Dr George McGavin knew exactly how to bring his message home when he introduced the bee as a contender in a debate on “The World’s Most Invaluable Species.” He first reminded his audience of the news that Britain was running out of honey, and there would be none left by Christmas.

George McGavin, who knows a thing or two about insects, having devoted 25 years to entomology at the Oxford University Museum of Natural History, and who also starred in this summer’s BBC TV series Lost Land of the Jaguar, was one of five top scientists invited by the environmental charity Earthwatch to the Royal Geographical Society, London, to speak at their eighth annual public debate.

For the sake of the fun, yet informative event, the field was narrowed to five possibilities — bats, bees, fungi, plankton or primates.

Championing bats was Dr Kate Jones of the Zoological Society of London; Dr. McGavin made the case for bees; Professor Lynne Boddy, Cardiff School of Biosciences, for fungi; Professor David Thomas, School of Ocean Sciences, University of Bangor, plankton; and the Chairman of the Ape Alliance, Ian Redmond OBE, for our zoological next-of-kin, the non-human primates.

Each had to persuade the 650-strong voting audience that their ‘species’ was truly irreplaceable to humanity and the world as we know it.

Back to the bees. Not that honey was the real issue, George said. Nor wax for that matter.

These pale into insignificance compared to bees’ service to the ecosystem.

The real problem for a world without bees — and bee populations world-wide are in freefall, he told us — would be lack of pollination for the majority of our food crops.

“Bees are the major pollinators of plants. Grasses are wind pollinated, but the major food groups, the fruit, vegetables, nuts, and the drug plants, are all pollinated by bees.

“It’d be a drab world without them. And we’d be living on gruel.”

Nearly three-quarters of all the food we eat (not volume) is directly or indirectly dependent on bees. The economic value of the bees’ pollination services for the world’s main food plants was estimated in 2005 at about £130bn.

“Mass production of food, agricultural pesticides, diseases and loss of natural habitat are driving wild and managed bee populations into a steep decline. If this continues, the effects will be nothing short of catastrophic.” On top of that, there is the run of wet summers we have had in the UK and fewer people keeping bees. Bees came into Kate Jones’s talk too, or pretty nearly, for she introduced us to the bumblebee bat, the smallest mammal in the world, and then to the megabats, the giant flying foxes of Africa and Asia.

Kate, vice-chairman of The Bat Conservation Trust, invited us to imagine a world without her beloved bats. She took us away from the imagery of Dracula and Hallowe’en to the intricacies of their extraordinary design, their role as pollinators and dispersers of plants, and importance as biodiversity indicators.

The primates could have no better advocate than globetrotting conservationist Ian Redmond, who has spent more than 30 years working with mountain gorillas on research, filming, tourism and conservation.

He heads up many ape and wildlife organisations, and was the man who introduced Sir David Attenborough to the gorillas in 1978.

His credentials could not be stronger. Nor could his arguments for saving primates and the forest habitats on which they depend — on which the world depends, as a carbon store, source of clean water, and for climate regulation.

But despite primates’ ecological role as keystone species, and the use of heart-string-pulling pictures, the audience knew their vote must go elsewhere. Ian’s “gardeners of the forest” need urgent protection, yet the world could probably survive without them. The focus turned to the healthy functioning of the land and water systems of the world. Which matters most: fungi or plankton? Hypothetical of course, scientists fully acknowledge the interconnectedness of ecosystems.

Lynne Boddy thinks we should all love fungi. We could not exist without them.

Despite their poor reputation as poisoners, rotters of food and homes, causes of plant disease and human infections, very few fungi in fact cause a nuisance, she said.

They are the chief waste disposal agents and recyclers of the natural world.

Most terrestrial plants depend on fungi on their roots to get their nutrients and water from soil.

And they are hugely important as food for soil animals and in human food production, she said, whisking out a shopping bag full of cheese, bread, chocolate, soft drinks, beer and wine to prove it.

Fungi also produce ‘wonder drugs’, including penicillin and statins. And plankton? David Thomas had a killer argument here, you would think. He said: “If plankton were removed from the biosphere, none of the others would survive, or for more than a few years.” Plankton is a collective term for a myriad of bacteria, viruses, plant-like microbes and small animals that drift on winds and ocean currents. They are the base of the whole food web.

We find them in any body of water, from bird bath to farm pond, inland lake to ocean. We can see them from space.

“They not only fuel aquatic ecosystems, but some release gases that can influence cloud formation and therefore global climate,” David said. “They are vital to running Planet Earth.”

He added: “They are also beautiful to look at . . . an inspiration to artists and designers ever since the first microscopes were invented.”

It was a lively debate, Earthwatch’s best yet. The audience faced difficult choices.

But, who won the vote? Well, the bees did (plankton, runners up), the voters presumably persuaded by the immediacy of the bee problem.

George summarised his case: “A world without bees is unthinkable. But it’s happening now.

“In the next 50 years we’ll find out which of these species is truly irreplaceable.”

On a different note, almost a century ago the poet Rupert Brooke asked: “And is there honey still for tea?” When he wrote the line in 1912 it was rhetorical; nothing could be more certain than honey.

George hopes he is wrong about his 50-year prediction. Yet, unless urgent action is taken to safeguard the bee, we may have to answer “No”.

You can hear the full debate in a special broadcast on BBC Radio 4 on Christmas Eve at 8pm.

For information visit www.earthwatch.org/europe