Wild bees provide crop pollination services worth more than $3,250 (2,880 euros) per hectare per year, a study reported Tuesday.

Their value to the food system is "in the billions, globally," its authors wrote in the journal Nature Communications.

Over three years, researchers followed the activities of nearly 74,000 bees from more than 780 species.

The team looked at 90 projects to monitor bee pollination at 1,394 crop fields around the world.

They found that on average, wild bees contribute $3,251 per hectare ($1,315 per acre) to crop production, ahead of managed honey bee colonies, which were worth $2,913 per hectare.

The probe adds to attempts to place a dollar figure on "ecosystem services" -- the natural resources that feed us -- to discourage environmental plundering.

Amazingly, two percent of wild bee species, the most common types, fertilise about 80 percent of bee-pollinated crops worldwide, the team found.

The rest, while crucial for the ecosystem, are less so for agriculture -- so conservationists may undermine their own argument by promoting a purely economic argument for the protection of bee biodiversity, the authors said.

"Rare and threatened species may play a less significant role economically than common species, but this does not mean their protection is less important," said David Kleijn, a professor at Wageningen University in the Netherlands, who led the study.

A healthy diversity of bee species is essential, given major fluctuations in populations, he added.

Honey bees in many parts of the world are suffering a catastrophic decline, variously blamed on pesticides, mites, viruses or fungus.

Last month, US watchdogs reported that US beekeepers lost 42 percent of their colonies from the previous year, a level deemed too high to be sustainable.

"This study shows us that wild bees provide enormous economic benefits, but reaffirms that the justification for protecting species cannot always be economic," said co-author Taylor Ricketts of the University of Vermont.

"We still have to agree that protecting biodiversity is the right thing to do."

- Busy bees -

According to the Food and Agricultural Organisation (FAO), about 80 per cent of flowering plant species are pollinated by insects, as well as by birds and bats.

At least one third of the world's agricultural crops depend on these unpaid workers, the UN agency says on its website.

Crops which require pollination include coffee, cocoa and many fruit and vegetable types.

The economic value of pollination was estimated in a 2005 study at 153 billion euros, accounting for 9.5 percent of farm production for human food.

Commentators not involved in the study said it may play an invaluable part of the campaign to save bees.

"Crucially, the commonest wild bees are the most important, which gives us the 'win-win' situation where relatively cheap and easy conservation measures can support these and give maximum benefit for the crops," said Pat Willmer, a professor of biology at Scotland's University of St Andrews.

"For example, planting wild flowers with wider grassy margins around crops, as well as less intensive or more organic farming, all enhance abundance of the key crop-visiting bees," he told Britain's Science Media Centre (SMC).