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Does loss of biodiversity affect human health?

The United Nations Environment Programme believes it does - the notion was one of the top lines in the last edition of its massive five-yearly , which came out in 2007.

The nuts and bolts of the link, though, can come across as a bit tenuous - loss of species may affect the discovery of new drugs; biodiversity can impact water quality; and so on. They're not necessarily the most convincing arguments to those who pride themselves on having hard heads.

This week, I came across something a bit more concrete - and what makes it more interesting is that it relates to one of the really poor cousins of the medical research field, .

Also known as bilharzia, this is a disease which receives so little attention and money that malaria is a rich prince by comparison. Yet it affects about 200m people and is said to be the second most devastating parasitic disease in the world - malaria being the first.

The parasites - flatworms of the genus Schistosoma - spend part of their lives in water-borne snails, and people - usually children - contract the infection from the water when the parasites swim free.

There's no vaccine, and there are really only two modes of attack - either giving regular doses of drugs such as praziquantel, or trying to eradicate the snails that carry the parasite, with chemicals such as copper sulphate.

Some people have looked at introducing crayfish to eat the snails - I hope something of an alarm bell rang there given the problems that invasive species have caused in some places around the world - or by introducing certain plants.

So , a researcher at the University of Colorado, asked a simple question; could the diversity of the snail population affect the number of parasites?

His team rigged up a series of experimental chambers in their lab. All had the same number of Planorbidae snails that carry the parasites, but he put in different numbers of other snail families that can't carry it.

As he reported in the Royal Society's journal this week, there was a definite impact. The number of Planorbidae infected fell by between a quarter and half when other types of snail were around.

The reason is probably what parasitologists call the "decoy effect". Some parasites will attempt to enter the wrong kinds of host - they can't, they die, and so there are fewer parasites around to infect the real hosts.

Now, this is a laboratory experiment - but if the results do hold true in the wild, here would be both a striking demonstration of the principle that biodiversity can beat disease, and something practical that the millions of people affected by schistosomiasis could use to protect themselves to some extent.

Simply keeping their ponds and streams in a state that preserves the range of native snails might reduce the number of people infected.

Implementing that remedy, however, might not be so straightforward given other environmental trends.

Agriculture is changing in many of the countries affected by schistosomiasis, even in its African heartlands.

Excess fertiliser running off farmland into water stimulates the growth of algae; and this appears to be an advantage to the disease-bearing snails, who can thrive on the green stuff, whereas other types die off.

(It's the same thing in microcosm that's happening to ; too much nutrition for algae brings the death of important native species - in this case, the coral polyps.)

You could argue, of course, that simply wiping out the wrong kind of snail would be more effective. But it's been tried, it has side effects, and it's a procedure that needs doing time and time again.

And wiping out the hosts wouldn't be an option for another condition where the link from biodiversity to human health has been demonstrated - .

and his collaborators have shown that a diverse ecology reduces the number of white-footed mice, an important carrier of the ticks that transmit the disease.

Lyme disease is frequently in the news in North America, and I'm not surprised, having met a conservationist in Canada a few years ago who was still suffering the effects more than a decade after infection.

Schistosomiasis is rarely in the news anywhere. But it should be; it is one of the factors holding back the health and education of children in the poorest countries, and if simply keeping the right mix of snails alive would indeed help keep the parasite down, why not?

In the meantime, it's not my job to do the UN's publicity; but if they're looking for concrete evidence to show why biodiversity matters to the human race, perhaps the snail-ridden waters of Africa and Asia are places worth looking.

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