How much are ecosystems worth? It's next to impossible to work out the monetary value of all their products - such as food, water, timber, medicine, climate regulation, recreation and aesthetics - but they are, of course, essential to our lives and economies.

One attempt to calculate the value of ecosystems suggests that the planet's forests are worth $4.7 trillion, about one-tenth of the world's gross product. To look at it another way, about half of all Western medicines - worth many billions of dollars - originated in nature. Aspirin, for example, was originally synthesized from willow bark.

Humanity has recently made an unprecedented impact on natural ecosystems in order to supply itself with food, water, fibre and energy. Sometimes the result has helped improve millions of lives, but more often, quite apart from the loss of biodiversity, nature's vital services have been weakened. Wetlands have often been drained for farmland - at the cost of devastating the fisheries that use them for nurseries - and lose their ability to cleanse water of pollution.

Similarly, mass deforestation - from the Amazon to Indonesia - yields timber and clears land for agriculture, but at a terrible price. Local people are hardest hit as the forest goods and services on which they depend are lost. But people living downstream can be flooded out of their homes because the forests that once held, and gently released, rainwater have been felled. And there are worldwide effects too; fewer trees mean more carbon dioxide in the atmosphere, and therefore more global warming.

It is not easy to work out to what degree natural systems can be safely exploited since they are not fully understood - partly because they are rarely valued. But once they are lost, it is difficult, often impossible, to restore them.

This is all likely to get worse in coming decades. So what can be done? Quite a lot, actually. Information campaigns can alter attitudes. Certification programmes that identify sustainably produced products help consumers buy green. Some governments now pay landowners who manage their lands to preserve water quality, absorb carbon dioxide or conserve other ecosystem services. Creating national parks and other protected areas - and providing more money and management support to the existing ones - also helps. Many businesses already develop environmentally friendly technologies, and they can be encouraged to do more. Protecting biodiversity, in its turn, requires such diverse responses.