Mass extinction refers to the loss of about three quarters of all species on the planet over a short period of time. There’s a scientific consensus that five mass extinction events have occurred within the last 450 million years and that we’re currently in the midst of a sixth mass extinction.
Mass Extinction Definition
Mass extinction is defined as a loss of about three quarters of all species in existence across the entire Earth over a “short” geological period of time. Given the vast amount of time since life first evolved on the planet, “short” is defined as anything less than 2.8 million years.
Mass Extinction Events
There’s a scientific consensus that the planet has undergone five major mass extinction events within the last 450 million years, with each destroying 70-95% of the species of plants, animals and microorganisms that existed previously. Each of the five mass extinction events happened on average about every 100 million years or so, ranging between 50,000 years to 2.76 million years.
The causes behind each mass extinction event vary, as do the resulting consequences. In the past, mass extinctions have been caused by massive volcanic eruptions, depletion of ocean oxygen or collision with an asteroid, and each took millions of years to recover the numbers of species comparable to those before the extinction event.
The First Mass Extinction Event
The first ever mass extinction event occurred about 443 million years ago, which wiped out more than 85% of all species on the planet at the time. Referred to as the Ordovician–Silurian extinction event, the event saw 27% of all families, 57% of all genera, and 60%-70% of all species including marine species like graptolites, brachiopods and conodonts, destroyed.
Studies have suggested that the first extinction event is caused by a global-scale ice age, followed by rapid global warming. However, according to a 2020 study, it counters that the event is in fact a result of volcanism, which led to cooling, warming, or both, and anoxia (referring to conditions of low oxygen).
The Second Mass Extinction Event
About 374 million years ago during the Late Devonian period, the second mass extinction event occurred, taking 75% of all species along with it. This extinction period decimated marine species and bottom-dwelling invertebrates in tropical seas in particular.
Though it’s difficult even to this day to accurately pinpoint the cause of the Devonian extinction, scientists have linked the event to several stresses including excessive sedimentation, rapid global warming or cooling, and high variations in sea levels. Plants during this period have also started taking over dry land, resulting in a drop in global carbon dioxide levels.
The Third Mass Extinction Event
The third mass extinction event is also the most devastating extinction event in history, killing off more than 95% of all species living at the time. Referred to as the “great die-out” or the “great dying”, the event took place around 250 million years ago at the end of the Permian period, and wiped out 96% of all marine species and around 70% of terrestrial species, including plants and insects.
The general consensus view of scientists is that the volcanic activity at the end of the Permian period, specifically in relation to the volcanic eruptions in the Siberian Traps, led to significant emissions of greenhouse gases into the atmosphere, resulting in rapid global warming, as well as increased ocean acidification. However, other studies have suggested that impact from an asteroid filled the air with pulverised particles, which possibly blocked up the sun and generated intense acid rains.
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The Fourth Mass Extinction Event
The fourth mass extinction event, or the Triassic-Jurassic extinction, occurred about 252 million to 201 million years ago, eliminating some 76% of all marine and terrestrial species from the planet. It is thanks to this extinction event that dinosaurs became the dominant land animals on Earth.
Like with other events, there is much debate surrounding the cause of the Triassic-Jurassic extinction, ranging from rising sea levels and spike in carbon dioxide emissions due to volcanism, to trapped methane gases in permafrost released rapidly warming the planet, to colossal geological activity in the Atlantic Ocean that elevated atmospheric carbon dioxide concentrations and acidified oceans. There are even some that argued that the fourth mass extinction event was not the product of a single major event but “simply a prolonged turnover of species across a considerable amount of time and thus should not be regarded as a mass extinction event.”
The Fifth Mass Extinction Event
The last recorded mass extinction event happened about 65.5 million years ago, and famously wiped out the dinosaurs – with the exception of birds – from existence. During the end-Cretaceous extinction, nearly 76% of all species were destroyed, including flying pterosaurs and other important marine invertebrates such as ammonites and groups of cephalopods. Similar to how the Triassic-Jurassic extinction allowed dinosaurs to dominate the land, the end-cretaceous extinction brought on a new age for mammals, allowing them to diversify and occupy new habitats.
There is strong geological evidence that the demise of dinosaurs and the fifth mass extinction event are caused by an asteroid that was as large as 10 km in diameter hitting the Earth, specifically in the Yucatan Peninsula of Mexico. The impact of the asteroid produced a dust cloud that encircled the planet, which prevented sunlight from reaching the Earth’s surface, thereby reducing plant’s and plankton’s ability to photosynthesis, and affecting the rest of the food chain. Dust and debris from the impact would have also triggered widespread wildfires.
There has been recent debate regarding the role of volcanic activity in the Deccan Province of west-central India in the fifth mass extinction event. However, a number of new studies including one published in September 2021 have debunked those theories, confirming that a giant meteorite impact is what caused the great biological crisis 66 million years ago.
A Sixth Mass Extinction Event?
An estimated 2% of the species that ever lived remain alive today following the five mass extinction events. But due to ever-increasing global populations, consumption rates, human activity including deforestation and worsening impacts of climate change, we are driving even more animals and plants to the brink of extinction.
A recent UN report found that species extinction rates are record high and “accelerating”. The world is currently losing species hundreds of thousands of times faster than the ‘normal’ rates occurring in the last tens of millions of years, meaning that we’re very much in the midst of a sixth mass extinction event.
More than 500 species of land animals are under threat of extinction and will likely be lost within the next two decades; the same number were lost over the whole of the last century, according to a 2020 study. As more species disappear, it affects its entire ecosystem and the services that they provide to humanity, threatening the survival of the human race as well.
Featured image by: Max Pixel