BOOK REVIEW / NONFICTION : Another Alarm Sounds in the Great Doomsday Debate : THE SIXTH EXTINCTION: Patterns of Life and the Future of Humankind by Richard Leakey and Roger Lewin, Doubleday, $24.95, 271 pages

In 1979, ecologist Norman Myers published “The Sinking Ark,” in which he argued that deforestation was removing as much as 2% of the world’s trees each year, wiping out enormous numbers of species on the planet and ultimately threatening human survival.

Since then, a debate has raged among scientists over whether Myers’ doomsday estimates are accurate or alarmist. The hand-wringing environmentalists are sure that Myers is right, and the growth-is-good folks are equally sure that he is wrong. As usual, no one’s mind is ever changed in these debates.

Now comes Richard Leakey, the noted anthropologist and conservationist, assisted by Roger Lewin, a science writer and theorist, to weigh in on the alarmist side. In “The Sixth Extinction” Leakey argues that the long history of life on Earth has seen five extinctions so far--the most famous being the disappearance of the dinosaurs, possibly caused by an asteroid hitting the Earth and disrupting its climate.

But if population growth continues and people keep destroying natural areas, he says, there will be a sixth extinction--humanity itself.

“We suck our sustenance from the rest of nature in a way never before seen in the world, reducing its bounty as ours grows,” he writes. “Human-driven extinction is continuing today, and accelerating to alarming levels.”

Unless we change course, Leakey says, fully half of the 30 million species in the world will disappear in the next hundred years. He has adopted the figure proposed by Myers himself, although Myers has conceded that “we have no way of knowing the actual current rate of extinction in tropical forests, nor can we even make an accurate guess.”

The fact that nobody knows exactly what is happening does not dissuade Leakey. He disagrees with those who think that just “because ecologists cannot say precisely how many species are endangered, it is premature to be alarmed about the putative impending collapse of biodiversity.”

Leakey’s prime opponent in this debate is Julian Simon of the University of Maryland, who has argued repeatedly and forcefully that the planet can sustain many more people than it currently does, and, besides, the more people there are, the more people will be at work figuring out how to produce more food to feed them.

Actually, this argument did not begin in 1979. It began nearly 200 years ago with Thomas Malthus, an English clergyman who warned that population growth would outstrip the growth of the food supply and people would starve to death.

But today, Malthus’ dire predictions have not come true.” This is because humanity has consistently and cleverly figured out how to produce more food. The question is, can we continue doing so?

The Julian Simons of the world say this pattern of the last 200 years will continue into the foreseeable future. The Richard Leakeys say that no matter how clever we are, we can’t keep using up resources indefinitely. Sooner or later, the day of reckoning will arrive.

I have no idea who’s right.

Leakey devotes a fair amount of ink to answering Simon’s points. Last year, Simon and Myers published “Scarcity or Abundance?” (Norton), in which they went at each other tooth and nail and covered much of the same ground that Leakey does.

What Leakey adds to the argument is historical analysis, discussion of the five previous extinctions and accumulation of arguments put forward by the scientists who agree with him, notably Stephen Jay Gould and E.O. Wilson.

He makes very good use of recent developments in evolution theory, particularly Gould’s contention that evolution is not a slow, steady process, as Charles Darwin thought, but a series of rapid changes followed by long periods of no change.

None of this, however, leads inexorably to Leakey’s conclusions. Leakey, nevertheless, synthesizes a great deal of material in support of his world view. Regardless of whether you accept his conclusions, you can learn a lot from his story.