Hummingbird, Tambopata Reserve, Peru. Photo: GUNTER ZIESLER /STILL PICTURES

At the heart of this Millennium Ecosystem Assessment is a stark warning. Human activity is putting such strain on the natural functions of the Earth that the ability of the planet's ecosystems to sustain future generations can no longer be taken for granted.

The provision of food, fresh water, energy and materials to a growing population has come at considerable cost to the complex system of plants, animals and biological processes that make the planet habitable.

As human demands increase in coming decades, these systems will face even greater pressures - and the risk of further weakening the natural infrastructure on which all societies depend.

Protecting and improving our future wellbeing requires wiser and less destructive use of natural assets. This in turn involves major changes in the way we make and implement decisions.

We must learn to recognize the true value of nature - both in an economic sense and in the richness it provides to our lives in ways much more difficult to put numbers on.

Above all, protection of these assets can no longer be seen as an optional extra, to be considered once more pressing concerns such as wealth creation or national security have been dealt with.

This assessment shows that healthy ecosystems are central to the aspirations of humankind.

Garbage pickers, Brazil Sifting through the detritus of modern civilization is a hazardous occupation. Yet it is a common income-generating pursuit for children in many of the world's megacities. And, though there are healthier ways to do it, recycling waste is vital for sustainable cities. Photo: BELERRA-UNEP/STILL PICTURES

Shopping mall, Kuala Lumpur, Malaysia The richest sixth of humanity - mostly living in the industrialized countries of the North - has a disproportionate impact on the world's ecosystems through its huge consumption. If each person alive today consumed at the rate of the average US citizen, we would need three more planets to meet the demand. Photo: GIL MOTI/STILL PICTURES

Burning tyre dump, Mexico Burning tyres release highly toxic smoke, polluting the air. How long before every element of the industrial system can be reused, recycled and reprocessed without harm to human health and the natural environment? Photo: SCHALHARIJK-UNEP/STILL PICTURES

Sand dunes, Ijnaoune, Mauritania Villagers in Mauritania secure sand dunes in an attempt to stave off desertification. The UN Convention to Combat Desertification notes: 'Forced to take as much as they can from the land for food, energy, housing and income, the poor are both the causes and the victims of desertification. In reverse, desertification is both a cause and a consequence of poverty.' Photo: CLIVE SHIRLEY/ PANOS

In the streets of a crowded city, in the aisles of a giant supermarket, or on the floor of a gleaming electronics factory, the biological state of Earth's rivers, forests and mountains may seem a remote concern. Yet despite the breakneck pace of technological change many of us have seen in our lifetimes, we each depend far more than we may realize on the web of life of which we are a part.

The food and fresh water that keep us alive, the wood that gives us shelter and furniture, even the climate and the air we breathe: all are products of the living systems of the planet.

As forests and savanna made way for farms, as rivers were diverted to irrigate fields, and as new technology enabled fishing vessels to haul ever-greater harvests from the oceans, the recent changes made to natural systems have helped not just to feed a rapidly growing human population, but to improve the lives of billions.

In the midst of this unprecedented period of spending Earth's natural bounty, however, it is 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.

Nearly two-thirds of the services provided by nature to humankind are found to be in decline worldwide. In effect, the benefits reaped from our engineering of the planet have been achieved by running down natural capital assets.

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.

The cost is already being felt, but often by people far away from those enjoying the benefits of natural services. Shrimp on the dinner plates of Europeans may well have started life in a South Asian pond built in place of mangrove swamps - weakening a natural barrier to the sea and making coastal communities more vulnerable.

Unless we acknowledge the debt and prevent it from growing, we place in jeopardy the dreams of citizens everywhere to rid the world of hunger, extreme poverty and avoidable disease - as well as increasing the risk of sudden changes to the planet's life-support systems from which even the wealthiest may not be shielded.

We also move into a world in which the variety of life becomes ever more limited. The simpler, more uniform landscapes created by human activity have put thousands of species under threat of extinction, affecting both the resilience of natural services and less tangible spiritual or cultural values.

Yet this need not be a counsel of despair. The natural balance sheet we bequeath to future generations depends on choices made at every level and in every corner of the planet - from the head of a village in Bangladesh to a corporation board in a New York skyscraper, from international gatherings of finance ministers to consumers in a Brazilian furniture store.