We know what extinction is: the complete and permanent loss of a species. This kind of thing is happening all the time on Earth; it’s just part of how our planet works. While it’s always sad to see a unique form of life go, the losses are usually balanced out by the appearance of new species, as organisms continue to evolve and change.
Sometimes, though, an unusually large number of extinctions happen in a relatively short period of time, in an event known as a mass extinction. Though there’s no exact definition for what counts as a mass extinction, it’s generally agreed that there have been five really big ones in the past 450 million years of our planet’s history.
The largest of these, the Permian-Triassic event, killed over 90% of the species that were living on Earth at that time. It was this massive disappearance of life that paved the way for the rise of the dinosaurs.
Because the dinosaurs are the most famous of the former inhabitants of our planet, the event that killed them – the Cretaceous-Paleogene event – is the most famous of the mass extinctions.
Scientists are still unsure what caused these relatively-sudden waves of extinction. Possible causes include naturally-occurring climate change; geological events, like volcanic eruptions; disasters originating in space, like asteroid strikes; and, in the case of the more recent extinction events, hunting by early humans.
Many scientists agree, though, that we are witnessing a major extinction event right now. The Holocene extinction has been going on since 1900, with species vanishing 1,000 times faster than they normally do. Scientists likewise agree that this is due to human activity, including human-caused climate change; deliberate killing of animals through overhunting and overfishing; widespread destruction of habitat; and introduction of non-native species, which can overwhelm and outcompete species that haven’t met them before.
This enormous loss of diversity on our planet is sad, and it is avoidable, if we choose to take action. If we choose to do nothing, the results may be catastrophic. Because of the complex ways in which we rely on other forms of life, experts say that if we continue to lose species at the current rate, we ourselves are likely to be one of the casualties.