Wed. Oct 27th, 2021

It is now common to refer to the current biodiversity crisis as the sixth mass extinction. But is this true? Are we in the midst of an event on the same scale as the five ancient mass extinctions that the Earth has experienced?

Humans actually drive animals and plants to extinction. Soil cleaning, habitat change and, above all, climate change all put biological diversity under pressure.

Many species have become extinct since the arrival of man, and many more are endangered.

But to answer this question fully, we need to look at the speeds at which species became extinct before the appearance of humans and compare it with the speed of today.

Life on Earth has diversified from a single cell more than 3.7 billion years ago to the estimated 8.7 million species living today.

But as I describe in my book Extinctions: living and dying in the margin of error, this journey has been a roller coaster ride. There have been times when biodiversity exploded with many new species evolving relatively rapidly. Conversely, there have been extremely short time intervals in which biodiversity crashed into a mass extinction.

The extent of biodiversity loss in a mass extinction is extraordinary. In the five mass extinctions on Earth, estimates of species loss range from about 70% at the end of the Cretaceous to 95% at the end of the Permian, the largest of the mass extinctions.

The Gubbio section, found in Italy, represents a complete overview of the mass extinction of the Cretaceous.
The Gubbio section, found in Italy, represents a complete overview of the mass extinction of the Cretaceous. The exact moment when about 70% of the species became extinct is marked by the fact that the gorge runs diagonally across the image.
Author informed

Each of these events resulted in a wave of extinctions that swept across all of the planet’s ecosystems. Reefs were wiped out, dinosaurs disappeared, insect species were decimated and plants underwent massive upheavals. It took up to a million years for ecosystems to recover from a mass extinction.



Read more: Ocean ecosystems take two million years to recover from mass extinction – new research


Old and modern extinction rates

Estimation of prehuman extinction rates from the somewhat rough fossil record is fraught with. Nevertheless, scientists have managed to do so, albeit using only vertebrate fossils.

Their estimate suggests that before vertebrate species were on the verge of extinction at a rate of about two per cent. Million species lost each year.

In 2015, another research team took this estimate and compared it to today’s vertebrate extinction rate. They found that vertebrates became extinct 53 times faster today than they were before humans arrived.

If the increase in extinction rates recorded in vertebrates is on a similar scale across the entire planet’s biota, humans have triggered a significant increase in the number of extinct species.

But is that enough to consider our current biotic crisis a mass extinction?

Trilobites once filled the world's oceans, but died out at the end of the Permian.
Trilobites were ancient arthropods that filled the world’s oceans from the early Cambrian, about 520 million years ago, until the mass extinction at the end of the Permian, 252 million years ago.
Shutterstock / Tami Friday

To answer this question, we need to consult the Red List, which is run by the International Union for Conservation of Nature (ICUN). This list is an attempt to assess the threat of extinction of all known species by assigning each a category of declining threat: extinct or extinct in the wild, serious threat of extinction, endangered and so on.

Are we there yet?

A look at the red list confirms that, as with the old mass extinctions, today’s species loss affects the entire biosphere. However, the situation changes when we compare the current level of extinction with those of the five major mass extinctions.

A fossil of a plant that became extinct 550 million years ago.
This fossil (Arborea arborea) is part of the Ediacaran fauna from South Australia, which became extinct about 550 million years ago.
Author informed

As mentioned above, the loss of species during the ancient mass extinctions is massive. Data from the red list indicate that we have not come close to them. For example, the Red List assigns only 1.46% of mammal species to the extinct or extinct in the wild categories. It believes that less than 1% of amphibians are extinct or extinct in the wild. For insects it is 0.65%, bivalves 4% and corals 0%. This species loss level is not close to the losses in the fossil record.

While the rate at which species are becoming extinct has increased and the entire ecosystem is affected, we currently have at least only low extinction levels.

Unfortunately, species extinction levels show only part of the problem. To see the full extent of the crisis, we must add the species that the Red List considers endangered to those that are already extinct.

When we do, the picture changes. Together, the percentage of mammals that are extinct or threatened with extinction rises from 1.46% to 23.48%, amphibians rise to 33.56%, insects to 19.23% and corals to 26.85%. These numbers show the true extent of the threat facing the planet’s biosphere.



Read more: Animals disappear from forests, with serious consequences for the fight against climate change – new research


I do not like to refer to today’s crisis as a mass extinction because it allows us to focus entirely on extinction levels, and they are low. Others have invented a new concept to reflect the fact that although many species are extinct, many more are threatened with extinction: defaunation.

Defaunation better describes the crisis unfolding in the planet’s biosphere. In order to avoid slipping into a complete extinction of mass, we must not allow default to continue. We know how to do this: reduce emissions, protect vulnerable ecosystems and regenerate degraded ones.

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