Four previously abundant species of bumblebee are close to disappearing in the United States, researchers reported Monday in a study confirming that the agriculturally important bees are being affected worldwide.

They documented a 96 percent decline in the numbers of the four species, and said their range had shrunk by as much as 87 percent. As with honeybees, a pathogen is partly involved, but the researchers also found evidence of inbreeding caused by habitat loss.

“We provide incontrovertible evidence that multiple Bombus species have experienced sharp population declines at the national level,” the researchers reported in the Proceedings of the National Academy of Sciences, calling the findings “alarming.”

“These are one of the most important pollinators of native plants,” Sydney Cameron of the University of Illinois, Urbana, who led the study, said in a telephone interview.

In recent years, experts have documented a disappearance of bees in what is widely called colony collapse disorder, blamed on many factors including parasites, fungi, stress, pesticides and viruses. But most studies have focused on honeybees.

Bumblebees are also important pollinators, Cameron said, but are far less studied. Bumblebees pollinate tomatoes, blueberries and cranberries, she noted.

“The 50 species (of bumblebees) in the United States are traditionally associated with prairies and with high alpine vegetations,” she added.

“Just as important — they land on a flower and they have this behavior called buzz pollination that enables them to cause pollen to fly off the flower.”