Smithsonian.com

Several species of bumblebees have declined in recent decades (image courtesy of flickr user Vicki's Nature)

“The bees are disappearing.” It’s such a well-known fact that it even became a key plot point in season 4 of Doctor Who (with the explanation that the bees were aliens simply returning to their home planet). Most of the concern has centered on honeybees and the problem now known as colony collapse disorder. But there have also been worrisome reports from Europe of declining bumblebees, and a new study in PNAS finds widespread decline among North American bumblebee species as well.

A group of biologists from Illinois and Utah examined the current and historical distributions of eight species of bumblebees from the genus Bombus, looking at thousands of museum records and data from recent nationwide surveys. They found that the abundances of half of those species (B. affinis, B. occidentalis, B. pensylvanicus and B. terricola) have declined by up to 96 percent and their ranges have contracted by 23 to 87 percent in the last 20 years. The other four species, however, remain abundant and widespread.

Bumblebees, like honeybees, are important crop pollinators, especially for tasty foods such as tomatoes and berries. The researchers call the reduction in their range and abundance “striking and cause for concern.” As for what might be causing it, there’s evidence that the species in decline have higher levels of infection of the fungal parasite Nosema bombi and lower genetic diversity than the healthy bumblebee populations. But whether these patterns are related to the cause of the decline or simply another outcome of it is not yet known.