SCIENCE / The answer lies in the soil: Do we need so many species? Hugh Aldersey-Williams on a remarkable new experiment

EXACTLY 150 years ago this month, three Icelandic fishermen set out on an infamous mission. Jon Brandsson and Sigourer Isleffson clubbed the two last Great Auks to death; Ketil Ketilsson put the boot into the last egg.

What seems remarkable about this incident today is not so much the fact of the extinction - we may be losing one species of bird or mammal a year according to latest estimates - but the detail that it was recorded by the perpetrators. We no longer regard an extinction as a personal achievement. Instead, we seek to conserve all species of plants and animals more or less indiscriminately.

The first reason for conserving species is moral, but often comes down to aesthetics. We should do what we can to ensure their survival for the same reason that we preserve buildings or works of art, cherishing them as a resource for human enjoyment. In practice, this means that glamorous species - large mammals, colourful butterflies - often get preferential treatment. We now mourn the loss of the passenger pigeon, but not the passenger pigeon louse that must have gone at the same time.

The second argument is fuelled by more immediate self-interest. We should conserve species because they might prove a convenient source of medicine or other substances useful to humanity. The difficulty here is that we do not know which species might be the useful ones. So the aim is to maintain 'biodiversity' - the range of plant and animal species - as a whole. This is the policy advanced by organisations such as the World Bank and the United Nations Environment Programme.

The final argument for conservation also centres on biodiversity, not on the off-chance that some species that has so far eluded scientific analysis might offer us something of commercial worth, but because biodiversity in itself is a good thing and may be a factor in ensuring our own long-term survival: the theory being that each species plays a part in the working of the global ecosystem. However, scientists' opinions have differed on whether a high level of biodiversity is necessary and, until recently, they have had no way to test the hypothesis.

One of the most promising experiments to investigate the usefulness of biodiversity was set up three years ago by Professor John Lawton, director of the Centre for Population Biology at Silwood Park near Ascot, which is part of London University's Imperial College. He built what he called the Ecotron, a series of chambers with full environmental control, into which various species of plants and animals can be introduced and then monitored.

The Ecotron consists of a number of separate chambers, each containing a patch of soil one square metre in area. In each of these scientists created a particular 'environment': three types in all, the first containing nine species of plants and animals, the second 15 species, and the third 31. As is usual in biological experiments, the three environments were replicated, using several chambers, in order to gather data that represented an accurate average result.

This was a precaution notably absent from Biosphere 2, the notorious 'sealed' ecosystem containing humans in the Arizona desert. The Ecotron environments were modest in comparison, the richest being about as varied as a typical chalk grassland. A sophisticated airconditioning system enabled scientists to simulate a wide range of day-lengths and seasons by varying light levels, humidity and temperature.

The researchers planted seeds in April 1993 and introduced species of invertebrates at suitable dates thereafter when they would stand the greatest chance of surviving. All three environments contained species at four different levels of the food chain - plants, earthworms and other animals that decomposed plant material, herbivores such as snails and aphids, and other insects that preyed on these. The only difference between the three environments was the number of species they contained, in other words their biodiversity. The Ecotron ran for 206 days, by which time all the plants had flowered and the animals had reproduced.

Professor Lawton's research group measured various biological processes within the 14 chambers. The scientists weighed dead plant matter for evidence of decomposition by the worms. They looked at how the ecosystems retained water and nutrients. They analysed the input and output of carbon dioxide in order to assess the plants' respiration. Finally, they measured the plants' efficiency at being able to use light in order to grow.

They found that the chambers with the greatest diversity of species were best at absorbing carbon dioxide and at making use of the light for growth. They thus showed for the first time that reduced biodiversity, often as a result of human intervention, does indeed alter the performance of ecosytems.

Loss of biodiversity is broadly what happens when we clear a forest, says Professor Lawton. A failure to maintain the earth's diversity of species could make the environment less able to mop up the excess carbon dioxide we create from the burning of fossil fuels.

There is another, more subtle, implication from this research. Ecologists have long suspected that smaller areas of land tend to be less diverse than larger areas of an identical ecosystem. Combine this idea with the Ecotron findings, that loss of diversity is likely to lead to further extinctions, and the conclusion is clear: continuing to eat into natural ecosystems - by creating smaller and smaller nature reserves, for example - could have a cumulative effect on extinction rates. Species cannot be maintained by corralling them into safe havens that are too tiny or are incomplete re-creations of former natural habitats.

We know we are losing species of all kinds at a rate of perhaps one a day. We know, too, that the human activities that are hastening this loss are not about to cease. If the result of Professor Lawton's experiment proves to be applicable generally (and he is working now in collaboration with laboratories across Europe to expand the range and richness of Ecotron environments more nearly to model the complexity of nature itself in order to resolve this question), we know too that any species loss is likely to impair the working of the ecosystem.

This research raises intriguing questions about how to set priorities in our efforts to conserve wildlife. Should we, Noah-like, try to preserve a few of everything? Should we concentrate our limited resources on those species most at risk, or should we let them go and pick winners instead?

The aesthetic justification for conservation is as good a guiding principle as any, says Professor Lawton. 'Fortuitously, better ecosystem services such as meadows or mixed woodlands are the kind of places people prefer to be. It does not make sense to replace them with conifers or wheat fields.'

Yet today, less than two per cent of the land area of the United Kingdom is managed for conservation. Professor Lawton calculates that 10 per cent would be sufficient to maintain the biodiversity necessary for a satisfactory local ecosystem. A start has been made with the Government's plans to increase the size and quality of some threatened habitats.

Furthermore, aesthetics motivate people. If we have to preserve habitat in order to save some conspicuous species, then we will also find ourselves conserving numerous other species in that same habitat. If we had saved the passenger pigeon we would have saved its louse without even trying.-

(Photograph omitted)