Bees and the wild flowers they pollinate have declined significantly across large tracts of northern Europe, according to one of the most comprehensive surveys of pollinating insects.
Scientists examined hundreds of wildflower sites in Britain, the Netherlands and Germany and found the diversity of bees has fallen in 80 per cent of them over the past 25 years.
The researchers said wildflowers that rely on specialist species of bees for pollination have also declined, suggesting the two are caught up in a vicious cycle of decline.
The authors of the report, in the journal Science, believe intensive farming and pesticides, as well as the loss of wild habitats, may be helping cause the widespread fall in the diversity of bees and wildflowers.
"We were shocked by the decline in plants as well as bees," said Dr Koos Biesmeijer, a researcher at the University of Leeds. "If this pattern is replicated elsewhere, the pollinator services we take for granted could be at risk, and with it the future of the plants we enjoy in our countryside.
"Whatever the cause, the study provides a worrying suggestion that declines in some species may trigger a cascade of local extinctions amongst associated species."
Bees and other nectar-loving insects such as hoverflies are essential for pollinating many plants, including commercially important crops. Globally, the economic value of pollination is thought to be worth between £20bn and £50bn a year.
In Britain, where bee diversity has fallen and hoverfly numbers have held steady, the amount of wildflowers that require insects for pollination has declined by 70 per cent, but the number of wind-pollinated plants or those that self-pollinate have remained steady or increased.
The scientists found a slightly different pattern in the Netherlands where bees have, on average, declined but hoverfly diversity has increased. Here, there has been a decline in wildflowers that rely on bees for pollination, but not in other plant species that make use of other insect pollinators.
Raul Ohlemüller, a member of the research team at the University of York, said the difference between the two countries suggests there is a causal link between the decline of bees and plants. "The parallel declines of wildflowers and their pollinators seem too strong to be a coincidence," Dr Ohlemüller said.
Bill Kunin, co-ordinator of the project from Leeds University, said: "We looked at plant changes as an afterthought, and were surprised to see how strong the trends were. When we contacted our Dutch colleagues, we found they had begun spotting similar shifts in their wildflowers as well."
Another example of a bee in trouble is the field scabious bee, Andrena hattorfiana, which raises its young on the pollen of the field scabious flower. The study found both the bee and the flower have gone into decline in Britain and the Netherlands, which may be partly caused by grazing and early cutting of hay meadows which prevent the plant from flowering.
As the plant has fallen in numbers, this has had a knock-on effect on the bee, whose declining numbers may have added to the flower's problems of how to produce viable seeds.
Yet another example of bees in decline are the longhorn bees, which have all but disappeared in many parts of Britain and the Netherlands, possibly because of a parallel decline in the wild peas that were once cultivated widely as animal fodder.
Stuart Roberts, of the University of Reading, said: "Here in Britain, pollinator species that were relatively rare in the past have tended to become rarer still, and the commoner species have become even more plentiful. Even in insects, the rich get richer and the poor get poorer."
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