A coming sixth mass extinction?

Of the four billion species estimated to have evolved on the earth over the last 3.5 billion years, some 99% are gone. It is quite clear therefore that organisms evolve and go extinct constantly; extinction and evolution are two sides of the same coin.

Evolution is defined as change in the heritable characteristics of biological populations over successive generations. Evolution leads to biodiversity. On the other hand, extinction is the end of a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and recover may have been lost well before that point.

The concept of mass extinction is, however, different. Mass extinction entails the death of all, or a majority of, living organisms existing on the planet in a very short period of time — in a matter of days or months. However, in truth, a mass extinction event may extend over many thousands of years.

In a landmark paper published in 1982, Sepkoski and Raup identified five mass extinctions. They said that extinction rates have appeared to be more elevated at certain times in the history of the planet. They defined a mass extinction event as one where there was a loss of at least 75% of species within a geologically short period of time. There have been five mass extinctions, and these were near the end of the Ordovician, Devonian, Permian, Triassic and Cretaceous periods.

It must be mentioned that the assessment of the previous mass extinctions is based entirely on fossil data, which maybe wildly inaccurate. There are also several differing hypotheses for the mass extinction events. These range from fluctuations in the sea levels, global warming, global cooling, floods, volcanic eruptions and a giant asteroid impact. Some scientists also state that mass extinction may have been because of an inability to adapt.

This brings us to the question: are we in the middle of a mass extinction event? We know that the present rate of extinction may be upto 1,40,000 species per year, which is estimated at 100-1,000 times higher than the “base” or the typical rate of extinction. This differs from the previous mass extinction events in that one species is deemed to be responsible for the current rate of extinction.

Humans have been responsible for climate change and overpopulating the planet. The arrival of humans on different continents has coincided with extensive extinction of fauna. Madagascar is an example of mega extinction of fauna, which coincided with the arrival of humans. Humans have been considered to be a ‘global superpredator’ species that preys on the adults of other apex predators. Hunting the big cats is a classic example of this behaviour. This destructive nature of humans has worldwide effects on food webs.

Faster extinction rate

A critical question is whether current rates of extinction would produce yet another mass extinction in the same geological period of time? The answer is, ‘probably yes’.

The current extinction rates for mammals, amphibians, birds and reptiles as calculated over the last 500 years are faster than during the ‘Big Five’ extinctions of the past. Critics of the mass extinction theory would argue that the previous assessments of mass extinctions have been done over spans of millions of years. Here, we are assessing only over a few thousand years.

Further, the previous mass extinctions involved relatively primitive animals whereas in the present era, we are predominantly studying mammals. However, despite all these arguments, the fact remains that disappearance of species is much faster now as compared to that in the past.

Several scientists believe that we are in the middle of a mass extinction event, and have sought to come to terms with what it means. In 2012, a coalition of scientists announced that one-third of the world population must die to prevent wide-scale depletion of the planet’s resources — and that humankind needs to figure out immediately how it wants to go reducing its numbers by more than two billion!

They suggested that this is the only way to reverse the destructive effects of humankind. These scientists represented multiple fields of study, including ecology, agriculture, biology and economics. The theory behind this mad idea is that when the population of a species grows beyond the capacity of its environment to sustain it, nature further reduces that capacity below the original level, ensuring an eventual population crash.

Will humanity destroy one third of its population or would we like to wait for ‘mother nature’ to take the call? Or, are the massive advances in technology capable of helping humankind become invulnerable to the vagaries of nature? Unlikely.

Tsunamis and epidemic diseases are increasing in number and intensity. We should accept that we are living on the edge of chaos and that the balance will tilt sooner or later to one side. The questions that remain are, ‘when’ and to ‘which’ side the balance will tilt.

(The writer is a Senior Consultant, Surgical Pathology and Molecular Diagnostics, Neuberg Anand Reference Laboratory)