Nothing signals the start of spring quite like the emergence of the first fuzzy bumblebees. But as Earth’s temperatures continue to warm, spring is coming earlier and earlier in some places—and scientists are worried about how these changes could affect bees.
A new study published Friday in the journal Ecology Letters suggests that earlier springs could be an indirect threat to certain bee populations.
It may sound counterintuitive—after all, earlier springs mean longer growing seasons for flowering plants, which should be a boon for the bees that rely on them for food. But the study suggests that longer springs also come with certain risks, such as frost events early in the season or droughts later in the summer. These events can cause flowers to decline at certain points throughout the season. That’s bad news for bees, which need constant, abundant resources to keep their colonies alive until winter.
The research has “pointed out a new mechanism that helps us look for reasons why climate change and bee declines could be linked, and I think that’s a really valuable contribution,” said Jeremy Kerr, a biodiversity expert at the University of Ottawa who was not involved with the study.
Indeed, previous research has suggested that climate change may already be influencing bee populations in a variety of ways. Kerr co-authored a major 2015 paper that suggested the ranges of bee populations in Europe and North America have been dramatically shrinking over the last century, likely due to the changing climate.
The new study focuses on a specific region in Colorado’s Rocky Mountains, where flowery meadows and bumblebees are both abundant. The research draws on more than 40 years of climate and flower data—a long-term project of David Inouye, a professor emeritus at the University of Maryland, College Park—and eight years of bumblebee monitoring, focused on three common bee species.
The long-term data suggest that the mountain snow may be melting slightly earlier than it used to, meaning the spring season is starting sooner and the flower season is lengthening. On the other hand, the abundance of flowers available in the meadows can fluctuate through the season, and the data also suggest there’s been an increase in the total number of days with low numbers of flowers available.
“Years that have a lot of days with low floral abundance seem to be years that have really low snowfall and early snowmelt,” said study co-author Rebecca Irwin of North Carolina State University. And there could be several reasons for this phenomenon, she added.
First, a longer growing season might be stretching flower resources a bit thinner than before—if the same number of flowers are spread out over more days, then some days are going to have lower flower densities than in the past. But earlier snowmelt may also be triggering climatic disturbances that can hurt the plants.
If flowers come out too early in the season, for instance, they may still be vulnerable to cold snaps or frosts. And if the snow melts and begins to run down the mountain too soon, freshwater resources may also begin to dwindle earlier in the summer, causing droughts.
All of these factors can lead to temporary troughs or dips in flower resources throughout the springtime—and these fluctuations can have consequences for bee populations. The past eight years of bumblebee monitoring, which tracked the number of bees buzzing about in the field, suggest that “low bee years are associated with years that have a lot of days of low floral abundance,” Irwin said.
The reason is likely tied to the bees’ unique life cycles, according to Jane Ogilvie of Florida State University, another of the study’s authors.
Over the winter, all the bumblebees in a colony die except for the queen bees, which hibernate through the cold season. At the start of spring, the queens emerge, lay their eggs, and spend the majority of their time foraging and bringing back food for their new families.
Eventually, the eggs hatch and a new colony is born. Throughout the rest of the warm season, the female offspring, or worker bees, take over the foraging responsibilities. Each day, they must bring back enough food to sustain the colony, which is why abundant floral resources are so important. Even a temporary dip in flower density can threaten a colony’s survival.
And it’s important for the colony, and particularly the queen, to survive all the way to the end of the season. Only then does the queen bee produce her “reproductive” offspring—male bees and new female queens, who will go out, find mates and ultimately create next year’s colonies. At the end of the season, the new queens must eat enough food and store up enough fat to make it through the winter.
“So you can see how food availability is really important over this whole time,” Ogilvie said.
Despite the fluctuations from year to year, the bumblebee populations seem to be more or less stable for now, the researchers note. And the longer growing season, in and of itself, could be seen as an advantage for bumblebees for the time being, said Nicole Miller-Struttmann, an ecology expert at Webster University who was not involved with the new research.
But the researchers worry that a continued trend in earlier springs—accompanied by more “low” floral resource days—could seriously hurt the bees in the long term.
“I think it is concerning that we are seeing these low floral resource days become more frequent through time,” Ogilvie said. “I think that it’s really critical that we understand very directly and mechanistically what effect that will have on bee populations. The data suggest that it might not be good.”
There are other questions about the timing of spring. While the new study focuses on bee abundance, it’s also worth considering how the season’s timing could affect the hibernating queens, said Leif Richardson, an ecologist and bee expert at the University of Vermont. (Richardson was not involved with the new research but was one of the Irwin’s former Ph.D. students.)
For instance, if the winter is too warm, hibernating queens may experience an increased metabolism rate and burn through their fat reserves too quickly, he suggested.
And Miller-Struttmann added that an important next stage for this line of research could be “trying to really understand what climate change means for the whole life history of these organisms”—not just for the workers’ abundance during the spring.
For the time being, it’s unclear whether the same concerns apply to other bumblebee species not included in this study, particularly species that live in warmer regions with less snow. According to Kerr, the University of Ottawa scientist, the new study’s findings don’t necessarily provide a “global explanation for why we have problems with bees with respect to climate change.”
Rather, they identify yet another specific pathway through which climate change can influence certain populations, he said.
The researchers plan to continue their monitoring efforts, and they may yield more insights about the bee populations’ long-term prognosis as time goes on. In the meantime, their findings may give yet another reason to keep a special eye on pollinators in the face of global warming.
“They do a lot of awfully careful work to uncover a mechanism that might give us some insight as to why climate change matters to these bees,” Kerr said.
Reprinted from Climatewire with permission from E&E News. E&E provides daily coverage of essential energy and environmental news at www.eenews.net.