By Tim Hirsch BBC environment correspondent in Kyoto

We should not ignore nature's solutions, the IUCN says

Experts are increasingly realising the huge benefits provided by these ecosystems in preserving and purifying fresh water, yet they are under threat all over the world.

The World Conservation Union (IUCN) highlights the example of the Nakivubo wetland in Uganda, which receives about half of the raw sewage from the capital city Kampala.

The plants and soils in the marsh purify the water before it drains into Lake Victoria, which provides drinking water for the city. Yet the Nakivubo is threatened by drainage and reclamation.

'Ecosystem approach'

IUCN's director-general Achim Steiner told BBC News Online: "We estimated last year that the value of that one wetland is around $15m 'services equivalent' provided, if you had to build sewage treatment plants to achieve the same kind of treatment of water."

Another example is the forests of Mount Kenya, which protect the water catchments of two of the five river basins in the country - providing nearly three-quarters of the country's electricity through hydropower dams, and irrigating agriculture.

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The "services" provided by these forests are put at $50m a year.

Of course, protecting these areas also prevents the destruction of a wide range of plant and animal life - but their benefits to people are too often ignored.

Some groups fear that this "ecosystem approach" to managing fresh water will get little recognition in the final statement from ministers to emerge from this forum.

But the message is that nature can often prove far more cost-effective in delivering clean water than concrete dams and metal pipes.