The last known Tasmanian tiger died in captivity in 1936. Gone, too, are the zebralike quagga, passenger pigeon and golden toad -- each of them, like the tiger, photographed before extinction. These lingering images serve as windows into the past, reminders of what our recklessness has subtracted from the Earth.
Since the demise of these four species, humans have made an effort to preserve more of the planet's endangered species through zoos and improved wildlife management. Yet a number of animals currently hang in the balance due to habitat loss, pollution, overhunting, invasive species and environmental change.
As of May 2010, the United Nations-recognized International Union for Conservation of Nature (IUCN) listed 75 species as extinct in the wild. Like a hospital patient on life support, they only live on due to human intervention. They simply don't exist naturally in the ecosystem any longer. Of course, some of these species breed exceedingly well in captivity. Thailand's red-tailed black sharks, for instance, may have lost their natural habitat, but are exported annually in the tens of thousands to aquariums [source: IUCN].
The black soft-shell turtle, on the other hand, is only known to survive in a single artificial pond on the grounds of a Buddhist temple in Bangladesh [source: IUCN]. While the turtles have long thrived in this environment, such a centralized population leaves them particularly vulnerable to extinction and -- in a cruel twist of irony -- susceptible to overpopulation deaths [source: Baruah]. Even if captive populations of a threatened species increase enough to permit reintroduction into the wild, the lack of genetic diversity also poses a problem. This issue is currently playing out among reintroduced populations of Père David's deer in China, which originally went extinct in the wild at least 1,000 years ago. [source: IUCN].
Still other species find themselves in an even worse predicament: their numbers sufficiently reduced to make them, in the words of some conservationists, functionally extinct. In other words, there may be survivors out there, but the population is so reduced that extinction is inevitable. According to recent surveys of the Yangtze River, the population of China's freshwater baiji dolphin has likely dipped so low that any remaining mated pairs will be unable to repopulate the species [source: BBC]. Similarly, wildlife surveyors predict that the West African black rhino as probably extinct. If any of these animals still walk the Earth today, they are likely the last. Neither classification, however, is an official IUCN category.
Still other endangered species remain in severe decline, despite human efforts to conserve them. In November 2009, the Siberian Tiger Monitoring Program reported just 56 animals, down from 500 just four years prior [source: BBC]. Fortunately, experts believe that this rare species can still bounce back from possible extinction.
Endangered plants, unlike turtles and deer, generally don't inspire conservation efforts. After all, when's the last time you empathized with a rutabaga? But large numerous plant species are dangerously close to disappearing due to loss of habitat, overexploitation, invasive species, pollution and climate change.
As of 2006, the IUCN listed 8,000 plant species as threatened, including such species as the Phillip Island Hibiscus. By the 1980s, feral pigs, goats and rabbits had grazed this plant to the brink of vanishing forever on its Phillip Island, Australia habitat. A mere two clumps of the flower remained [source: Sydney Botanic Gardens Trust]. Subsequent removal of feral animals has allowed the plant's population to rebound, but it still remains one of the most endangered plants on Earth.
Explore the links on the next page to learn even more about endangered species.