Northeastern bees have suffered population declines over the last century and a half, largely due to human encroachment, which has fragmented their environments. But none has faced a more devastating, rapid and recent collapse than the genus Bombus - the humble bumblebee - say entomologists in the Proceedings of the National Academy of Sciences online, March 5.
Combing through the Cornell University Insect Collection - with some 7 million insect specimens representing more than 200,000 species - and other collections, such as the American Museum of Natural History in New York City, Rutgers University and the University of Connecticut, entomologists were able to track patterns of bee abundance in eastern North America over 140 years.
"Our goal was to database all major bee collections from the northeastern United States - and to seek trends in relative abundance over time," said co-author Bryan Danforth, Cornell professor of entomology. "Indeed, there are declines among some bee species - host-plant specialists, in particular, are worse off than generalist bees, and several species in the genus Bombus appear to have declined rapidly in the most recent time periods.
"This study shows how important natural history collections are for documenting and studying long-term trends in plants and animals over time," he said.
Studying more than 30,000 museum specimens representing 438 bee species present in the Northeast from 1872 to 2011, the researchers found slight declines in the number of bee species over time. Also, they found that more than half of all bee species changed in relative size proportion over time. Species found in lower proportions now tend to have larger body sizes and more restricted diets, and are active fewer months of the year. Bees without these characteristics do not show evidence of decline.
The researchers found that three species in the genus Bombus suffered recent and rapid population collapses: the rusty-patched bumblebee, Bombus affinis, a crop pollinator for potato and apple; B. pensylvanicus; and B. ashtoni. Of special worry is the Macropis patellata, an oil bee, which had shown gradual, historical decline before 1950 and not been collected in the Northeast since 1991.
Regional species found at lower latitudes increased in abundance, possibly due to global climate change. The results, say the authors, could help guide conservation efforts aimed at protecting native bee populations and the pollination services they provide.
The research, "Historical Changes in Northeastern U.S. Bee Pollinators Related to Shared Ecological Traits," was also co-authored by Shannon Hedtke and Jason Gibbs, Cornell postdoctoral researchers, entomology; and John Ascher, Ph.D. '03 of the American Museum of Natural History; lead author Ignasi Bartomeus and senior author Rachael Winfree, both from Rutgers, conducted the data analysis. The National Science Foundation funded this research.