In 1997, a team of biologists and economists tried to put a value on the "business services" provided by nature - the free pollination of crops, the air conditioning provided by wild plants, the recycling of nutrients by the oceans. They came up with an estimate of $33 trillion, almost twice the global gross national product for that year. But after what today's report, Millennium Ecosystem Assessment, calls "an unprecedented period of spending Earth's natural bounty" it was time to check the accounts.

"That is what this assessment has done, and it is a sobering statement with much more red than black on the balance sheet," the scientists warn. "In many cases, it is literally a matter of living on borrowed time. By using up supplies of fresh groundwater faster than they can be recharged, for example, we are depleting assets at the expense of our children."

Flow from rivers has been reduced dramatically. For parts of the year, the Yellow River in China, the Nile in Africa and the Colorado in North America dry up before they reach the ocean. An estimated 90% of the total weight of the ocean's large predators - tuna, swordfish and sharks - has disappeared in recent years. An estimated 12% of bird species, 25% of mammals and more than 30% of all amphibians are threatened with extinction within the next century. Some of them are threatened by invaders.

The Baltic Sea is now home to 100 creatures from other parts of the world, a third of them native to the Great Lakes of America. Conversely, a third of the 170 alien species in the Great Lakes are originally from the Baltic.

Invaders can make dramatic changes: the arrival of the American comb jellyfish in the Black Sea led to the destruction of 26 commercially important stocks of fish. Global warming and climate change, could make it increasingly difficult for surviving species to adapt.

A growing proportion of the world lives in cities, exploiting advanced technology. But nature, the scientists warn, is not something to be enjoyed at the weekend. Conservation of natural spaces is not just a luxury.