What is the most valuable species on earth?

We could all name our favourite species, whether endearing, exciting or indeed endangered.

But what of the less well-known species we take for granted but whose loss would have an immeasurably greater impact on our world?

This was the premise of a recent debate hosted by the charity Earthwatch. Five experts battled it out to prove that their chosen species was the most invaluable to life on earth.

The chosen species were: bats, bees, fungi, plankton and primates. You can read and hear audio clips of the experts' main arguments below. Then have your say on the message board at the end.

Bats

While rarely seen by humans due to their nocturnal habits, bats play a key role in the ecosystem, according to Dr Kate Jones of the Zoological Society of London.

"They provide a number of ecosystem functions including pollination and seed dispersing and they also regulate and control insect populations," she said.

Recent research in the US estimated that one-third of the value of the main crops grown there would have to be spent on pesticides if it were not for the pest control undertaken by bats.

Unlike most species, they are found across the globe, from parks to jungles. In fact, everywhere apart from the Poles. Contrary to what you might have read, only three species of bat drink blood and they all live in South America and not Transylvania.

Bees

Like bats, bees play a crucial role in the ecosystem, according to Dr George McGavin from Oxford University Museum of Natural History.

They pollinate a third of what we eat, from apples and raspberries to barley and runner beans as well as a vast array of wild plants.

In fact, it is estimated that 75-80% of the world's major crops are dependent on bees for pollination. A study in 2005 put the economic value of pollination services provided by bees for the world's main crops at £130 billion.

"If you take away bees, you've got no flowering plants, fruit or vegetables and then there is also a knock-on effect on the species that feed on those, including humans," said Dr McGavin.

There are 20,000 species of bees in the world with 265 different species in the UK.

Fungi

Food, wonder drugs and beer - fungi have a surprisingly important usefulness in our everyday life. Bread, beer, chocolate and cheese to name just a few are all made using fungi.

"More importantly they are also crucial to the running of the planet," said Professor Lynne Boddy from Cardiff School of Bioscience. "More than 90 per cent of plants live with fungi which helps them take up water and nutrients from the soil," she added.

Fungi's usefulness does not stop there. They also help animals digest food and gave us penicillin, one the major breakthrough drugs of the 20th century, which saves millions of lives every year.

Plankton

Coming from the Greek word 'planktos' meaning drifters of the ocean and lakes, plankton is basically any organism that drifts with the current.

It covers a huge range of organisms from bacteria to jellyfish.

As well as being critical to the ocean food web (for example, whales eat 1,000kg of them every day) they also play an important role in regulating the amount of gas in our atmosphere by absorbing carbon dioxide and producing oxygen.

"Land-based plants do produce oxygen but they are only half the equation," said Professor David Thomas, from the University of Bangor. "Carbon dioxide levels are in a very tightly knit balance and if you take out half of the equation the whole thing crashes.

"If plankton were removed from the biosphere all other species would die out within a few years," he added.

Primates

As well as being our closest relatives on planet earth and fascinating to study and observe, primates also play an important role in preserving their rainforest habitat.

"They are key species in the tropical and sub-tropical forests. As global warming proceeds with increasing speed every year more and more people are realising that these forest are not just nice decorations on the earth," said Ian Redmond, chairman of the Ape Alliance. "They are part of the biosphere that we depend upon.

"The primates disperse the seeds, prune the trees and are part of the living diversity of the forest and we need to save primates for the forests just as much as we need to save the forests for primates," he added.