Proceedings of the National Academy of Sciences
shows that wild bees may be disappearing in many important U.S. farmlands—including California’s central valley, the Midwest’s corn belt, and the Mississippi river valley. If losses of these crucial pollinators continue, the study indicates that farmers will face increasing costs—and that the problem may even destabilize the nation’s crop production.
The researchers created the new maps by first identifying 45 land-use types from two federal land databases, including both croplands and natural habitats. Then they gathered detailed input from 14 experts on bee ecology about each type of land, and how suitable it was for providing wild bees with nesting and food resources. Averaging the experts’ input and levels of certainty, the scientists built a bee habitat model that predicts the relative abundance of wild bees for every area of the contiguous United States, based on their quality for nesting and feeding from flowers. Finally, the team checked and validated the model against bee collections and field observations in many actual landscapes.
The research team estimates that wild bee abundance between 2008 and 2013 declined in 23% of the contiguous United States. The study also shows that 39% of U.S. croplands that depend on pollinators—from apple orchards to pumpkin patches—face a threatening mismatch between rising demand for pollination and a falling supply of wild bees.
The new study identifies 139 counties in key agricultural regions of California, the Pacific Northwest, the upper Midwest and Great Plains, west Texas, and the southern Mississippi river valley that have the most worrisome mismatch between falling wild bee supply and rising crop pollination demand. These counties tend to be places that grow specialty crops—like almonds, blueberries, and apples—that are highly dependent on pollinators. Or they are counties that grow less dependent crops—like soybeans, canola, and cotton—in very large quantities.
Pesticides, climate change, and diseases threaten wild bees, but the new study also shows that their decline may be caused by the conversion of bee habitat into cropland. In 11 key states where the new study shows bees in decline, the amount of land tilled to grow corn spiked by 200% in five years—replacing grasslands and pastures that once supported bee populations.