For at least 17 years, the Mexican wolf has trotted only as a memory among the juniper- and pion-swathed mountains of the Blue Range, which straddle the Arizona-New Mexico border.
Hunted to the brink of extinction, the remaining 149 wolves can be seen only in zoos. By year's end, however, 20 of these animals could be released into the wild in what the US Fish and Wildlife Service and the conservation group, Defenders of Wildlife, hope will be a successful reintroduction.
But the wolves are not out of the woods yet, despite more than 20 years of human preparation for the event and experience in reintroducing wolves elsewhere. "We're still not sure of the recovery area's carrying capacity or even which parts of the area the wolves will use," says Bob Ferris, director of Defenders' species conservation division.
His concern about habitat is echoed by ecologists around the country as they struggle to implement more than 600 recovery plans for endangered species across the United States.
As a basic principle, the need for some place hospitable to put endangered plants and animals seems obvious, researchers say. But the details can be as prickly as cactus needles.
"We need to understand in a more holistic way what's needed for recovery.... The dynamics of ecosystems are poorly understood," says Peter Raven, professor of biology at Washington University in St. Louis, Mo.
Ecologists confront two broad, often overlapping habitat issues as they try to put the biological equivalent of Humpty Dumpty back together again. In one, the habitat may be in good shape physically but the threatened or endangered species face competition from other species, including humans. In other cases, ecologists must try to restore habitat itself.
"Suppose you have a perfectly suitable habitat, but the species doesn't occur there," says Dan Falk, executive director of the Society for Ecological Restoration, based in Madison, Wis. "Your first question becomes: If this is such a perfect habitat, why isn't the species here?"
Failure to confront this question can thwart species reintroduction. Richard Primack, a Boston University biologist, notes that this failure has undercut otherwise well-meaning efforts to rebuild sea-turtle populations along the Gulf Coast.
"There's been a major project to collect eggs, hatch them, nurture the hatchlings through their most vulnerable stages, then release them," he says. "Tens of thousands have been released, but never has there been a documented case of a released turtle returning to the same beach.
"The effort makes people feel good," he says, but it fails to deal with the cause of the decline: turtles trapped as "by-catch" in fishing nets.
Human activities, however, may not always be the culprit. "Maybe there are real ecological reasons a species is endangered, like random colonization by other species that you also may want to preserve," Mr. Falk says. "As common, if not more so, are situations where you have to correct something other than people with shotguns."
Efforts to reintroduce black-footed ferrets to parts of the Great Plains, for example, have run into difficulty in part because they fall prey to coyotes, which now roam regions wolves once inhabited. Mr. Ferris explains that wolves, hunting in packs for larger prey, in effect kept coyotes at bay. This left the ferrets freer to fill their niche in the plains ecosystem. Coyotes, which are now more common, also compete with ferrets for food - in the form of prairie dogs, whose populations also are declining.
Reintroduction also can be complicated by the need to "train" species raised in captivity to cope with human technology it may encounter, even in nominally remote areas. After wildlife officials released captive-born California condors into the wild in 1992, one bird died after drinking from a puddle of power-steering fluid, while several more ran afoul of power poles. Two years ago, researchers at the San Diego Zoo began a training program for young condors on power-pole etiquette - how avoid them.
More challenging, Falk continues, are situations where ecologists must take a once-suitable habitat, with its subtle relationships between organisms, and make it hospitable again.
"By definition, restoration has to start with a reference ecosystem," he says. "But no restoration tries to return a habitat to, say, 1820, because the ecosystem would have evolved in any case. Restoration is more like asking: Where can I meet up again with a moving train?"
Bringing scientific expertise and technical prowess - global-positioning satellites, computer modeling, data bases, and mapping systems - to bear on these issues means "we're not shooting in the dark; but we are shooting in the dim," Falk says.
"We need a better knowledge base to begin with," agrees Dr. Raven. "Then we need to do a better job of anticipating threats" to ecosystems.
Perhaps the largest effort to do just that is under way in southern California, where scientists and state and local officials are trying to develop the ecological equivalent of an urban master plan.
The effort focuses on the coastal sage scrub ecosystem in San Diego, Riverside, and Orange Counties. It grew out of a more species-specific attempt to list a bird known as the California gnat catcher as endangered, according to John Rieger, an ecologist with the state department of transportation who is focused on San Diego County's portion of the program.
"The ecosystem thrives along the coastal plains," he says, adding that this is prime territory for a thriving species: the housing tract. Yet the region also caters to "a larger number of endangered species than anywhere else in the continental United States," he says. Mapping soils and vegetation, or identifying populations and the corridors they use to migrate and breed requires hours of work, he says. "This is still a work in progress."
And it represents the essence of habitat restoration and conservation, says the SER's Falk. "Geographical information systems and satellite photos help orient you to the landscape in a quantitative way. They come about as close to a revolution in this field as you can find," he says.
"But in the end, this is not a technical undertaking. The essence of the field is someone slogging through the underbrush with a shovel, a clip board, and a good idea."