That phrase has evolved to mean that public-land managers should endeavor to preserve plants, animals and other natural features so they remain within the range that they exhibited before Europeans first arrived in North America. Any meddling that occurs in protected areas, therefore, must be in the service of a perceived previous natural order.
"It instituted in the Park Service in a way a kind of respect for nature that was apart from gardening," Graber says. "Before the Leopold Report, I called it cowboy biology. We made it up as we went along. If Yellowstone wanted more buffalo, they got it."
Under the new regime, it became necessary to prove that such a bison introduction would be "natural."
Notwithstanding some controversies - such as "natural" wolves versus "introduced" ranchers in the Yellowstone area - this approach has met with monumental success. Nearly a century after Congressman William Kent introduced the legislation that created the National Park Service, the 295-acre ravine he donated to create Muir Woods National Monument remains much as it was a millennium ago, filled with redwoods, ferns and ladybugs.
"We don't move anything unless it falls on someone," notes Muir Woods interpretive specialist Lavin.
Another impressive legacy of the new ethos lies 20 or so miles to the west. At the turn of the century, egg hunters and pelt gatherers had reduced the wildlife-rich Farallons to a relatively barren state. Since they became protected as the Farallon National Wildlife and Wilderness Refuge in 1969, the islands have become the largest seabird colony outside Alaska and Hawaii. Northern fur seals, which once populated the Farallons by the tens of thousands, were hunted to extinction there following the Gold Rush. They, too, have returned in force. A single pup was born on the islands in 1996. Last year, there were 100 pups.
The starving Cassin's auklets, however, point to a possible future when this let-it-be strategy will no longer produce the desired results.
Strategies that do something effective - that don't just let nature succumb to climate change - are hard to come by.
Two hundred and fifty miles southeast of San Francisco, new studies resulting from decades of research show that giant sequoia saplings are thriving less robustly in the warming central Sierra Nevada. So do officials in Sequoia National Park build sequoia sapling greenhouses? Do they install sprinkler systems around the great sequoia monarchs? Or do they prepare a new habitat farther north, removing other species to make space for sequoia saplings? Should such moves even be contemplated, given the still-fledgling nature of predictive climatology?
And what of the rest of the trees in the West - the ones doomed to die from drought, fire and beetle infestation?
Scientists studying forest diebacks say one response to the dying might be to thin forests, so that individual trees are hardier and more beetle-resistant. It remains to be seen how well this would go over with an environmental movement accustomed to opposing logging. Other controversial ideas include intensive breeding and genetic engineering to create insect-resistant tree species, combined with the aggressive use of herbicides and pesticides.
Wildlife managers have long believed that local plant species should be kept genetically pure. But climate change may ultimately call for a sophisticated type of wildlife gardening, in which heat-loving southern plant species are brought north and encouraged to crossbreed with cold-loving cousins.
Already, a massive die-off of pinon pine trees in the Southwest is being called a "global warming type event." Again, selective logging might be one answer, scientists say: If fewer trees share scarce water, they just might survive in the new climate.
But for plant species that simply can't survive in their old habitat, some scientists are floating the idea of a forced march north.
Animals whose habitat dwindles as the climate changes might just scurry elsewhere, explains Nathan Stephenson, a research ecologist at the Western Ecological Science Center at Sequoia and Kings Canyon National Parks. But trees cannot get up and walk away. "The National Park Service has to decide: Are we going to assist species migration?" says Stephenson.
Helping plants and animals migrate north isn't just a matter of leasing fleets of flatbed nursery trucks. Many species under threat aren't easy to dig up and put in a pot. Soil microorganisms, fungi, butterflies, and other small creatures critical to the functioning of ecosystems may also find their traditional homes unlivable. Assisting species migration would mean setting aside broad swaths of wild land to provide an uninterrupted pathway north for entire habitats.
"I've had a number of conversations with land managers, identifying all the land in California that could conceivably be used as refugia, and what would be the appropriate species to go where. The magnitude of the problem is mind-boggling," says Graber, the Park Service scientist. "There is a vocal minority of people in the conservation community who believe that things should unfold on their own. The theory being, we don't know what we're doing, and we're bound to screw things up.
"What we're talking about is an order of intrusion greater than anything we've done in the past."
Already the nonprofit Nature Conservancy is considering buying land and ecological easements to create north-south habitat-migration superhighways. "We need to take into account this vulnerability to large vegetation shifts," says Patrick Gonzalez, a forest ecologist who works with The Nature Conservancy under the title "climate change scientist." "One way in which we're using that data is in the establishing and maintaining of corridors that link areas in the network."
Doing this on any sort of meaningful scale, however, would require making the preservation of American grasses, trees and rodents an expensive national priority. And it would mean treating habitat-choking urban sprawl as even more of an environmental calamity than is currently recognized.
Putting America on this sort of ecological wartime footing - to prepare for an environmental future that nobody can fully predict - will likely prove a hard sell in Washington. Almost as difficult will be convincing the environmental community to abandon a hard-won national consensus about what it means to preserve the natural world.