Climate change is the biggest threat to wild bees

In a new study from Penn State, researchers have found that climate change reduces the abundance and diversity of wild bees. The experts report that wild bees are more affected by climate change than by disturbances to their habitats.

Furthermore, the research suggests that addressing land-use issues will not be enough to protect wild bees.

“Our study found that the most critical factor influencing wild bee abundance and species diversity was the weather, particularly temperature and precipitation,” said Professor Christina Grozinger. 

“In the Northeastern United States, past trends and future predictions show a changing climate with warmer winters, more intense precipitation in winter and spring, and longer growing seasons with higher maximum temperatures. In almost all of our analyses, these conditions were associated with lower abundance of wild bees, suggesting that climate change poses a significant threat to wild bee communities.”

According to Melanie Kammerer, few studies have considered the effects of both climate and land use on wild bees.

“We thought this was an oversight because, like many organisms, bees are experiencing habitat loss and climate change simultaneously,” said Kammerer. “By looking at both factors in the same study, we were able to compare the relative importance of these two stressors.”

The investigation was focused on a 14-year dataset from the United States Geological Survey that documented wild bee occurrences from more than 1,000 locations in Maryland, Delaware, and Washington, D.C. 

Based on this data, the researchers examined how different bee species respond to land-use and climate changes.

“To really understand the effects of weather and climate, particularly as weather patterns become more variable with climate change, we need to use these very large, long-term data sets,” said Grozinger. “We hope that our study, and others like it, will help encourage the collection and integration of these data sets for future research.”

The team used land cover maps and spatial models to analyze the landscape surrounding each of the sampling locations. The experts also used machine learning to identify the most important climate variables that affect wild bees. 

“We found that temperature and precipitation patterns are very important drivers of wild bee communities in our study, more important than the amount of suitable habitat or floral and nesting resources in the landscape,” said Kammerer.

According to Grozinger, different bee species were most impacted by different weather conditions. For example, areas with more rain had fewer spring bees, she explained.

“We think the rain limits the ability of spring bees to collect food for their offspring,” said Grozinger. “Similarly, a very hot summer, which might reduce flowering plants, was associated with fewer summer bees the next year.”

“This result coincides with studies showing that, with earlier spring onset, overwintering adults had higher pre-emergence weight loss and mortality and shorter life span post-emergence.”

Kammerer noted that these weather changes will likely worsen in the coming years.

“In the future, warm winters and long, hot summers are predicted to occur more frequently, which we expect will be a serious challenge to wild-bee populations,” said Kammerer. 

“We are just beginning to understand the many ways that climate influences bees, but in order to conserve these essential pollinators, we need to figure out when, where and how changing climate disrupts bee life cycles, and we need to move from considering single stressors to quantifying multiple, potentially interacting pressures on wild-bee communities.”

The study is published in the journal Global Change Biology.

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By Chrissy Sexton, Earth.com Staff Writer