Wild bees pollinate many crops, but some bees are busier than others.
On average, only 2% of wild bee species were responsible for 80% of the pollination visits witnessed by researchers around the world, according to a study published Tuesday in the journal Nature Communications.
"This study puts a spotlight on how few species actually do all the work," said Mace Vaughan, co-director of the Pollinator Program at the Xerces Society, a nonprofit devoted to protecting invertebrates and their habitats.
David Kleijn, an ecologist at Wageningen University in the Netherlands, had an inkling that something like the 80-20 rule might be at work with wild bees. As he was studying the insects in farm fields in the Netherlands and in Southern Italy five years ago, he noticed something striking.
"Different crops, different countries and climatic regions, 1,500 kilometers apart, and yet the bees pollinating these crops were mostly the same," he said.
Could this be part of a global trend? To find out, Kleijn gathered a team of 58 researchers who had studied bees on five continents. Together, they had observed and identified nearly 74,000 individual wild bees foraging for pollen across 1,394 farm fields on agricultural crops like apples, cranberries and coffee.
About half the pollination work the researchers witnessed — as measured by economic value — was done by commercial bees, Apis mellifera, that beekeepers manage to pollinate crops and produce honey.
The rest of the work was done by wild bees. But in every site, they found that fewer than eight wild bee species focused on crop pollination, according to Kleijn. All the other types of wild bees foraged on wild plants.
Among the wild bees, the common Eastern bumblebee (Bombus impatiens) made the greatest contribution to crops, supplying pollination services valued at $390 per acre, on average, in the places where it was found.
Worldwide, both wild and domesticated bees each contribute an average of about $1,215 in pollination services per acre of crops, the study found, though this figure varies drastically from place to place and crop to crop.
In recent years, the economic value of pollination has been a prominent argument for conserving bees.
One implication of this research, study authors said, is that efforts to protect wild bees can't rest on their economic value alone. If that were the case, 98% of bee species would be deemed unworthy of protection.
Over 20,000 wild bee species exist worldwide, including many that are rare or threatened. Ranging from jet black and hairy to metallic green and shiny, wild bees tend to live alone in holes in the ground, in hollow reeds or in tunnels they bore into wood. California alone hosts about 1,600 species of wild bees, said Leithen M'Gonigle, a bee researcher at Florida State University.
Kleijn's research team also found that farmers can more than triple the likelihood that wild bees will pollinate their crops by planting hedgerows of wildflowers and keeping strips of wild grass around the edges of their fields.
Even the wild bee species that do not pollinate agricultural crops have a vital ecological role, said Vaughan of the Xerces Society, who was not involved with the study. The plants they pollinate provide food for birds and other wildlife, for instance.
The study authors also noted that different species of bees may become more important to crop pollination in the future under different climatic conditions.
Whether they save farmers money or not, wild bees have intrinsic value, Kleijn said.
"They are spectacularly beautiful and fascinating creatures," he said. "I want my grandkids to be able to see them."