Researchers claim that `` humans have caused mass extinction since the end of the Cretaceous period ''
It is said that life was born on the earth about 3.5 billion years ago. Since then, many creatures have prospered and diversified to form various ecosystems. Of course, there are some extinct creatures behind the birth of new creatures, but the mass extinction of a large number of creatures at a certain time is called “ mass extinction ”. Researchers claim that such mass extinction is caused by humans.
What is a 'mass extinction' and are we in one now?
Mass extinction is usually defined as the extinction of more than 75% of the species living on the earth in a short period of geology, “less than 2.8 million years”. Considering from 540 million years ago when biodiversity greatly expanded due to the “ Cambrian explosion ”, there were five cases that met this mass extinction standard.
The past five mass extinctions occur on average at intervals of about 100 million years, but there is no specific pattern in the timing of occurrence, and the period of extinction proceeds from 50,000 to 2.76 million years . The first mass extinction is said to be the end of the Ordovician mass extinction that occurred about 444 million years ago. At this time, 85% of all species are extinct, and it is considered that the Earth has entered the ice age.
The second mass extinction was a late Devonian mass extinction that occurred about 374 million years ago. At this time, 75% of the species were extinct, mainly in invertebrates living on the tropical seabed. . During this period, various environmental changes seemed to have occurred, such as sea level rise, temperature changes, and the number of plants in dry areas began to increase and the concentration of carbon dioxide in the atmosphere decreased.
The third mass extinction was the end of the Permian mass extinction that occurred about 251 million years ago, which is said to be the largest mass extinction in the history of the earth, with 95% of the species extinct. The The cause of mass extinction is that there are theories that greenhouse gases from volcanic activity have drastically changed the global environment, and that the amount of carbon dioxide in the atmosphere has increased and oxygen in the ocean has become deficient.
The end of the Triassic mass extinction that occurred 50 million years after the end of the Permian mass extinction is thought to have been caused by large-scale crustal movements that formed the Atlantic Ocean and accompanying volcanic activity, and 80% of the species The neighborhood was extinct.
In the end of the Cretaceous mass extinction known as the most famous mass extinction , 76% of species including dinosaurs were extinct. The most prominent cause of this mass extinction is said to be an asteroid that collided near the Yucatan Peninsula , Mexico. In this way, you can see that the Earth has been in danger of mass extinction many times.
At first glance, this mass extinction is a large-scale event involving a drastic change in the global environment, and it seems that it does not apply to the modern earth. However, researchers such as Frédérik Saltré, a researcher at Flinders University , pointed out that a large number of species have been extinct due to human activities on the earth in recent years. I think it can be. '
Whether or not the present earth is in the midst of mass extinction can be examined by comparing it with “the extinction rate of living things at normal times, not the time of mass extinction”. Researchers estimated how many organisms had been extinct in the past using fossil records, etc., and estimated a certain range, `` the number of species that are extinct in 1 year is 0.1 per 1 million species The ratio of “two” was derived.
When applying the extinction rate in recent years to the “extinction rate in the normal state” derived in this way, it was found that in the last few hundred years, it was found that the organism was extinct at a rate of 10 to 10,000 times. Pointed out.
According to a 2014 study, there are 322 vertebrates living on land that have been extinct since 1500, and 1.2 species have been extinct in about two years. In addition, in the modern age when many animals and plants are said to be in danger of extinction, there is always a significant decline in population and distribution area before the organism is extinct. More serious. There have been reports that the number of vertebrate individuals has declined by 60% across the globe when viewed since 1970.
Biologists are debating how modern extinction rates exceed normal ones, but even a fairly conservative estimate can show the loss of biodiversity typical of mass extinctions. It is clear. From these results, Saltré argued that 'the sixth mass extinction is already underway.'
by Tom Swinnen