by Perry McCarney

Created on: September 08, 2009   Last Updated: April 09, 2012

While it is rarely, if every, possible to categorically prove a negative, most endangered species are endemic, living only in a specific region of the world. Cosmopolitan species, those living in many different parts or over extensive regions are far less likely to die off unless there is a major catastrophic event that affects the entire planet. As such, when looking for living individuals of a critically endangered species, we normally have a limited region to search.

Multiple factors determine how accurately we can estimate the probability or certainty of extinction. The habitat natural to the species and its remoteness from human population centers is of particular importance  Plants, even the smallest, can usually be found if they exist, if a thorough enough examination of their region is done. Animals are much more difficult because they move, and success may be dependent on how stealthy the species is. Even if the species has not been particularly stealthy in the past, any remaining survivors may be survivors because they have become so, have become extremely adept at hiding from man!

The probability that the Yangtze river dolphin is extinct is very high. They were a relatively large animal with a limited habitat, despite the length of the river. An extensive six week search in 2006 found no living dolphins and none have been observed since. The Yangtze river is a crucial waterway in China, with substantial populations living along its banks. Therefore, the probability that they are extinct is as near to a certainty as it is possible to get.

The last known Tasmanian tiger died in 1936, yet unsubstantiated sightings of this fabled carnivorous, marsupial predator still occur. The island of Tasmania is the twenty-sixth largest island in the world, with a land area of 62,409 square kilometers or 24,096 square miles. Of that land, 22.6% is reserved for conservation, the largest proportion of any state in Australia. Most of that land is mountainous or very rugged and unpopulated except for some rangers, researchers and visiting hikers. The Tasmanian tiger is believed to have been driven to extinction primarily due to extensive hunting encouraged by offered bounties. Any survivors would therefore want absolutely nothing to do with Man and would pass this learned behavior down to their offspring. Although the size of a large dog, their distinctive striped coat was well suited to camouflage them in their habitat. All these factors leave some possibility that the species is still extant, although it is still highly unlikely. While the tigers themselves can move and hide, paw-prints, dens and the remains of their prey would not. No such evidence of their continued presence has come to light.

With small, secretive animals it can be impossible to say with any certainty. Several species thought extinct have subsequently been found still living in small populations in remote regions. Their status changing from extinct to critically endangered and immediate research into their ecological needs begun to try to sustain and expand their populations. But most searches for individuals of species believed possibly extinct fail. Habitat loss and fragmentation, introduced species and diseases, whether deliberate or accidental, over-hunting, human pollution and climate change are all savage challenges for any wild species in today's world to overcome. As time passes without a substantiated sighting or the finding of "forensic" evidence of their presence, the probability that they have indeed gone extinct increases.

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