Science: Lessons in good louse-keeping - It is not only tigers and whales that need saving from extinction; tiny insects are also important, says Stella Wiseman

The New Zealand lesser short-tailed bat died out in the Sixties, and the Caerulean Paradise flycatcher met the same fate in 1978. Rhinos, giant pandas, tigers and gorillas are well-known names on the endangered species list. Among the many other threatened species is a small, chubby creature with claw-like legs and striped antennae. It's an unprepossessing animal and tends to be ignored, but it could die out. It's a gorilla louse.

Conservationists call on us to save the gorilla - but save the gorilla louse? Lice on the whole do not receive a good press, and in marketing terms the idea of campaigning for their protection is unlikely to catch on. Yet, if nothing else, they indicate some of the complexities of conservation. Frequently, one species of animal is linked to another or to a particular plant. When one goes extinct, another may do so, too.

Mammals and birds are especially vulnerable. One extremely gloomy prediction made last year is that half of all known birds and mammals will become extinct in the next 200 to 300 hundred years - that is around 7,000 species, 40 times the number known to have died out in the last 400 years. But as we sponsor whales and campaign to protect the tiger, there are people who would have us take more notice of insects, some species of which may be just as threatened.

Lice are among the most at risk, with fleas a close second, say Chris Lyal and Nigel Stork from the Natural History Museum in London. Both groups depend on other animals for their existence and, in most cases, are therefore as common or as rare as the animals they live on. Humans have three species of lice which are not found on any other animal. When the elegant passenger pigeon became extinct in 1914, at least two species of lice died out as well, but this went almost unnoticed until recently. If that is the case, Dr Lyal and Dr Stork say, how many other species of insect have become extinct? Certainly more than the 61 recorded since 1600.

But who cares? 'Every species has the same right to survival,' Dr Lyal says. 'People naturally want to conserve fluffy creatures with large, trusting brown eyes, but doesn't the gorilla louse, for example, deserve some concern, too, even if its closest relative is the human crab louse? It's superbly designed, and just as fascinating as anything with fur and feathers. Lice also play an important part in a gorilla family's life. They are one of the reasons gorillas groom each other, and so help to form bonds between the members of the family.'

Rare animals are often taken into zoos for conservation and breeding, and on arrival they are usually disinfected to kill any parasites. Since the parasites are often just as endangered as their hosts, Drs Lyal and Stork suggest that conservationists should ask how necessary the disinfection really is. 'The parasites may not be doing the animal a lot of good, but they may not be endangering it,' Dr Stork says. Not that he would place the parasite before its host. 'If the parasite was going to kill the host, then I would encourage disinfection, but we should be aware of the consequences.'

This can be easier said than done. The amount we know about insects is minimal. There are probably 10 million species of insects in the world and entomologists have only scratched the surface when it comes to gathering data about them. However, a glance at some of what is already known shows how complex the relationship between species may be and how widespread the effects of our actions can be, not just on parasites but on many other animals.

In both the plant and animal kingdoms there are 'keystone species' which can play a vital role in the existence of other plants and animals. Fig trees, for instance, are affectionately known as 'jungle burgers' because so many animals and plants rely on them for food or shelter or to reproduce in. In the highly unlikely event of fig trees dying out, so would several species of fig wasp, since every species of fig has its own specific wasp and each wasp has its own set of parasites. While fig trees are not endangered, this does underline the need for careful planning in forestry conservation.

In some tropical areas, native trees are being cut down and replaced by fast-growing species, such as pine and eucalyptus. However, while the number of animals and plants living in the forest may remain high as a result of this, some of the original species may have died out because they were dependent on a particular tree, and have been replaced with others from outside. 'We may,' Dr Stork says, 'be losing species we want to conserve.' While it may be more important to replace the forest quickly than to conserve all the original animals and plants, it is also important to weigh up all the factors involved.

Complex relationships between species take place much closer to home. Break open any common wood ant's nest and you will find beetles and other species of ants, exploiting the nest's resources. These other insects have evolved alongside the wood ants and often depend on them for existence. Destroying the ant's nest will therefore destroy them, which may seem a small price to pay to anyone who dislikes ants, but rare, beautiful animals can be put at risk.

The Large Blue Butterfly, for instance, became extinct in Britain but has been reintroduced and is found close to both wild thyme and the nests of small red ants. The butterfly larvae begin by feeding on the thyme but, as they grow, they develop a 'honey' gland which attracts the ants. The ants carry the larvae to their nests where they treat them as if they were potential queen ants, lavishing the greatest care on them. In turn, the larvae feed them the honey-like substance that they secrete. While conservationists used to think it was merely the thyme that was important to the Large Blue, they are now also actively preserving ants' nests.

The fact that it is generally easier to sympathise with those who wish to preserve the Large Blue Butterfly than with those who want to save lice shows the type of value judgements we make about animals. There may be good reasons for this - but Drs Lyal and Stork would at least like us to make decisions about conservation based on facts rather ignorance.

'Sometimes, we may decide a species should go extinct - smallpox for instance - or that our priority is to preserve a particular animal or plant, and it may be impossible to do so without destroying its parasites,' Dr Lyal says. 'If that is the case, we should act from a position of knowledge and thought. I regret the passing of any species, but particularly when it's a matter of casual indifference.'

(Photographs omitted)