The prevalence of four common species of bumblebees in the United States has dropped by 96% in the past few decades, according to the most comprehensive national census of the insects.
Scientists said the alarming decline, which could have devastating implications for the pollination of both wild and farmed plants, was likely to be a result of disease and inbreeding.
Bumblebees are important pollinators of plants and crops, thanks to their large body size, long tongues and high-frequency buzzing, which helps release pollen from flowers.
Bees in general pollinate about 90% of the world’s commercial plants, including most fruits, vegetables and nuts, and are the start of a food chain that also sustains wild birds and animals.
But the insects and other crucial pollinators, such as moths and hoverflies, have been in serious decline around the world since the last few decades of the 20th century. Scientists think it is from a combination of new diseases, changing habitats around cities and the increasing use of pesticides.
Sydney Cameron, an entomologist at the University of Illinois, led a team on a three-year study of the changing distribution, genetic diversity and pathogens in eight species of bumblebees in the US.
By comparing his results with those in museum records of bee populations, he showed that the relative abundance of four of the sampled species (Bombus occidentalis, B pennsylvanicus, B affinis and B terricola) had declined by up to 96% and that their geographic ranges had contracted by 23% to 87%, some within just the past two decades.
Cameron’s findings reflect similar studies across the world. According to the Centre for Ecology and Hydrology in the United Kingdom, three of the 25 British species of bumblebees are already extinct and half of the remainder have shown serious declines, often up to 70%, since the 1970s. Last year scientists inaugurated a £10-million programme, called the Insect Pollinators Initiative, to look at the reasons behind the devastation in the insect population.
Cameron’s team also showed that declining species of bees had higher infection levels of a pathogen called Nosema bombi and lower genetic diversity compared with the four species of bees that were not in decline — B bifarius, B vosnesenskii, B impatiens and B bimaculatus. The infection reduces the lifespan of bees and results in smaller colony sizes.
The reduction in genetic diversity seen in the declining bees means that they are less able to fight off new pathogens or resist pollution or predators.
‘Higher pathogen prevalence and reduced genetic diversity are, thus, realistic predictors of these alarming patterns of decline in North America, although cause and effect remain uncertain,” Cameron wrote this week in Proceedings of the National Academy of Sciences.
Insects such as bees, moths and hoverflies pollinate about a third of the crops grown worldwide. If all of the UK’s insect pollinators were wiped out, for example, the drop in crop production would cost the British economy up to £440-million a year, equivalent to about 13% of the UK’s income from farming.
The collapse in the global bee population is a major threat to crops. It is estimated that a third of everything we eat depends upon pollination by bees, which means they contribute about £26-billion to the global economy.
Other identified causes of bee decline include parasites, such as the bloodsucking varroa mite and viral and bacterial infections, pesticides and poor nutrition stemming from intensive farming methods.
‘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 bumblebee decline, in coordination with informed conservation strategies, will go a long way to mitigating further losses,” Cameron wrote. — Guardian News and Media 2011