or decades, scientists have been warning that human activities were extinguishing species, altering the climate and degrading landscapes. Now a group of experts has reframed the issue, releasing a sweeping report that measures damage not to nature itself, but to the things nature does for people.

In the report, part of a continuing project called the Millennium Ecosystem Assessment, more than 1,300 ecologists and other researchers from 95 countries focus on the capacity of ecosystems to perform valuable functions like filtering water, providing food and pollinating crops.

Their conclusion is bleak: over all, 60 percent of those functions are being degraded by human activities, both through direct actions like overfishing and through indirect ones, like the tendency of deforestation to raise the risk of floods.

The report - which was released last week and online at www.millenniumassessment.org - lists some instances in which destructive practices have changed and damage has been prevented, but says far more action is needed in the next several decades.

"We must learn to recognize the true value of nature - both in an economic sense and in the richness it provides to our lives," said an accompanying statement by the board of scientists who led the project.

"Above all," it continued, "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."

Under the current method of measuring progress, the report said, "a country could cut its forests and deplete its fisheries and this would show only as a positive gain." And in too many instances, it said, that is exactly what is occurring.

The study considered various kinds of "ecosystem services": simple provisioning, like supplying water and protein; regulatory functions, including a forest's ability to store and filter water and to cool and humidify the air; cultural services, like providing a place for recreation; and life-support services, including photosynthesis and soil formation.

Many of the regions where such natural assets are being most rapidly degraded are also the world's poorest, the scientists said. And as a result, deteriorating environments are likely to hamper efforts to stem poverty, disease and hunger in developing countries.

But the study also said wealthy countries were contributing greatly to some problems - for example, in soaring increases in agricultural runoff containing nitrogen, a fertilizer that can create oxygen-starved "dead zones" in coastal waters.

The assessment, which cost $24 million, was commissioned five years ago by the United Nations and by countries adhering to global environmental treaties on preserving wetlands and migratory species, preventing the spread of deserts and conserving the diversity of species on earth.

Some ecologists not involved with the project credited the authors for avoiding old arguments that tended to set people against nature.

"We have to start thinking about nature as a design issue," said Dr. Daniel B. Botkin, an ecologist and author of several books charting ways to mesh human activities and life on earth. "For too long we've been seeing everything people do as a negative. This is a break from that. They're trying to bring people and nature together."

The study said the degradation of potentially renewable natural resources was being fueled in part by destructive subsidies, uncoordinated policies of government agencies dealing with overlapping activities like forestry, farming and land tenure, lawlessness in frontier regions and the persistent treatment of nature's bounty as free for the taking.

Subsidies and other artificial incentives to overharvest resources are especially vexing problems, said Dr. Harold A. Mooney, a biologist at Stanford and a lead author of the report.

"A third of the global value of farm production in 2000 was the result of subsidies," he went on. "In many places we spend more catching fish than we make selling fish," Dr. Mooney said.

Unlike many earlier environmental assessments that have compiled trends for losses of forests, reefs and other wild places, this one focused on how such losses directly affected human welfare, using as its yardstick trends in "ecosystem services" rather than simply lost species or acreage.

Besides identifying losses in familiar trouble spots like rain forests and reefs, it focuses on less known danger zones, like dry-land ecosystems, where human populations are growing fastest and depend most heavily on fragile natural systems.

A prime example is the parched band of Africa below the Sahara Desert, where drought, combined with ever-growing demands for water, has contributed to recent social upheavals and bloodshed in Sudan.

Around the world, Dr. Mooney said, "the dry-land problem really jumps out at you."

"You have two billion people there and huge limits on water," he continued. "Some of the world's highest population growth rates are in these dry regions and in mountain systems that are the least productive. That creates conditions for conflict."

He added that global warming, which is expected to disrupt weather patterns in the same dry regions, will make matters only worse.

Dr. Botkin said an unavoidable weakness in this kind of assessment was that the complexity of global ecology and economic activity made it hard to specify causes and effects.

The authors of the report acknowledged huge gaps in data, but pointed to small successes that helped crystallize the idea that nature is more than pretty pictures.

Dr. Mooney cited several recent studies that put a monetary value on natural services. In one study in Costa Rica, Dr. Gretchen C. Daily of Stanford and other researchers measured the increase in coffee yields to a plantation from the pollinating efforts of bees living in two nearby fragments of forest.

From 2000 to 2003, they calculated, the presence of the forest bees lifted the plantation's income $60,000 a year.