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The Big Five Extinctions… and are we causing #6?

The dodo – that poor, lumbering pigeon of Mauritius – has long been the poster child for extinction, but it was certainly not the first animal species to go extinct. Not by a long shot. In fact, of all the species that have ever lived, 99.9% are now extinct. Some of these died out completely, unable to compete and survive in a changing environment, whereas others evolved into entirely new species (the latter scenario is sometimes known as pseudoextinction and considered separately).

The ill-fated dodo, surrounded by other similarly-doomed bird species, Source: Wikimedia Commons

Extinctions in the fossil record

Imagine you’re taking a careful look at the fossil record. It’s a bit like taking a stroll through geological time, a historical journey spanning millions upon millions of years. You watch as certain species suddenly disappear from the record, never to be seen again, and this is quite a normal occurrence. But there are periods in our past when something happened: life on Earth received a shake-up of absolutely catastrophic proportions, and entire groups of species were abruptly wiped from existence. These are called mass extinctions.

The most impressive of these events are known as The Big Five:

The “Big Five” extinction events

Time (millions of years ago)

Proposed causes

Estimated % of species that went extinct

End Ordovician

450-440

global cooling, sea-level fall

84

Late Devonian

375-360

sea-level changes, ocean anoxic event

79

End Permian

251

flood basalt event (volcanism), methane hydrate gasification

95

End Triassic

205

climate change, volcanic eruptions

79

End Cretaceous

65.5

asteroid impact event

70

You’ll be familiar with at least one of these: the End Cretaceous event was triggered by an asteroid impact and is responsible for the demise of the dinosaurs. However, since modern-day birds are directly descended from dinosaurs, you could argue that some dinosaur species are only pseudoextinct.

But that’s small-fry compared to what happened 200 million years earlier. The champion of extinction events took place at the end of the Permian period, just before dinosaurs came onto the scene. Its causes are still up for debate, but what we do know is that 70% of terrestrial vertebrates and 96% of all marine species went extinct. This was catastrophic, a disaster that stripped our planet bare and left it lying in the gutter. In fact, it took life on Earth about 10 million years to recover from so dramatic a blow.

the rate of marine-life extinction (families/million years) across geological time, showing the “Big Five” mass extinctions: End Ordovician (a), Late Devonian (b), End Permian (c), End Triassic (d), and End Cretaceous (e). Source: Hallam, A.W. & Wignall (1997)

The rate of marine-life extinction (families/million years) across geological time, showing the “Big Five” mass extinctions: End Ordovician (a), Late Devonian (b), End Permian (c), End Triassic (d), and End Cretaceous (e). Source: Hallam & Wignall (1997)

Are we causing #6?

We’d like to think that the worst is behind us, but I’m not so sure. We’re currently living in what could easily be considered the sixth mass extinction, and regrettably it’s all our fault. Hunting, habitat destruction, and human-induced climate change have caused extinction rates to sky-rocket to between 100 and 1000 times the “background” extinction rate. Just to give one example, about one-third of all amphibian species are currently at risk of being lost forever.

So great has been our influence that many scientists have adopted the term “Anthropocene” to describe the period when humans started radically interfering with natural ecosystems. We’re now about 200 years into the Anthropocene, and with no end in sight. Life on Earth is already on the ropes – are we delivering the knock-out blow from which it will never recover?

What do you think?

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