by Richard Gray

Nearly 17,000 species are now considered to be threatened with extinction and 869 species are classed as extinct or extinct in the wild on the International Union for Conservation of Nature's Red List. In the last year alone 183 species became more endangered.

Now, in the face of the growing threat posed by environmental changes around the globe, five leading scientists are to argue whether there is a single type of plant or animal which the planet really cannot afford to lose.

The debate, titled Irreplaceable - The World's Most Invaluable Species, will see five experts present the case for the world's most important animals and plants from a shortlist of five: primates, bats, bees, fungi and plankton.

Primates, which are among the most threatened of animals, are likely to win hearts due to their cuddly exterior while those with a sweet tooth for honey will doubtless sympathise with the bees, which are suffering near catastrophic declines.

Fungi are among the most abundant organisms on the planet and include amongst their numbers the Earth's biggest living organism, a giant fungus known as Armillaria ostoyae which stretches for 2,384 acres in Oregon's Blue Mountains.

Bats are the biggest family of mammals and the only one that can fly, but are threatened by habitat loss and persecution by humans.

Plankton provides food for some of the smallest and biggest animals on the planet, including the Blue Whale.

Here we examine the contenders in detail and asks if we can afford to lose any of them at all.

PRIMATES

Number of different species: 394

Weight: 1 ounce to 440 pounds

Strength in numbers: 400,000 great apes, around a billion other primates

Threats: 114 species are threatened with extinction. Bushmeat hunters and habitat loss are the main threats

PRIMATES are our closest cousins. By studying them and watching their behaviour, humans have been able to gain a remarkable insight into our own beginnings and how our complex cultures have developed.

Primates share more than 90 per cent of our DNA. For Chimpanzees, our closest relatives, the similarities in our genetic code has surprised even the experts.

They are also of great economic importance in many countries - in Rwanda and Uganda the Mountain gorillas are now the number one source of foreign currency income through tourism.

Ian Redmond, chairman of Ape Alliance, an international coalition of organisations and individuals working for the conservation and welfare of apes, said: "Primates are a keystone species in tropical rainforests. They are major dispersers of seeds as they eat fruits and then dispense the seeds in little packets of fertiliser around the forest.

"We need to protect primates today in order to have forests tomorrow that can absorb carbon dioxide from the atmosphere and prevent the erosion of soil."

BATS

Size: 2 grams (0.07 ounces) to 1.5kg (3 pounds)

Number of different species: 1,100

Strength in numbers: billions

Threats: 1 in five species are threatened from habitat loss and persecution

Legends of Dracula and tales of vampire bats have done little to enhance these creatures' reputations. Only one species is the notorious blood sucker, while most feed on insects and fruits.

Bats are the only mammal capable of flying and are so highly evolved to be capable of pinpointing a single insect flying in the pitch black and plucking it out of the air using echo location.

For this reason they are a major predator of insects and play a key role in controlling insect numbers. They are also the most abundant mammal on the planet - one in five mammals is a bat.

"Bats have an extraordinary diversity, which makes them an essential part of the ecosystem," said Dr Kate Jones, a bat expert from the Zoological Society of London. "They are also a key indicator species that can provide information on the health of an ecosystem.

"They occupy a wide range of habitats from urban areas to caves and forests.

"Most crucially, bats are major agents of pollination and seed dispersal. Without them many crops would fail because they play such an essential part of the ecosystem."

BEES

Size: Around half an inch

Number of different species: 20,000 known species of bee

Strength in numbers: Billions of individuals - a single honey bee hive can contain 40,000 bees

Threats: Disease and climate change have seen populations plummet by up to 80%

Without bees, humans would starve. These industrious little insects are the world's greatest pollinators, carrying a dusting of pollen from flower to flower as they gather nectar for their hives. Millions of years of evolution has seen many plants become almost entirely reliant upon bees to help them breed.

Crops such as almonds, peaches, avocados and apricots are totally reliant upon bee pollination.

The total worldwide economic value of pollination has been estimated to be around £130 billion a year, and that is without the honey and wax that bees also produce.

Bee numbers have, however, fallen by up to 80% in some parts of the world due to disease, climate change and pesticide use. The situation has grown so critical that beekeepers are warning there will be no British honey left in the shops by Christmas.

George McGavin, an honorary research associate at Oxford University's Museum of Natural History, said: "The planet could go on functioning quite happily without any large animals such as primates.

"We rely upon bees for just about every vegetable, flower and fruit around. They are a crucial terrestrial group and we would face mass starvation without them."

FUNGI

Size: a single cell to 2,300 acres

Number of different species: Up to 1.5 million

Strength in numbers: millions of billions

Threats: Probably the least threatened group and the cause of threat to many other species in the form of disease

FUNGI are a much maligned group of species. They include pests that can kill gardeners' plants, diseases that are responsible for ailments such as athletes foot and moulds that leave unsightly stains in our houses.

But without fungi we would not have gardens, houses or even feet at all. It was fungi that first allowed plants to move out of the oceans and on to land by establishing a symbiotic relationship that still exists today.

It is a fungi, known as mycorrhiza, that allows plants to obtain nutrients and water from the soil. Rather than directly sucking these essential building blocks of life into its roots, plants have to rely upon the fungi to gather it for them from the surrounding soil.

"It was fungi that allowed plants to move onto land around 600 million years ago," explained Professor Lynn Boddy, a mycologist at the Cardiff School of Biosciences. "Without fungi we would still be living in the ocean."

The other main role that fungi perform is as nature's recyclers. They clean up remains of dead plants and animals by decomposing them and returning the nutrients they hold back to the environment to be used again.

"They are involved in the production of many foods too," added Professor Boddy. "Mushrooms are fungi, but also bread, beer, cheese and chocolate all rely upon fungi to be produced. Many drugs such as penicillin come from fungi too."

PLANKTON

Size: 10 micrometres (0.0004 inches) to 1 millimetre (0.04 inches)

Number of different species: 50,000 in the light zone of the ocean alone

Strength in numbers: Billions of trillions

Threats: Pesticides and pollution can damage plankton blooms

It is hard to feel too attached to plankton. A drifting soup of microscopic algae, creatures and bacteria, they are not even one group of species but bridge entire taxonomic kingdoms. Plankton is essentially anything living in water that is too small to swim against the current, including krill and algae.

But despite its small size, blooms of plankton are visible from space and can sustain billions of marine creatures. The plant-like organisms in plankton, known as phytoplankton, are found close to the surface of the water where there is sufficient light to allow photosynthesis.

"Half of the world's oxygen is produced by these organisms," explained Professor David Thomas, from the school of ocean sciences at the University of Bangor. "If you took that away you would lose the basis of life on the globe. There simply wouldn't be enough oxygen to support life."

The bacteria also provide a vital role by breaking down organic material in the water and recycling dead organisms. The zooplankton, which encompass a wide range of little organisms from single-cell protozoa to creatures such as jellyfish, krill and copepods, provide the basic link in the ocean food chain.

Professor Thomas said: "If you go back far enough in time, life started in the plankton, so we owe it a remarkable debt."

The Irreplaceable debate is being organised by environmental research charity Earthwatch and is being held at the Royal Geographical Society in London on Thursday 20 November. Entry is free and doors open at 6pm.

For tickets and information please call 01865 318856 or email events@earthwatch.org.uk [1]

And here are a few species we may be happy to do without

Wasps

Capable of injecting venom from the end of their sting even after they have died, it is a popular question faced by entomologists - what are wasps actually good for?

Rats

They carry plague and live in the sewers. Even Sir David Attenborough, the wildlife presenter, does not like them.

Feral Pigeons

Known as the rats of the sky, they are considered pests in most city centres around Britain

Woodlice

These scuttling crustaceans thrive in the warm damp corners of houses and are reputed to be a good substitute for prawns in seafood sauces

Stinging Nettles

The bane of all schoolboys who have ever been forced to wear short trousers. Although nettle soup is a known delicacy.