“We as a society have to decide if it is going to be ethically and morally appropriate to simply display animals for entertainment purposes,” said the director of the Smithsonian Conservation Biology Institute, part of the National Zoo in Washington. “In my opinion, that model is broken. There needs to be an explicit role for zoos to champion species.”

He wants zoos to raise more money for the conservation of animals in the wild and to make that effort as important as erecting fancier accommodations for their captive collections. Zoos, he said, should build facilities — not necessarily open to the public — that are large enough to handle whole herds of animals so that more natural reproductive behavior can occur. And less emphasis should be placed on animals that are popular attractions but are doing fine in the wild, like African elephants and California sea lions, adding that they should be replaced with animals in desperate need of rescuing.

He wants all zoos within the Association of Zoos & Aquariums to aim higher on conservation efforts. “I am comfortable with raising the standards for zoos so that eventually it will be harder and harder to be accredited unless you are doing that,” he said. “If you can’t keep up, then you probably need to be dropped off the bottom.”

In their first century, American zoos plucked exotic animals from the wild and exploited them mainly for entertainment value, throwing in some wildlife education and a touch of preservation. When wilderness began disappearing, causing animals to fail at an accelerating pace, zoo officials became rescuers and protectors. Since the 1980s, zoos have developed coordinated breeding programs that have brought dozens of animals, like the golden lion tamarin of Brazil, back from the brink.

The increasingly difficult challenge is to be a force for conservation while continuing to put on a show.

“We are always balancing the public experiencing with conservation needs,” Dr. Bonner said. “If you ask me why I have camels, I would say that we need something interesting for people to see at the back of the zoo in winter, and they are always outside.”

Currently, there are 214 accredited zoos in the United States, from tiny eight-acre attractions to world-renowned destinations like the San Diego Zoo, whose annual budget approaches $200 million. The main organization binding these zoos is the Association of Zoos & Aquariums. Since 1985, it has been setting higher standards, raising the bar for animal care, conservation in the field and cooperative breeding programs.

Less Room for Animals

But while the association can remove accreditation, which it does from time to time, it has few other enforcement powers. So decisions on just how conservation-oriented to be — from how many imperiled animals to save to how much money will be spent on species still in the wild — are largely made zoo by zoo.

As standards for animal care rise and zoos install larger, more natural-looking exhibits, there is room for fewer animals. In the 1970s, the primate house in St. Louis held 36 species of monkeys and apes. Now it has 13.

And that narrowing of the species list is likely to continue for another reason. Zoos have come to understand that for animals to reproduce successfully for the long term without inbreeding, they need to maintain much wider gene pools for each animal. There are 64 polar bears in captivity in American zoos, far short of the 200 considered optimal for maintaining the population over 100 years. So zoos have been adding to the numbers of some species while culling others at the same time. St. Louis says it houses 400 more animals but 65 fewer species or subspecies than it did in 2002.

How the shrinking slots are allocated is becoming more considered and scientific. As conservation pressures mounted in the 1990s, the association began putting together advisory groups of zookeepers that look across entire families of species and advise on which ones should be made priorities and which ones should be phased out.

All sorts of criteria are considered, including uniqueness, level of endangerment in the wild, importance of the animal’s ecological role, and whether there is an adequate population in captivity for effective breeding.

Zoos are essentially given a menu of endangered species that the association is trying to maintain and can then choose according to their particular needs. But final decisions are often as much about heart as logic.

St. Louis, for example, has committed $20 million — or the equivalent of 40 percent of its annual operating budget — to building an enormous exhibit for polar bears — complete with a fake ice floe — even though its last polar bear died in 2009 and the Marine Mammal Protection Act makes it illegal to remove or rescue the bears from the wild. The zoo hopes that in the five years needed to open the exhibit, it can argue for an exemption, import orphaned bears from Canada or perhaps secure the cubs of captive bears.

Dr. Bonner acknowledges that the polar bear project runs counter to many of his more practical convictions on the role of the modern zoo. He has insisted that his keepers spend what limited field conservation dollars they raise on threatened animals that are most likely to make a comeback in the wild. With sea ice disappearing at an alarming rate, polar bears do not fit the profile.

But he justifies the exemption as a lesson for zoo visitors: “I want people to see this beautiful creature and ask, ‘How could we have let this happen?’ ”

The IUCN estimates that nearly one-fourth of all mammals are at risk of becoming extinct, roughly in the course of the next three generations. The situation is even more dire for amphibians and seabirds. The organization, for example, has declared the disappearance of the Australian gastric brooding frog, which ingested its eggs, gestating them in its stomach and eventually spitting out tadpoles.

The problem for zoos is that even small animals require more of a commitment than a plastic container if a return to the wild is to be realized.

Several large buckets of dirt are now home to the threatened American burying beetle, so named because it buries the corpses of small animals, like birds and squirrels, and lays its eggs around them. Once, the beetles, with their brilliant red markings, ranged over 35 states. By the time the United States Fish and Wildlife Service listed them as endangered in 1989, there was one known population left, in Rhode Island.

At the government’s behest, the St. Louis Zoo, in conjunction with a zoo in Rhode Island, has been successfully breeding them and returning them to the wild.

Mr. Merz says the effort was worthwhile because the beetle might play an irreplaceable role in the ecological web. He considers picking species worth saving akin to life-or-death gambling. “It is like looking out the window of an airplane and seeing the rivets in the wing,” he said. “You can probably lose a few, but you don’t know how many, and you really don’t want to find out.”

In 1989, scientists realized that the world’s frogs were dying in alarming numbers. They deduced that about a third of all 6,000-plus amphibian species were threatened with extinction, many being felled by a fast-moving fungus that has probably been spread in large part by trade and pet sales.

After that, scientists developed a protocol for determining which species should be rescued first. At this point, only 42 percent of the world’s amphibians have been assessed. But zoos and aquariums have taken note and are following the protocol’s recommendations.

So far, only about 10 percent of the amphibian species requiring immediate rescue have been given homes. Still, many scientists, like Robert C. Lacy, a conservation breeding specialist, believe the frog mobilization will become a model. Yes, he said, there are endless moral and ethical debates. For example, should priority be given to animals that cannot survive outside zoos or to those in need of temporary shelter to restock their numbers?