American zoos, once just a place where visitors could gawk at exotic animals, are increasingly playing a role in preventing the extinction of endangered species. But as the populations of more and more rare breeds dwindle, U.S. zoos are running out of space and money to devote to conservation, according to a report in The New York Times. That's forcing many zoo officials to make tough decisions on which animals they should try to save, and which they should phase out. Here, a guide to this increasingly common — and always difficult — choice:

How many animals are endangered?
There are roughly 5,400 mammal species in the world. The International Union for Conservation of Nature estimates that a quarter of them are in danger of becoming extinct within the next few generations. Amphibians and seabirds are in even worse shape.

Can zoos keep these animals from disappearing?
Some of them, yes. Since the 1980s, coordinated breeding programs at American zoos have rescued dozens of species, such as Brazil's golden lion tamarin. But the number of breeds that need protection is soaring, and U.S. zoos can't come close to saving them all.

Why not?
There are too many animals that need saving, and just 214 accredited zoos in America. Some zoos are just tiny eight-acre parks that can spread awareness but lack the resources to do much to breed rare animals. That means that the bigger zoos — from the 100-year-old St. Louis Zoo to the world-famous San Diego Zoo, which has an annual budget of close to $200 million — have to pick up the slack. But these facilities have limited resources, so they're having to make the bitter choice of which species to save, and which to let go.

How do zoos make these choices?
The Association of Zoos & Aquariums draws up a priority list to help zoo officials choose species for their conservation efforts. The animals on the list must be unique and truly endangered, and they must have an important ecological role. Perhaps most importantly, there has to be a captive population big enough to grow the population without inbreeding.

Sources: Aquatic Community, Gawker, New York Times