"We are tinkering with our life support systems" and "we don't realize the consequences of what we are doing," cautions Jane Lubchenco, former president of the American Academy for the Advancement of Science and contributor to the magazine's 31-page report.

The report states:

Humans have altered or cleared 40 to 50 percent of the earth's land surface, drastically altering the plant and animal communities in those areas.

Because of land conversion and competition from invading species, 18 percent of mammals, 11 percent of birds, 8 percent of plants and 5 percent of fish species are threatened with extinction.

One fourth of all bird species are already extinct.

Of the world's fisheries, 66 percent are being used to their maximum or depleted faster than they are being replaced.

Fossil fuel burning adds some 5.5 billion metric tons of carbon dioxide to the atmosphere every year.

Man consumes half of the world's usable fresh water, using most of it for farming. We have built 36,000 dams worldwide, and 98 percent of all U.S. rivers have been diverted or dammed.

Several major rivers, including the Nile, the Ganges and the Colorado, are mostly used up before they reach their mouths. (Sources: Science, Knight-Ridder News Service.)