Wild bee populations have declined significantly since 2008 in Central California and some other key areas of the United States, according to a newly published study co-authored by UC Davis researchers.

The new study suggests that wild bee populations likely declined in areas comprising 23 percent of the nation between 2008 and 2013, a decline associated with conversion of natural wild bee habitat into intensive agriculture.

The paper, titled “Modeling the Status, Trends and Impacts of Wild Bee Abundance in the United States,” will be published online Dec. 22 in the Proceedings of the Natural Academy of Science.

This was the first-ever attempt to simultaneously map the trends in wild bee abundance based on recent land conversion and the demand for pollination of different crops across the nation.

The seven-member team, organized by lead author Insu Koh of the Gund Institute for Ecological Economics at the University of Vermont, Burlington, integrated a wild bee habitat model, land cover data and expert knowledge to map U.S. bee abundance and trends.

Wild bees and pollination demands

“We see striking mismatches in many places between the demand for pollination and the ability of wild pollinators to support that need,” said pollination ecologist Neal Williams, an associate professor in the UC Davis Department of Entomology and Nematology. Williams helped design the study and led efforts to assess bee habitat quality as part of the Integrated Crop Pollination Project.

“Indeed it is crops where demand has most increased that we estimate greatest decline in wild pollinator supply,” Williams said.

“The research is also unique in including uncertainty in our knowledge of the quality of habitat for pollinators and thus recognizes where more effort is needed to understand the vulnerability of pollination services,” he added.

Williams noted that the paper has the potential to bring wider attention to the correlation between the status of wild bee communities and crop pollination demands nationally.

Mismatch in pollination supply and demand

The researchers determined that 139 key counties, comprising 39 percent of U.S. pollinator-dependent crop area, exhibit a “mismatch between pollination supply and demand,” with large areas of pollinator-dependent crops and low expected abundance of wild bees.

The authors noted that the supply of managed honeybees has not kept pace with pollination demand and needs, due to management challenges and colony loss over the last decade. “There is growing evidence that wild, unmanaged bees can provide effective pollination services where sufficient habitat exists to support their populations,” they wrote.

Some of the most important crop pollinators are bumblebees, which have declined over the past decades. “Our mapped index of bee abundance clearly shows that areas of intense agriculture (e.g. the Midwest Corn Belt and California’s Central Valley) are among the lowest in predicted wild bee abundance,” the authors pointed out.

Helping agriculture preserve wild pollinators

The authors agreed that the study results may help farmers and agricultural stakeholders develop management efforts to preserve wild bee populations and their pollination services in farmland. Just like honeybees, wild bees are important pollinators and face many of the same threats including habitat loss, environmental pesticide exposure, and climate change.

In addition, the results may help wild bee researchers focus attention on poorly understood regions, the authors said.

Responding to White House call

The study comes on the heels of a 2014 U.S. presidential memorandum calling for a national assessment of pollinators to promote the health of honeybees and other pollinators. One goal of that federal strategy is to set aside 7 million acres over a five-year period for pollinators.

“The study’s new assessment is highly valuable in response to these calls at the federal level to direct research and management efforts to wild pollinators,” Williams said. “I really hope the paper will generate additional original efforts to improve our understanding of the challenges facing pollinators and their critical role in sustainable food supply.”

Collaborators and funding

In addition to Koh and Williams, the team included Claire Brittain of the Neal Williams lab, UC Davis Department of Entomology and Nematology; Eric Lonsdorf of Franklin and Marshall College, Lancaster, Pennsylvania; Rufus Isaacs and Jason Gibbs of Michigan State University, East Lansing; and Taylor Ricketts of the Gund Institute and the University of Vermont.

The study was completed with support from the Specialty Crop Research Initiative of the U.S. Department of Agriculture’s National Institute for Food and Agriculture.