Some people love science for the crazy ideas, the ones that transport you beyond the everyday grind: black holes, alien life, anything with the word "quantum." Others prefer the not-crazy ideas, the practical solutions: zippier computers, 100-mpg cars, cures for cureless diseases.

So what do you make of an idea like Pleistocene rewilding? It manages to be both crazy and not crazy at the same time. As the article by C. Josh Donlan beginning on page 70 describes, a team of biologists has proposed a decades-long project to restock North America with large mammal species like those that roamed the continent before humans crossed the Bering Strait--species such as camels, lions and elephants (the nearest thing to mammoths). The undertaking would culminate in a vast national park--1,000 square miles or more--stretching across the Great Plains. The plains states are depopulating anyway, whereas Africa and Asia are filling up. So the project would transplant wildlife from where it gets in the way to where it would have plenty of room.

To be sure, Midwesterners might not see it that way. Elephant families running free under big skies sounds romantic--unless you have to dodge them on your morning commute. Lion cubs are so very cute--except when they wander into your backyard. Farmers worry about rampaging rogues, cattle ranchers about novel diseases. Proponents have addressed some of the concerns but clearly have a lot more work to do.

Whether or not cheetahs ever chase pronghorn across the continent again, the rewilding concept has drawn attention to the fact that the loss of biodiversity is not just a problem for the rain forest; it affects less exotic locations, too. The demise of large animals has thrown entire ecosystems out of balance. Even if humans decided now to leave these ecosystems alone, they are too far gone to recover on their own. The prairie would revert not to its Pleistocene glory but to a scraggly weedland.

Instead of merely bemoaning nature's plight, the proponents of rewilding are doing something about it. The reintroduction of wolves in Yellowstone a decade ago brought the population of moose and elk under control, with a cascade of benefits for vegetation, birds and beavers. Other projects have reintroduced tortoises, bison and falcons into their old haunts. Wildlife-stocked private ranches let you go on safari within half an hour's drive of the Alamo. More broadly, biologists are also working to restore fisheries, forests and wetlands.

Most of these efforts are too piecemeal to nurse whole ecosystems back to health. They need a broader framework, and a grand rewilding project can provide it. Visionary schemes have a checkered history. The ones that work combine big ideas with baby steps. They need both: for lack of attention to detail, a grand project can easily fail; equally so, for lack of an overarching plan, incremental steps can seem like spitting in the sea. Recent history offers many examples of enterprises that, deprived of a real goal, lost their way. One that comes to mind is the human space program. After the moon landings, NASA became little more than a delivery van service. A thriving ecosystem can underpin economic prosperity and enhance our quality of life, but it won't happen on its own. We have to make it happen.