A quarter of the known bee species have not been sighted since 1990

Bees feed us. Many of the 20,000 species pollinate 85 percent of food crops and fruits around the world—everything from garlic and grapefruits to coffee and kale.

But, it seems, these crucial insects aren’t doing very well. A study published today in the journal One Earth reveals that in recent decades, the number of bee species reported in the wild has declined globally. The sharpest decrease occurred between 2006 and 2015, with roughly 25 percent fewer species spotted—even as sightings by citizen scientists were increasing rapidly.

Halictid bees—also called sweat bees for their attraction to our perspiration—pollinate important crops such as alfalfa, sunflowers, and cherries. Observations of these tiny metallic fliers have fallen by 17 percent since the 1990s, the study found. Bees in the rare Melittidae family, which provide us with blueberries, cranberries, and orchids, have plummeted by as much as 41 percent. (The world’s bees are divided among seven families.)

Though lesser known, such wild bees supplement the work of honeybees in managed hives.

“Even if honeybees can be efficient pollinators of many crops, heavy reliance on a single species is very risky,” says study leader Eduardo Zattara, a biologist at the Institute for Research on Biodiversity and the Environment, in Bariloche, Argentina.

For instance, during a disease outbreak in 2006, the U.S. lost about half its honeybees. Had only domesticated bees been present, “the yield loss would have been enormous,” Zattara says. (Read how dying bees spell trouble for U.S. agriculture.)

The study drew on an open-access website called the Global Biodiversity Information Facility, which contains bee-observation records sourced from museums, universities, and private citizens going back to the 1700s.

Most studies on bee diversity focus on a specific area or species, which is what inspired this broad analysis.

“There’s no long-term, very accurate, precise sampling of bees all over the world,” Zattara says. “We wanted to see if we could use this kind of data to get a more global answer, and the answer we got is, yes.”

Nevertheless, he cautions, the records that underpin the study don’t give us enough information to determine if certain species have gone extinct. “What we can say is that wild bees are not exactly thriving.”

Threats to bees

The analysis shows a decline in species sightings on all continents except Australia, where there’s a comparative dearth of data, Zattara says. Bees don’t inhabit Antarctica.

During the second half of the past century, a global agricultural boom led to habitat loss, while widespread use of pesticides killed off many plants bees rely on for food. Meanwhile, warming temperatures have forced bee species out of their native ranges or killed them outright. (Related: Bumblebees are going extinct in a time of “climate chaos.”)

Another cause of declines: When countries introduce non-native bees to pollinate particular agricultural crops, pathogens may come with them, “creating insect-style pandemics,” Zattara says.

He points to two European bumblebees brought into Chile and Argentina that have driven the Patagonian bumblebee—nicknamed the “flying mouse” on account of its size—to endangered status because of competition for its food and susceptibility to novel diseases.

Crunching bee data

To derive order from a staggering amount of data—there could be as many as 100,000 bee records per year—Zattara and his colleague, biologist Marcelo Aizen, of Argentina’s National University of Comahue, first divided the information by year. Then every species reported that year was counted.

Zattara says what mattered wasn’t how many individual bees were sighted in a year, but the frequencies of the species themselves. This approach helped reduce inconsistencies among countries—a much larger fraction of data comes from North America than, say, in Africa, so tallying up raw numbers of sightings could skew the results.

“Species that were more common would be almost always reported, while species that were harder to find would be more likely to be missing from a given year,” he says.

Then too with such a plethora of information going back hundreds of years, there’s the likelihood of errors and personal biases creeping in, such as when an observer who is looking for a specific bee may ignore other species, says Rachael Bonoan, an ecologist at Providence College, in Rhode Island. Bonoan specializes in insect pollinators and wasn’t involved in the research. (Read nine ways people can help pollinators at home.)

Even so, “the authors really did a good job dealing with possible biases,” Bonoan says.

Zattara acknowledges that, when wrangling information on 20,000 bee species, mistakes and errors can occur.

The rise of the citizen scientist

Given the overall picture of declines, he hopes scientists will make more research and collection data public, including contributions by citizen scientists to help fill knowledge gaps.

Indeed, during the coronavirus pandemic, many citizen science websites in the U.S. have shown more activity, particularly ones that track insects. “It’s really useful to have many eyes watching out for change,” Zattara says.

“We’ve definitely hit this point in time where people are starting to care about insects, which is fantastic,” Bonoan adds.

Sounding the call to “care about these really charismatic, useful insects,” she says, “can do nothing but good for the environment and other insect pollinators as well.”