bySydney A. Cameron, Jeffrey D. Lozier, James P. Strange, Jonathan B. Kochb , Nils Cordesa, Leellen F. Solter, and Terry L. Griswold
Bumble bees (Bombus) are vitally important pollinators of wild plants and agricultural crops worldwide. Fragmentary observations, however, have suggested population declines in several North American species. Despite rising concern over these observations in the United States, highlighted in a recent National Academy of Sciences report, a national assessment of the geographic scope and possible causal factors of bumble bee decline is lacking. Here, we report results of a 3-y interdisciplinary study of changing distributions, population genetic structure, and levels of pathogen infection
in bumble bee populations across the United States. We compare current and historical distributions of eight species, compiling a database of >73,000 museum records for comparison with data from intensive nationwide surveys of >16,000 specimens. We show that
the relative abundances of four species have declined by up to 96% and that their surveyed geographic ranges have contracted by 23– 87%, some within the last 20 y. We also show that declining populations have significantly higher infection levels of the microsporidian
pathogen Nosemabombi and lower genetic diversity compared with co-occurring populations of the stable (nondeclining) species. Higher pathogen prevalence and reduced genetic diversity are, thus, realistic predictors of these alarming patterns of decline in North America, although cause and effect remain uncertain.
The first author wishes me to point out that the bees have undergone reduction of genetic diversity (loss of genes because of shrinking populations), not inbreeding.
James C. Trager, Ph. D.
Biologist - Naturalist
Shaw Nature Reserve
P.O. Box 38
Gray Summit MO 63039
636-451-3512 ext. 6002
