How long since the last sighting of a creature for it (the species) to be declared extinct? There is no clear answer to this. It is very difficult to definitively say that all populations of the species have vanished across its entire range. The Indus dolphin Platanista minor is one of the few freshwater dolphin species in the world. Besides the Indus dolphin the Indian subcontinent also has the Ganges Brahmaputra freshwater dolphins. Both species are endangered with numbers estimated to be at most in the several hundreds each. The Indus dolphin's range extends beyond the Indus main channel to its many tributaries such as the Sutlej and Beas. It was last seen in the Punjab rivers in the 1930's which is why the sighting of this animal in the Harike wildlife sanctuary south of Amritsar couple of weeks ago has caused delight among wildlife enthusiasts. The image below shows the Harike wetlands formed at the confluence of the Sutlej and Beas rivers.
It is astonishing that there was not a single confirmed sighting of this dolphin in the Punjab for about 70 years. These are not some isolated stretches of rivers. Punjab is heavily populated, with extensive agriculture land use along the banks of its rivers. Biologists are not sure if the Indus dolphin is really broken up into semi-isolated sub populations or there are regular encounters between populations in the Sutlej-Beas with those in the Indus main channel. Is it possible that the Sutlej population went extinct and these latest sightings represent a new population recently migrated from the Indus channel. Do the many barrages on the Punjab rivers pose a serious barrier to migration or can the dolphin negotiate these barriers especially at flood times? Many questions and I hope wildlife authorities take this opportunity to extensively study this species. Just to give you an idea of the range of this species the image below shows the Indus river basin from its delta up to the Harike wetlands.
Sightings such as these makes me think about how many species are yet to be discovered. The focus in recent years and correctly so has been on the accelerated rate of extinction. But around 300 new species are being formally described by scientists daily across the entire range of life. A new mammal species is reported once every 3 years and a large new vertebrate from the open ocean once every 5 years. There are currently about 1.6 million species recorded but at the current rate of discovery these could represent about 10% of the total species currently living on earth. What rate of species loss can the biosphere sustain without shrinking overall? It is estimated that about 99% of species that have ever lived are now extinct. The diverse biota today must reflect then a slight excess of speciation over extinction. Using the fossil record the background extinction rate has been calculated to around 2.5 species per year which suggests a slightly higher speciation rate to maintain overall diversity. This estimate should be regarded as a canonical speciation rate and the variance around this mean speciation rate is more useful in understanding speciation patterns and diversity. A recent measurement suggests this approach to understanding the problem. If there are about 16 million species at present then every year the tree of life grows by an additional 16 million years of branch length. The average age of living species is about 5 million years. To maintain overall branch length i.e. to sustain current levels of biodiversity will require not more than 3 species per year going extinct. Measured extinction rates since the 1600's suggest that about 25 species have gone extinct every decade a figure that agrees with the above theoretical expectations. This rate however should be considered an underestimate since many extinctions would have gone unnoticed and has definitely increased recently. For example the last century has seen about 20 mammalian species alone going extinct. The effect of natural extinction on overall biodiversity is very different from a mass extinction such as the one human activity has almost certainly induced. Unlike natural background extinctions the current increased rates of extinction may be non-random i.e it may not just prune the branches of life but wipe out related branches of the tree of life causing similar types of organisms to selectively disappear. The overall structure of the tree of life may change with certain ecosystems or tiers within ecosystems disproportionately emptied than others, causing genuine reductions in biodiversity. Something like this may already be going on, for example amphibians have been assessed to be at greater risk from global warming than other vertebrate groups. Anyways its great to see the dolphins back in Punjab. May the force be with them.
Meanwhile in another wetland that dried up some time back, this sighting of an incredible rare creature.
From NASA Mars explorer image, what looks like a humanoid female clambering down the slope towards a dried up lake bed in search of .... maybe water, food or dry ice? The Indian media has had a field day with this with experts coming and giving long talks on the possibility of martian life. Amazing stuff!