A decline in pollinating insects in India is resulting in reduced vegetable yields and could limit people`s access to a nutritional diet, a study warns.
Indian researchers said there was a "clear indication" that pollinator abundance was linked to productivity.
They added that the loss of the natural service could have a long-term impact on the farming sector, which accounts for almost a fifth of the nation`s GDP.
Globally, pollination is estimated to be worth £141bn ($224bn) each year.
The findings were presented at a recent British Ecological Society meeting, held at the University of Leeds.
Each year, India produces about 7.5 million tonnes of vegetables. This accounts for about 14% of the global total, making the nation second only to China in the world`s vegetable production league table.
Lack of data
Despite the concern, no study had been done to assess directly the scale of the decline in natural pollinators, explained Parthiba Basu, from the University of Calcutta`s Ecology Research Unit.
"The ideal situation would have been if we were able to compare the overall pollinator abundance over the years, but that kind of data was just not available," he told BBC News.
Instead, his team compared the yields of pollinator-dependent crops with pollinator-independent crops.
"Data shows that the yields of pollinator-independent crops have continued to increase," Dr Basu said. "On the other hand, pollinator-dependent crops have levelled off."
He explained that certain crops did not depend on insects for pollination, including cereals. Instead, the plants used other mechanism - such as relying on the wind to carry the pollen.
However, many vegetables - such as pumpkin, squash, cucumber and gherkin - were reliant on insects, such as bees.
He added that the fall in yield per hectare was against the backdrop of a greater area being turned over to crop production each year.
In an attempt to identify an underlying cause for the pollinator decline, the team is carrying out a series of field experiments, comparing conventional agriculture with "ecological farming".
Defined as "a farming system that aims to develop an integrated, humane, environmentally and economically sustainable agricultural production system", ecological farming is almost a hybrid of conventional and organic farming, looking to capitalise on returns from modern farming methods as well as drawing on natural ecological services, such as pollination.
Dr Basu said: "There is an obvious indication that within the ecological farming setting, there is pollinator abundance. This method typically provides the habitats for natural pollinators - this is the way forward."
He added that if the team`s findings were extrapolated, this would offer a "clear indication" that India was facing a decline in natural pollinators, as ecological farming was only practiced on about 10-20% of the country`s arable land.
Figures show that agriculture accounts for almost one-fifth of the nation`s gross domestic product (GDP), compared with the global average of just 6%. The sector also provides livelihoods for more than half of India`s 1.2 billion population.
The UN Food and Agriculture Organization (FAO) estimates that of the slightly more than 100 crop species that provide 90% of food supplies for 146 countries, 71 are bee-pollinated, primarily by wild bees, and a number of others are pollinated by other insects.
In order to gain a clear insight into the scale of the global problem, the FAO has established the International Pollinators Initiative, which includes a project involving seven nations (India is among them) with the aim of identifying practices and building capacity in the management of pollination services.
In a 2007 assessment of the scientific data on the issue, the UN Environment Programme observed: "Any loss in biodiversity is a matter of public concern, but losses of pollinating insects may be particularly troublesome because of the potential effects on plant reproduction and hence on food supply security."
Dr Basu said food security was unlikely to be the main consequence facing India.
"There has been a debate within India about this, but most of the cereal crops are not pollinator dependent, so if there is a pollination crisis it is not going to affect food security as such.
"What is going to be affected is nutritional security."
The concept of food security was first established by a FAO committee in 1983. Nutritional security was soon added as a key pillar to ensure "access by all people at all times to enough food for an active and healthy life".
Dr Basu said that vegetables such as pumpkins, squash, cucumber, and gherkins were "quite substantial" in terms of delivering necessary nutrients to the population.
"But there are many other vegetable crops that are eaten by people who are around the poverty level, so-called minor vegetable crops like eggplant, for which is there is no or very little data," he explained.
About a quarter of India`s population is believed to live below the poverty level, which - under the UN`s Millennium Development Goals - refers to people who live on less than US$1 a day.
In industrialised nations, such as the US and in Europe, many farms employ the services of commercial hives to pollinate fruit trees and food crops, and ensure they harvest adequate yields.
But Dr Basu said the use of domesticated bees in this context was not widespread in South Asia.
"There are honey farmers, but using hives in the field to pollinate crops is not at all common in India," he said.
"That is why a lot of the political noise about a global pollination crisis came from the US and Europe, because their managed/domesticated bee population was declining."
In 2007, about one third of the US domesticated bee population was wiped out as a result of a phenomenon known as Colony Collapse Disorder (CCD), with some commercial hive owners losing up to 90% of their bees.
The exact cause remains a mystery, and last year a number of UK agencies - including the Biotechnology and Biological Sciences Research Council (BBSRC) - began a £10m project to help identify the main threat to bees and other insect pollinators.
A number of possible causes have been suggested, including the misuse of pesticides, habitat loss and fragmentation, and the spread of parasites and diseases.
Dr Basu said that as a result of his team`s field experiments, it was clear that India too was experiencing a decline.
However, he cautioned: "There are many kinds of natural pollinators. As a result, we - not only in India, but in other parts of the world - do not really know what is happening to natural pollinator populations."