Political movements are often built on literary foundations. Abolitionism owed much to “Uncle Tom’s Cabin.” Progressivism had Upton Sinclair and Ida Tarbell. Books, fiction or not, have the power to convince us impressionable readers that we face dire threats, such as unclean meat or pesticides. Political entrepreneurs, promising to protect us from those threats, can then work on the fertile ground of our fears.

The environmental movement has been very successful at making America afraid. Forty-five years ago, Rachel Carson’s “Silent Spring” convinced the public that DDT was a great threat to our ecosystem; more recently, Vice President Gore’s “An Inconvenient Truth” created widespread alarm about global warming. In 1968, Paul Ehrlich’s “The Population Bomb” terrified millions with its claim that humans had overtaxed the environment and that “in the 1970s and 1980s hundreds of millions of people will starve to death in spite of any crash programs embarked upon now.”

Inconveniently for Mr. Ehrlich, but luckily for the rest of us, his prediction did not come true. Yet still, despite its empirical failings, “The Population Bomb” was in many ways a great success. By convincing its many readers that ignoring the environment was a perilous course, the book advanced the cause of green activism and set the stage for the landmark environmental legislation of the Nixon era.

It seems particularly appropriate that in this year of rising commodity prices, exactly 40 years after “The Population Bomb,” Paul Ehrlich is back. Together with his wife Anne, he has written “The Dominant Animal” (Island Press, 428 pages $35), a book that is being billed as a new and vital environmental warning from the once consummate Cassandra.

Luckily, the book is better than its publicity. Paul Ehrlich is a distinguished entomologist, an expert on lepidoptera. The book’s first 200 pages provide a well-written presentation of evolutionary science that shows the depths of Mr. Ehrlich’s knowledge. Co-evolution is taught through the poisonous monarch butterfly and its mimic, the viceroy. Geographic speciation is explained with hermit thrushes. The Ehrlichs’ description of island equilibria is particularly compelling.

Stripped of the Ehrlichs’ political agenda, the book could have been a very nice piece of popular science on the rise of mankind to world dominance. Of course, from Thomas Huxley to Richard Dawkins, evolution has long attracted some of the finest popular science writers. And there was no guarantee that a new book on evolution would survive in the highly competitive world of Darwinian literature.

Perhaps as a result, “The Dominant Animal” goes beyond its evolutionary origins. The second part of the book once again sounds the environmental tocsin. I found this part of the book unobjectionable, but the warnings are hardly as exciting today as they were 40 years ago. Enough books on environmental doom have been printed to kill off a forest of giant redwoods. Moreover, the Ehrlichs are no longer making exciting, if irresponsible, claims about the imminent demise of millions. Instead, their more moderate warnings have become the conventional wisdom.

Most of my Republican friends would now agree with the Ehrlichs’ view that climate change is a real danger, and that people do not internalize the full environmental costs of producing toxic chemicals and driving. Today, we need sound policies that will make us better stewards of our “natural capital,” as the Ehrlichs call it, more than we need more alarms.

Unfortunately, the authors’ forays into policy making are the most painful part of the book. The authors have thrown together a left-wing wish list crammed with proposals that stray far from their science. How can environmental issues get better treatment in America? The Ehrlichs propose we “stop gerrymandering.” Ah yes. The best thing to save the spotted owl would be to spend millions of hours trying to pass a constitutional amendment that would prevent legislatures, which seem likely to be overwhelmingly Democratic after the next census, from redrawing the political map.

On foreign policy, they recommend that “congress should insist in the short term that the executive branch work with Russia on what may be the most crucial environmental problem of all — the threat of a humanly and ecologically catastrophic nuclear war.” Is it really wise, or constitutional, for Congress to pass a resolution that forces the hand of the executive branch in conducting diplomacy? Such a resolution would do wonders to ensure that the State department has as little bargaining power as possible in its dealings with Russia.

The authors are particularly ardent in their opposition to population growth. It is true, as they point out, that there are environmental costs of having more people — all of us use natural resources and energy and bear some responsibility for greenhouse gas emissions. But there are also benefits, especially to the people being born. Each new person has a brain that might come up with new technologies that could reduce humanity’s environmental impact. As an urban economist, my life’s research has focused on the many ways in which we are all enriched by the people around us. Are there many parents who think that the world would have been better off if they had decided to have one less child?

The Ehrlichs are right that we face real environmental threats, but there are better and worse ways of facing those threats. Today, we need sophisticated policies that weigh costs and benefits, not more warnings. Ironically, the very success of environmental alarmism has convinced many of us that the environment is too important to be left to the environmentalists.

Mr. Glaeser is the Glimp Professor of Economics at Harvard, director of the Taubman Center for State and Local Government, and a senior fellow at the Manhattan Institute.