Wild bees and hoverflies have suffered widespread losses in the UK, posing a potential future threat to agriculture, research has shown.
Between 1980 and 2013, a third of more than 300 species studied experienced population declines, while 11% became more abundant.
Even though 22 of the most important pollinators were among the species “winners”, scientists warned that the overall biodiversity loss could be storing up problems in years to come.
Dr Gary Powney, from the Centre for Ecology and Hydrology in Wallingford, Oxfordshire, said: “While the increase in key crop pollinators is good news, they are still a relatively small group of species. Therefore, with species having declined overall, it would be risky to rely on this group to support the long-term food security for our country.
“If anything happens to them in the future, there will be fewer other species to step up and fulfil the essential role of crop pollination.”
The value of pollinating insects to the UK economy has been estimated at £690 million per year.
While around 34% of pollination is carried out by honeybees, a scarcity of hives means crop farmers are highly reliant on their wild cousins and other insects, especially hoverflies.
The research is based on analysis of more than 715,000 observational records collected by volunteers between 1980 and 2013.
A total of 353 bee and hoverfly species, all known pollinators, were included in the study which focused on around 19,000 “cells” each covering a square kilometre of countryside.
The results, published in the journal Nature Communications, revealed a biodiversity trend equivalent to losing four bee and seven hoverfly species per cell over the study period.
On average, the geographic range of bees and hoverflies declined by about a quarter, with greater losses in the upland areas of northern Britain.
Environmental measures put in place by farmers, such as growing wild flowers in the margins of crop fields, were thought to have contributed to a 12% increase in dominant pollinators.
The scientists said it remained to be seen how pollinators might have been affected by insecticide restrictions brought in since the study was carried out.
In 2013 the European Union introduced a temporary ban on the widespread use of insecticides known as neonicotinoids in light of evidence suggesting they harmed bees.
Until then, the chemicals had routinely been used by crop farmers.
Last year, the ban on three of the main neonicotinoid types was widened to cover all crops grown outdoors and made permanent.
The researchers echoed previous calls by experts for garden owners to encourage pollinating insects by growing patches of wild plants and weeds.
Matt Shardlow, chief executive of the charity Buglife, said: “This new paper provides further evidence, were it needed, that our pollinators are in trouble and that the health of our environment and food supply cannot be taken for granted.
“Solitary bees, rare bees and bees and hoverflies that live in the uplands are in particular trouble and need urgent help.”
Here are three “losers” identified by scientists looking at pollinator biodiversity in the UK between 1980 and 2013.
– Bombus ruderarius, the red-shanked carder bee. Once widespread across much of England and Wales, the species has declined by an estimated 42%.
– Lasioglossum parvulum, the smooth-gastered furrow bee. A resident of southern Britain where it favours spring blackthorn flowers, its numbers have reduced by 40%.
– Panurgus banksianus, the large shaggy bee. The study suggests that this species, found in coastal regions of southern England and Wales, has declined by 54%.
– Andrena cineraria, the ashy mining bee. An important crop pollinator, especially for oil seed rape. Its abundance increased more than five-fold between 1980 and 2013.
– Colletes hederae, the ivy bee. A foreign visitor discovered in Dorset in 2001. The research suggests its numbers have increased by 16% per year since its arrival in mainland Britain.
– Lasioglossum pauxillum, the lobe-spurred furrow bee. Once rare, this species has shown a more than five-fold increase. It is now considered an important crop pollinator in England.
Environment scientist Dr Lynn Dicks, from the University of East Anglia, said: “This pattern of biodiversity loss is happening everywhere we look.
“Some common species increase, while many more species decline, and the decliners tend to be more specialised in their needs, associated with particular habitats or food sources, for example.
“It’s a process of homogenisation and leaves us with a natural world that is far poorer and less resilient to change.”
A Defra spokesperson said: “Insects are fundamental to the health of the natural world and the decline of these vital species on a global scale is deeply concerning.
“That’s why we are taking action to restore biodiversity and build on some of the successes recognised in this report. We are improving the status of pollinating insects and other species through our 25-year Environment Plan and National Pollinator Strategy. For example, we are restoring wildlife-rich habitats and supporting science-led restrictions on neonicotinoids to protect bees and other pollinators.
“It is critical that we act now to ensure that we leave our environment in a better state for future generations.”