Although these
species have become rare or absent throughout large areas of
their historical ranges, co-occurring species, such as B. bifarius,
B. vosnesenskii, B. impatiens, and B. bimaculatus, remain relatively
abundant and widespread.

We show that declining populations
have significantly higher infection levels of the microsporidian
pathogen Nosema bombi and lower genetic diversity compared
with co-occurring populations of the stable (nondeclining) species.

Patterns of widespread decline in North American bumble bees
Sydney A. Cameron, et al
www.pnas.org

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