How to know when a species becomes extinct
How do scientists know when to classify a species as extinct?
Animals such as the Yangtze River Dolphin, the Christmas Island Shrew and the Venezuelan Skunk Frog are all extinct, or close to the brink of extinction, but how do scientists decide when to label a creature as 'extinct'?
With the threat of climate change, deforestation, hunting, and pollution, the United Nations has said that we are on track for the fastest extinction rate since the dinosaurs disappeared 65 million years ago. However, trying to prove that a species is really extinct is a more difficult process that many would think.
"If there's one thing in my career I'd like to be proved wrong about, it's the baiji," said Sam Turvey of the Zoological Society of London, using another name for the Yangtze River dolphin.
Turvey spent almost 3 months this year interviewing Chinese fishermen in vain for sightings of the long-snouted dolphin, which has not been seen since 2002. Some colleagues in China are still looking.
The baiji was almost declared extinct in 2006 after an acoustic and visual survey of the river turned up nothing. Then, a blurry video gave experts pause, and it was rated "possibly extinct."
About 300 plant and animal species, including the Christmas Island shrew and the Venezuelan skunk frog are also "possibly extinct," the worst category short of extinction, according to the International Union for Conservation of Nature's (IUCN) Red List.
If Turvey's study turns up no firm evidence, it will likely push the Yangtze River dolphin into the "extinct" column, said Mike Hoffmann, who manages a global project to assess species for the IUCN and Conservation International.
It would be the first "megafauna" mammal -- one weighing more than 100 kg (220 lb) -- to die out since the Caribbean monk seal in the 1950s.
"To say something is extinct requires quite a lot of proof, of negative evidence, and may take many years to collect," said Craig Hilton-Taylor, who manages Red List.
Scientists working on the "possibly extinct" list rummage in the undergrowth for rare plants, frogs or rats, set up night-time traps for bats or moths, or scour the seabed for corals.
Some experts liken the difficulties to "proving" that the mythical Loch Ness Monster does not exist.
About 76 mammals have gone extinct since the year 1500, and currently 29 are on the 'possibly extinct' list for this year. The Christmas Island Shrew has not been seen in Australia since 1985 and the Venezuelan skunk frog has not been spotted recently either; however both are currently not on the extinct list.
There are some species that are pronounced extinct, and then show up years later as they have been left alone to breed again; an example of this is the 'Green Sphinx Moth' from Hawaii.
The trouble with labeling species as extinct is that when everyone thinks they are dead, no more money can be raised for their conservation and then another critically endangered species just moves up the list.
No one has an exact number of how many species there are on earth - the UN estimates anywhere between 5 to 30 million, but they have also stated that they could be vanishing faster than they are being discovered.