Extinction event

An extinction event is a period of rapid and widespread loss of biodiversity on Earth. Scientists also refer to these events as mass extinctions or biotic crises. These events occur when the rate of species disappearing exceeds the rate of speciation, which is the process of new species forming. A mass extinction is specifically identified by a sharp decline in the abundance and variety of multicellular organisms. While some species go extinct all the time, this is known as the background extinction rate. A mass extinction event represents a sudden spike far above that normal level.

The mechanics of these events often involve sudden changes to the global environment. For instance, during the Late Devonian extinction, nutrients were carried into the oceans by rivers. This process triggered massive algal blooms, where algae grew in huge quantities. As this algae died and decomposed, it consumed the dissolved oxygen in the water column. This created anoxic conditions, meaning the water lacked enough oxygen for life. Other events have been linked to volcanic activity, global warming, or changes in atmospheric carbon dioxide.

Scientists Jack Sepkoski and David M. Raup published a landmark paper in 1982. They identified five specific geological intervals that experienced excessive loss of diversity. These are commonly known as the "Big Five" mass extinctions. Originally, these periods appeared as outliers on a trend of decreasing extinction rates during the Phanerozoic Eon. As researchers applied more stringent statistical tests to fossil data, these five events were confirmed. These events are not necessarily separate incidents but may exist on a smooth continuum of extinction events.

The first major event was the Late Ordovician mass extinction, occurring 445 to 444 million years ago. This event killed 27% of all families, 57% of all genera, and 85% of all species. Some studies suggest volcanism caused global warming and anoxia, while others suggest cooling and glaciation were the drivers. The second was the Late Devonian extinction, occurring between 372 and 359 million years ago. This was actually a series of events, including the Kellwasser Event and the Hangenberg Event. Together, they eliminated 19% of families, 50% of genera, and at least 70% of species.

The third and largest event was the Permian–Triassic extinction, often called the "Great Dying." Occurring 252 million years ago, it wiped out 53% of marine families and 84% of marine genera. It also claimed about 81% of all marine species and 70% of terrestrial vertebrate species. This event was so massive it ended the dominance of early synapsids on land. In the oceans, the percentage of sessile animals, which are organisms unable to move, dropped from 67% to 50%. This massive loss of life created vacant niches that allowed archosaurs to eventually become dominant.

The fourth event was the Triassic–Jurassic extinction 201.3 million years ago. This event resulted in the loss of 23% of families and 48% of genera. It eliminated many large amphibians and archosauromorphs. This reduction in competition allowed dinosaurs to become the primary animals on land. The fifth and final "Big Five" event was the Cretaceous–Paleogene extinction, or K–Pg extinction. This event killed 17% of families, 50% of genera, and 75% of all species. It famously caused the extinction of all non-avian dinosaurs and many marine groups like ammonites and mosasaurs.

Understanding these patterns is vital because scientists believe a sixth mass extinction is currently underway. This modern event is driven by human activities, including population growth and the overconsumption of natural resources. Since 1900, extinction rates have occurred at over 1,000 times the background rate. A 2023 study showed that at least 73 genera of animals have gone extinct since 1500 due to human impact. Currently, an estimated 1 million plant and animal species are threatened with extinction. Studying the fossil record helps us understand how these biological crises reshape the entire history of life.