The eco chamber without the hype: A new 'biosphere' experiment shows that diversity of species is vital, says Sanjida O'Connell

Across the globe, entire species of plants and animals are going quietly into extinction. This loss of 'biodiversity' has become a catch-phrase for our eco-conscious times. There are perfectly valid reasons to lament the decrease in biodiversity, but many appeal to aesthetic or moral considerations, which tend to carry little weight in the realpolitik of international relations, despite the political rhetoric at last year's Earth Summit in Rio.

But, in the unlikely setting of the Home Counties, an experiment has just produced clear evidence that unless we protect our plant and animal species, we could exacerbate the greenhouse effect and increase global warming.

Life on earth is protected from the hostility of space by the biosphere - the layer of gases surrounding the planet. It is only two miles thick - the equivalent of wrapping a piece of paper around a model globe. The biosphere, temperature and humidity are maintained by plants and animals; without them the earth would resemble a cross between Mars and Venus, with barely a trace of oxygen.

It is predicted that by the year 2100 we could have lost 50 per cent of our fauna and flora. With fewer species, can the earth's atmosphere be maintained, and, if the climate is heating up, could a reduction in biodiversity lead to more global warming?

To try to find some answers, scientists at Imperial College, headed by Professor John Lawton, Dr Shahid Naeem, Dr Lindsey Thompson, Dr Sharon Lawlor and Richard Woodfin, built an 'Ecotron', at Silwood Park, near Ascot, Berkshire. The Ecotron resembled the much-hyped giant greenhouse in Arizona, Biosphere 2, which contained a sealed community of people, plants and animals. The Ecotron was a completely sealed unit housing plants and animals over a period of time, but there the similarity ended. For a start, the Ecotron was much smaller and contained fewer species. Nor did the Ecotron contain people: unlike Biosphere 2, it was not intended as a model for a future dwelling on the ocean floor or in space. 'We've been courted by space colonies,' says Dr Naeem, 'but we've got our feet firmly on the ground.' The whole set-up was also much more scientifically rigorous.

The pounds 1m Ecotron had 14 identical chambers, each containing one of three biodiversities: one had a low number of animal and plant species, the second, an intermediate number, and the third, a relatively high biodiversity. The most complex biodiversity was replicated in six of the chambers, the other two biodiversities were in four chambers each. Each chamber was hooked up to a computer network that controlled air, light and rainfall in the chambers. The experiment covered 200 square metres on two floors. 'I'm not used to spending this much money,' says Dr Naeem, 'I worked in Costa Rica and I used plastic bags to collect plants.'

The plants chosen for the Ecotron were ones that would do well living in such confines. The chambers, rather less glamorous than tropical rainforests, resemble weedy fields. The animals included soil decomposers, herbivores and predators, but these grand-sounding categories were filled by creatures like aphids, snails and earthworms. However, of the 10 million species estimated to be on earth, 9 million are invertebrates like these.

The thinking behind the experiment was that a certain fraction of energy from the sun is taken up by plants when they photosynthesise and is translated into plant matter. The more plant matter there is, the more carbon dioxide is absorbed, stabilising greenhouse gases.

When the experiment was set up in May, Professor Lawton was not convinced that a wide range of species would be better at stabilising the climate. He argued that species should be saved because they were aestheticalIy pleasing, for instance, or to provide a larger gene pool. In terms of plant matter, a rainforest with 150 species per hectare could produce as much as a less diverse environment, such as a saltmarsh or mangrove swamp. Professor Lawton's theory was that if one species was eliminated, another would take over the unused space, and the total productivity would remain the same.

Dr Naeem disagreed. He believed that the more species there were, the greater the productivity and the more stable the climate would be. Stability is vital.

'Global warming will happen; I don't think people are being alarmist.'

The experiment ended two weeks ago and most of the results have been collected. They clearly show that the Ecotron chambers holding a higher diversity of plants and animals are better at using up carbon dioxide, water and nitrogen. Those chambers contain plants with different types of leaves, growing to different heights, which means that the leaves have a greater ability to absorb light and so produce a larger amount of plant matter. The animals interact with each other and the plants and contribute to the 'soaking up' of nutrients and gas.

Did the results of the experiment change Professor Lawton's mind? Partially. From the conclusions drawn from the Ecotron, he recognises that 100 species will keep an atmosphere more stable than if there are only 10, but he adds that his theory still holds true in some cases. For example, if a field of prairie grass is replaced by a field of wheat, there may be little or no effect on the climate, as both species are roughly comparable in size and shape and hence in the amount of gas and water they need. But if a rainforest was replaced with two species of banana plants, the banana plants would not fulfil the role the forest had.

'If a reduction in biodiversity means a reduction in species, and that means a reduction in shapes - plants that have creepy stems or sturdy ones or vines - that is as an eco-system that works less well,' says Dr Naeem. 'In terms of how ecosystems affect the earth's atmosphere, from the results of this experiment, we should conserve species. I'm glad the result is positive. I prefer any argument that makes it possible to do this.'

Biodiversity may have become a catch- phrase, but if we do not pay more than lip service to the concept, we risk worsening global warming.

(Photograph omitted)