Should the condor, which had almost been wiped out by habitat loss, hunting, and eating carcasses that were poisoned by lead bullets, be left to die in the wild?
Or should scientists take the remaining 22 condors into captivity and breed them, which would cost millions of dollars?
Sanjayan's view was that humans had a moral responsibility to save North America's largest flying bird.
That's exactly what happened: Captive-born condors were reintroduced into the western United States in the early 1990s. There are now more than 200 in California, Arizona, and northern Mexico.
On a recent trip to the Grand Canyon, Sanjayan—now the lead scientist at the Nature Conservancy—looked up and spied one of the big black birds soaring above.
"That's pretty incredible if you think about it," he says. "They're really out there in the wild now." (See "Banning Lead Ammunition Could Give Condors a Chance.")
The condor's recovery shows that endangered species can be brought back from the extreme brink. And there are plenty of other examples.
Gray wolves, which by the 1970s were wiped out of most of their North American range due to hunting, have bounced back to more than 3,500, thanks largely to reintroduction efforts. Northern elephant seals, hunted down to fewer than a hundred individuals, now number 150,000 along the West Coast.
But with dozens of new species going extinct every day—scientists say that more than 20,000 plants and animals are on the brink of disappearing forever—deciding which species to save is a tricky question.
This week, National Geographic will spotlight some of the world's most innovative and unusual efforts to save disappearing species, from the mountains of Tanzania to the plains of Missouri, in a series called "Last of the Last."
The series will focus on campaigns to bring back species deemed worth saving. Which raises a basic question: How do we decide which species to save?
In some cases, scientists and economists use algorithms and logistical models to determine a return on investment for trying to save the last of the last: If x dollars are put toward saving the spotted owl, it's possible to determine how many might be saved.
In practice, though, scientists and conservations prioritize based on a mix of public perception and a species' economic value—for instance, whether it's a popular seafood or brings tourism dollars to a state.
And there's a another, more subjective factor: How they feel about a particular piece of flora or fauna.
"What we decide to save really is very arbitrary—it's much more often done for emotional or psychological or national reasons than would ever be made with a model," Sanjayan says.
As in the case of the condor, he says, "people end up saving what they want to save—it's as simple as that."
Some conservationists argue that how we choose which species live or die is deeply flawed, that our bias for preserving cute and fuzzy animals diverts precious resources from creatures that actually keep our planet humming.
Ants, for instance, are essential environmental helpers, distributing seeds, aerating soils, and eating other insects that are often human pests, says Marc Bekoff, an ethologist at the University of Colorado Boulder.
"If we're going to save pandas rather than ants, we need a good reason, and being cute is not a good reason," he says. (Also see "Is Breeding Pandas in Captivity Worth It?")
Hugh Possingham, an expert in environmental decision-making at Australia's University of Queensland, says our obsession with "celebrity species" is likely detrimental to as many as thousands of other creatures in need.
Snakes and Spiders Need Not Apply
Endangered species that get a lot of love are often those that elicit the broadest public interest.
As a result, the endangered species may have more money spent on it than any other. In 2010, the cost of managing tiger reserves alone cost at least $82 million, according to the Economist. (Take an endangered species quiz.)
Elephants are another animal fan favorite, even though there are still a half a million left on Earth.
Many lesser known species of fish and frogs are in more dire straits, with just 20 individuals left in some cases, says Jean-Christophe Vie, deputy director of the International Union for Conservation of Nature (IUCN)'s Global Species Programme.
A bias against smaller, less iconic animals also shapes the decisions of major donors.