The first in-depth national study of wild bees in the U.S. has uncovered major losses in the relative abundance of several bumble bee species and declines in their geographic range since record-keeping began in the late 1800s.
The researchers report that declining bumble bee populations have lower genetic diversity than bumble bee species with healthy populations and are more likely to be infected with Nosema bombi, an intracellular parasite known to afflict some species of bumble bees in Europe.
The new study appears this week in the Proceedings of the National Academy of Sciences.
"We have 50 species of bumble bees in North America. We've studied eight of them and four of these are significantly in trouble," said University of Illinois entomology professor Sydney Cameron, who led the study. "They could potentially recover; some of them might. But we only studied eight. This could be the tip of the iceberg," she said.
The three-year study analyzed the geographic distribution and genetic diversity of eight species of bumble bees in the U.S., relying on historical records and repeated surveys of about 400 sites. The researchers compiled a database of more than 73,000 museum records and compared them with current sampling based on intensive national surveys of more than 16,000 specimens.
The national analysis found that the relative abundances of four of the eight species analyzed have declined by as much as 96 percent and that their surveyed geographic ranges have shrunk by 23 to 87 percent. Some of these contractions have occurred in the last two decades.
Researchers have many hypotheses about what is causing the declines, but none have been proven, Cameron said. Climate change appears to play a role in the declines in some bumble bee species in Europe, she said. Habitat loss may also contribute to the loss of some specialist species, she said. Low genetic diversity and high infection rates with the parasite pathogen are also prime suspects.
"Whether it's one of these or all of the above, we need to be aware of these declines," Cameron said. "It may be that the role that these four species play in pollinating plants could be taken up by other species of bumble bees. But if additional species begin to fall out due to things we're not aware of, we could be in trouble."
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