A third of British wild bees and hoverflies are in decline, according to a new study.
If current trends continue, some species will be lost from Britain, scientists say.
The study found "winners" and "losers" among hundreds of wild bees and hoverflies, which pollinate food crops and other plants.
Common species are gaining at the expense of rarer species, with a general picture of biodiversity being lost.
Scientists warn that the loss of nature can create problems in the coming years, including the ability to grow food.
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The study looked at trends in 353 wild bees and hoverflies in Scotland, England and Wales over 33 years from 1980.
A third of the species experienced declines in terms of areas where they were found, while about 10% became more abundant, including bees that pollinate flower crops, such as oilseed rape.
While some pollinations are carried out by bees in hives, much of the pollination of food crops and wild plants is performed by their wild relatives and other insects, especially hoverflies.
Dr. Gary Powney of the Center for Ecology and Hydrology (CEH) in Wallingford, Oxfordshire, said that while the increase in the main crop pollinators is "good news," the species generally declined.
"It would be risky to rely on this group to support long-term food security in our country," he said.
"If anything happens to them in the future, there will be fewer other species to develop and fulfill the essential role of crop pollination."
The losses were concentrated among the rarest and most specialized species. Dr. Nick Isaac, also from the Center for Ecology and Hydrology (CEH) in Wallingford, Oxfordshire, said that this is "particularly bad news if you are interested in wildlife and conservation."
The "losers" include solitary bees, which live in burrows on the ground, and mountain bees, living in mountains and heathlands. Among the "winners" are 22 of the most important crop pollinators.
Experts say that the increase of some common species is in the context of a general loss of diversity.
"Every square kilometer in the UK has lost an average of 11 species of bees and hoverfly between 1980 and 2013, according to the new analysis," said Lynn Dicks of the University of East Anglia.
She said the pattern of biodiversity loss is happening everywhere we look.
"It's a process of homogenization and leaves us with a natural world that is much poorer and less resilient to change."
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Matt Shardlow, executive director of the Buglife charity, said the new study provides further evidence that our pollinators are in trouble and that the health of our environment and the food supply can not be taken for granted.
"Lonely bees, bees and rare bees and hoverflies living in the highlands are particularly troublesome and need urgent help," he said.
The research is based on the analysis of more than 700,000 volunteer records that record the presence or absence of insects in your area.
This gives a measure whether the bands of insect pollinators are getting smaller or larger, however, it does not look to drivers for that change.
What is causing the decline?
Scientists believe that habitat loss is the key. Other possible factors include climate change – which may be having an impact on mountain and northern species.
They say that the relative success of some species may be due to environmental measures put in place by farmers, such as sowing strips of wild flowers.
Or this may be because much more rape is now grown than in 1980.
Another factor is the use of insecticides. The scientists said it remains to be seen how pollinators may have been affected by the restrictions introduced since the study was conducted.
In 2013, the European Union temporarily banned the widespread use of insecticides, known as neonicotinoids, in the light of evidence suggesting that they harmed bees.
Last year, banning three of the major types of neonicotinoids became permanent and was expanded to encompass all crops grown outdoors.
What can be done to help?
Researchers say actions such as wildlife-friendly agriculture and gardening can have a positive impact on pollinators in cities and in the countryside.
They repeated previous requests from gardeners to encourage insect pollinators by growing wild plant patches and weeds.
The loss of insects has far-reaching consequences for entire ecosystems. Insects provide a food source for many birds, amphibians, bats and reptiles, while plants depend on insects for pollination.
A recent scientific review of the number of insects worldwide suggested that 40 percent of the species were experiencing "dramatic rates of decline," with bees, ants and beetles disappearing eight times faster than mammals, birds or reptiles.
Previous studies have found losses of butterflies, moths, beetles, bees and hoverflies across the UK.
The study was published in the journal Nature Communications.
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