Many U.S. bumble bee populations have declined significantly over the past few decades, with certain species dropping off by as much as 96 percent. While the decline is linked to low genetic diversity and disease, an underlying cause remains uncertain.
Scientists from multiple U.S. universities studied eight bumble bee species from across the country for three years, paying special attention to changes in their distributions, genetic diversity, and infection rates. Their report, published in the Proceedings of the National Academy of Sciences, is the first to comprehensively survey bumble bee populations in the U.S.
Bumble bees are important pollinators worldwide. Many wild ecosystems rely on bumble bees to ferry pollen from one flower to another. Their robust size, long tongues, and buzz-pollination method (high-frequency buzzing which encourages flowers to release their pollen), also make them highly efficient pollinators of crops.
Reports in Europe preceded those in the U.S. Of 19 species of true European bumble bees, three are locally extinct and eight are in serious decline, leaving only four widespread species in the entire region. Habitat loss, climate change, and pathogenic infection are thought to contribute to European declines.
The U.S. team, led by University of Illinois entomologist Sydney Cameron, compiled a database of more than 73,000 museum records and compared them to current population assessments determined through intensive surveys of more than 16,000 specimens at 400 sites. The study was the first national large-scale survey of bumble bee population health and found that four of the eight species studied had declined by as much as 96 percent, and that their ranges had shrunk by 23-87 percent.
“We have 50 species of bumble bees in North America. We’ve studied eight of them and four of these are significantly in trouble,” said Cameron. “They could potentially recover; some of them might. But we only studied eight. This could be the tip of the iceberg,” she said.
While a cause has yet to be confirmed, the presence of the bumble bee pathogen Nosema bombi in diminished populations supports previous hypotheses that escape ofN. bombi from captive pollination colonies is responsible for decimating wild populations. However, the paper cautions that more research is needed in order to determine whether the pathogen is capable of affecting a healthy population or simply taking advantage of ones already weakened by something else.
The researchers also found that the depleted populations exhibited low genetic diversity, increasing the risk of proliferation of harmful genes. The study states that low diversity could be alleviated by bees colonizing new areas and interacting with other populations, although this could also result in further spread of N. bombi.
Ultimately, more study is needed to figure out what is actually causing the declines, and how to fix it.
“Pollinator decline has become a worldwide issue, raising increasing concerns over impacts on global food production, stability of pollination services, and disruption of plant-pollinator networks,” Cameron told The Guardian. “In accordance with the goals of the United Nations convention on biological diversity to reduce the rate of species loss by 2010, such efforts to elucidate the causes and ecological impacts of bumble bee decline, in co-ordination with informed conservation strategies, will go a long way to mitigating further losses.”