There are grizzlies in Colorado's San Juan Mountains.

What is your reaction to that possible but improbable statement? Thrilled, scared or both?

It's likely similar to the complex reaction humanity has held for centuries toward predators capable of making a meal out of us: fear, respect, hatred and reverence, all at once.

Instead of thinking of predators as a threat, consider David Quammen's more researched and unsettling take: By the year 2150, all top predators will be dead or in a zoo where captivity and their declining gene pool will eventually render them mere shadows of their species.

" . . . People will find it hard to conceive that those animals were once proud, dangerous, unpredictable, widespread and kingly, prowling free among the same forests, rivers, estuaries and oceans used by humanity. . . . Children will be startled and excited to learn, if anyone tells them, that once there were lions at large in the very world."

This is the alarm-bell opening for Quammen's important, erudite and passionate book about the past and future for humans and alpha predators. In Monster of God, Quammen explores the complex issue of whether or not humans will continue to live in proximity with animals that are occasionally threatening and need at least as much room as people.

It's a dilemma reaching far beyond conservation, involving class, politics, economics, natural resources, biology and some powerful human emotions.

Quammen, best known for his award-winning columns for Outside magazine, is one of this country's best science writers. His four books of collected essays are among the most lively, funny and hard-hitting of any natural history writing. The Song of the Dodo remains a high-water mark for the urgency and accessibility with which it covers evolution and the biology of extinction.

Throughout Monster of God, Quammen compiles an exhaustive catalog of facts, history and opinions about the four animals and locations he visits: lions in India's Gir forest, crocodiles in northern Australia, brown bears in Romania and Siberian tigers in the Russian Far East.

The book is, in many ways, a series of biological and sociological case studies that examine the challenges people and large predators present each other. Quammen also devotes energy to examining our myths - Gilgamesh, Beowulf and even the film Alien - as well as our conscious history with predators via the artwork in the Chauvet Cave in southern France. The paintings are thought to be 35,000 years old and depict lions hunting.

To his great credit, Quammen avoids the romanticism that plagues much "environmental" writing. And he has the wit and curiosity to keep such a hefty book afloat:

"While we humans may be the most reflective members of the natural world, we're not (in my view, anyway) its divinely appointed proprietors. Nor are we the culmination of evolution, except in the sense that there has never been another species so bizarrely ingenious that it could create both iambic pentameter and plutonium."

There are plenty of human low points. In Romania, Quammen stumbles upon the Carpathian Hunting Museum, which is a trophy warehouse for ex-dictator Nicolae Ceausescu, a "pipsqueak autocrat who fancied himself a great hunter."

Ceausescu's shooting - it cannot be called hunting - was repugnant, though no better than how he treated Romanians: "His most notable fit of excess occurred in the autumn of 1983 when, during a single day, aided by four separate game drives toward his position, Ceausescu personally shot twenty-four bears."

Quammen draws interesting parallels between the politics of a country and how large predators fare. Generally, the rule is: the more centralized and imperial the government, the better large predators fare. After noting excessive hunting in British India of lions and in colonial Australia of crocodiles, Quammen floats a theory that "the extermination of alpha predators is fundamental to the colonial enterprise. . . . You haven't conquered a people, and their place, until you've exterminated their resident monsters."

Can we exist with large predators? On that point, Quammen points out, "Whom do we mean by 'we'?" In most of the places he visits, too many people face a future where their own survival is threatened. It is largely the poor who bear the brunt of living among lions and crocodiles, whether from a threat to their safety or, more often, a threat to their livestock and livelihood.

Population growth is inevitable and Quammen's point is that if an effort is not made to protect these large predators, then they will disappear beneath humanity's waves. It may happen faster and more quietly than we think and there may be repercussions we can't foresee.

Ecologists frequently call predators "keystone species," which means they are necessary for keeping the balance in an ecosystem. Remove the top predator, and a legion of nibblers takes over, eventually destroying the ecosystem.

Amara Bhai, a Maldhari herder in India's Gir forest, was injured two years in a row by the same pair of lions when he defended his buffalo. While he worries about that particular pair, he doesn't think any worse of lions in general. Lions hunt buffalo - it's what they do.

The insidious idea that wilderness should be wild but safe guarantees just one thing: no true wilderness.

Through a translator, the scarred and resilient Bhai said about the lions: "The animal is good. There's nothing wrong with the animal."

Tyler D. Johnson is a multimedia producer for the Rocky Mountain News.