"At the heart of this assessment is a stark warning," said the 45-member board of the Millennium Ecosystem Assessment.

"Human activity is putting such strain on the natural functions of Earth that the ability of the planet's ecosystems to sustain future generations can no longer be taken for granted," it said.

Ten to 30 percent of mammal, bird and amphibian species were already threatened with extinction, according to the assessment, the biggest review of the planet's life support systems.

"Over the past 50 years, humans have changed ecosystems more rapidly and extensively than in any comparable time in human history, largely to meet rapidly growing demands for food, fresh water, timber, fibre and fuel," the report said.

"This has resulted in a substantial and largely irreversible loss in the diversity of life on earth," it added. More land was changed to cropland since 1945, for instance, than in the 18th and 19th centuries combined.

GETTING WORSE

"The harmful consequences of this degradation could grow significantly worse in the next 50 years," it said. The report was compiled by experts, including from UN agencies and international scientific and development organisations.

UN Secretary-General Kofi Annan said the study "shows how human activities are causing environmental damage on a massive scale throughout the world, and how biodiversity -- the very basis for life on earth -- is declining at an alarming rate."

The report said there was evidence that strains on nature could trigger abrupt changes like the collapse of cod fisheries off Newfoundland in Canada in 1992 after years of over-fishing.

Future changes could bring sudden outbreaks of disease. Warming of the Great Lakes in Africa due to climate change, for instance, could create conditions for a spread of cholera.

And a build-up of nitrogen from fertilisers washed off farmland into seas could spur abrupt blooms of algae that choke fish or create oxygen-depleted "dead zones" along coasts.

It said deforestation often led to less rainfall. And at some point, lack of rain could suddenly undermine growing conditions for remaining forests in a region.

"We're seeing an increasing risk of abrupt changes in many ecosystems," Walt Reid, executive director of the assessment, told Reuters.

The report said that in 100 years, global warming widely blamed on burning of fossil fuels in cars, factories and power plants, might take over as the main source of damage. The report mainly looks at other, shorter-term risks.

The study, to be handed to governments, said big changes in consumption, better education, new technology and higher prices for exploiting ecosystems could brake damage.