Bees are disappearing—that much is certain. What's unclear is why. Pathogens and pesticides have been posited as potential causes, as has the loss of bees' preferred floral resources. This last reason has intuitive appeal: wildflowers are disappearing because of agriculture, and bees rely on the pollen and nectar in flowers, so the loss of flowers should be causing the loss of bees.

But a demonstration of this seemingly simple idea has been hard to come by. Different species of bees rely on different plants—the bee species that are disappearing have never been analyzed in terms of taste for the plants that are disappearing to see if they match up. And, once the bees or plants are gone, it's hard to figure out what relationship (if any) they might have had. Pesky details.

Researchers in the Netherlands have gotten around this problem by examining museum specimens of bees to figure out which bees like which flowers. They've demonstrated that the bee species that have declined are in fact those that like the pollen from flower species that have also declined.

The Netherlands was an appropriate place for this work because, in addition to conveniently being where the researchers live, it is (according to them) "one of the most human-modified and intensively farmed countries in the world." As a result, bees in the country have lost eighty percent of their habitat over the course of the twentieth century.

Bees are pickier in choosing their source of pollen, which supplies them with protein and minerals, than in choosing their source of nectar, which provides them with energy. Thus, the ecologists who designed this study looked at the pollen loads in the bodies of bees collected before 1950—before agriculture intensified and bee populations plummeted. This is important because by now, when both bee and floral populations have been declining for decades, bee species may be reduced to eating from plants that are not really their faves.

The scientists concluded that loss of preferred host plants is one of the main factors responsible for wild bee decline, along with body size (bigger bees need more food and are more sensitive to its loss). The variety of the bees' diet did not play nearly as large a role. The researchers played with a variant on the chicken-and-egg idea; just because bees and their favorite flowers declined in tandem the loss of the flowers did not necessarily cause the loss of the bees, as the flowers rely on the bees for pollination. So perhaps the loss of bees caused a subsequent decline in the flowers. But circumstantial evidence—the fact that these plants can be pollinated by a number of different insects, and plants pollinated by water and wind have also decreased dramatically due to agriculture—suggests that the loss of the flowers led to a loss of the bees.

Based on these results, enhancing floral resources may be a good strategy for mitigating bee loss. Conservationists just need to figure out which bees they want to save, and they can increase the proper host plant species.

PNAS, 2014. DOI: 10.1073/pnas.1412973111 (About DOIs).