There are nearly two million known species on the planet. But many of those won't be around much longer; one out of every eight known bird species, one in four mammal species, and one in three amphibian species are at risk for extinction, according to the World Conservation Union (IUCN), which maintains the Red List, a catalog of the world's species classified according to their risk of extinction.
"It's supposed to inform conservation practice, to be a wake-up call for the extinctions that are happening," says Caroline Pollock, a program officer with the Red List unit. Animals that are classified as "critically endangered" are at the highest risk--their numbers in the wild may be extraordinarily low or their territories incredibly small. "It is possible to bring them back," Pollock says, "but it is quite work-intensive and financially expensive." Here, a look at five species on the brink.