Four U.S. species of bumblebees - an insect that plays an important role in crop pollination - have suffered a sharp decline in abundance and geographic range over the past few decades, a study says.
Their relative abundance, depending on the species, declined 88% to 96% in the last 20 to 30 years, and their geographic ranges have shrunk by 23% to 87%, according to the study led by University of Illinois entomology professor Sydney Cameron.
Though direct causes haven’t been confirmed, the study’s authors said declining bumblebee species are more likely than stable species to be infected with a certain parasite and are more likely to have lower genetic diversity.
The authors studied eight of North America’s 50 bumblebee species, and of those eight, "four ... are significantly in trouble," Cameron said in a University of Illinois press release. The study was published this week in the Proceedings of the National Academy of Sciences.
"They could potentially recover; some of them might. But we only studied eight. This could be the tip of the iceberg," Cameron said.
Populations of some bumblebee and honeybee species have reported to be on the decline in North America and elsewhere for years, but until now, no evidence for large-scale range reductions of bumblebees had been collected, the authors wrote.
"The wide-scale reductions in range and abundance of North American species, which also confirm earlier studies of decline at local levels, are striking and cause for concern," the authors wrote.
The researchers counted bumblebees of eight species at 382 U.S. sites from 2007 to 2009 and compared the count to historical data that was collected from 1900 to 1999 and held by natural history museums across the country.
One species that was once found throughout the eastern United States and northern Midwest saw its geographic range reduced by 87%, according to the study. One species with a historically broad presence in the West "was largely absent from the western portion of its range," and another historically wide-ranging species, while still abundant in the Gulf states and in the western Midwest, was "not observed across most of its historical northern and eastern range."
The declining species were more likely than the stable ones to be infected with the Nosema bombi parasite, which is known to afflict some bumblebee species in Europe, the news release said.
"But confirming a direct link between N. bombi and North American (bumblebee) decline will require further research," the authors wrote in the study.
Scientists previously have hypothesized that parasites, viruses, climate change and habitat loss could contribute to bumblebee decline.
Bumblebees' pollination style make them among the most efficient pollinators of certain crops, such as cranberries, blueberries and tomatoes. In Europe particularly, billions of dollars are invested in greenhouse bumblebee pollination of certain crops, especially tomatoes, Cameron said in a phone interview Tuesday.
Bumblebees also are important pollinators for wild plants in temerpate zones, particularly in mountainous areas, "because they can fly in lower temperatures when other bees can't fly - particularly in the early spring, the bumblebees are going to be the only bees that can fly that early," Cameron said.
Other causes aside, Cameron said loss and destruction of natural habitat such as prairies and forests "clearly is going to have an impact on all species of bees. She said the planting of more native flowers and shrubs, including on plots a small distance from crops, would be helpful to bees.
The research was published two weeks after a separate study proposed that viruses possibly causing the collapse of U.S. honeybee populations might be spreading from hive to hive through pollen, as reported by National Geographic.