SafekipediaExtinction event
Adapted from Wikipedia · Discoverer experience
An extinction event, also known as a mass extinction or biotic crisis, is a time when many kinds of living things quickly disappear from Earth. During these events, the variety and number of plants, animals, and other organisms drop sharply. Scientists notice these events because the speed at which species disappear becomes much faster than the normal, slower rate of extinction.
Scientists think there have been many major mass extinctions in the last 540 million years, but they don't all agree on exactly how many. Some believe there were as few as five big events, while others think there might have been more than twenty. These differences happen because it can be hard to decide what counts as a "major" extinction and because scientists use different ways to measure how many types of life existed long ago.
These events help us understand how Earth changes and how life can recover after big challenges. Studying them also helps scientists think about how we might protect the variety of life today.
The "Big Five" mass extinction events
In 1982, scientists Jack Sepkoski and David M. Raup studied how life on Earth has changed over time. They found five big moments when many plants and animals disappeared quickly. These events are called the "Big Five" mass extinctions. They happened at different times during a long period called the Phanerozoic Eon.
The "Big Five" happened at the ends of four time periods and one big moment in the middle of a period. They are:
- Late Ordovician mass extinction, 445–444 million years ago
- Late Devonian mass extinction, 372–359 million years ago
- Permian–Triassic extinction event, 252 million years ago
- Triassic–Jurassic extinction event, 201.3 million years ago
- Cretaceous–Paleogene extinction event, 66 million years ago
Each of these events caused many kinds of animals and plants to disappear. For example, the extinction at the end of the Cretaceous period ended the time of the dinosaurs. After each big extinction, new kinds of animals and plants slowly appeared and filled the places left empty.
Sixth mass extinction
Main articles: Holocene extinction and Biodiversity loss
Scientists believe we are now living through a sixth mass extinction, which is happening because of human activities. Since 1900, animals and plants have been disappearing much faster than normal. This is largely due to how humans use the Earth's resources.
Many species are at risk of disappearing forever. Studies show that at least 1 million kinds of plants and animals could be lost in the next few years if we do not change our ways. This is happening much faster than it would naturally.
Extinctions by severity
Main article: List of extinction events
Scientists study extinction events by looking at changes in Earth’s rocks, the balance of plants and animals, and how many new species appear compared to how many disappear. Early studies often focused on groups of related animals, but later work used smaller groups for more accurate results. Some of the biggest extinctions in history are known as the “Big Five,” and they are highlighted in research papers. These events show how Earth’s biodiversity has changed over millions of years.
| Extinction name | Age (Ma) | Sepkoski (1996) Multiple-interval genera | Bambach (2006) | McGhee et al. (2013) | Stanley (2016) | |
|---|---|---|---|---|---|---|
| Taxonomic loss | Ecological ranking | |||||
| Late Ordovician (Ashgillian / Hirnantian) | 445–444 | ~49% | 57%[d] (40%, 31%)[e] | 52% | 7 | 42–46% |
| Lau event (Ludfordian) | 424 | ~23% | – | 9% | 9 | – |
| Kačák event (Eifelian) | 388~ | ~24%[a] | – | 32% | 9 | – |
| Taghanic event (Givetian) | 384~ | ~30%[a] | 28.5% | 36% | 8 | – |
| Late Devonian/Kellwasser event (Frasnian) | 372 | ~35% | 34.7% | 40% | 4 | 16–20% |
| End-Devonian/Hangenberg event (Famennian) | 359 | ~28%[a] | 31% | 50% | 7 | [f] |
| Serpukhovian | 330–325~ | ~23% | 31% | 39% | 6 | 13–15% |
| Capitanian | 260 | ~47%[b] | 48% | 25% | 5 | 33–35% |
| Permian–Triassic (Changhsingian) | 252 | ~58% | 55.7% | 83% | 1 | 62% |
| Triassic–Jurassic (Rhaetian) | 201 | ~37%[c] | 47%[c] | 73% | 3 | N/A[g] |
| Pliensbachian-Toarcian | 186–178 | ~14% | 25%, 20%[e] | – | – | – |
| End-Jurassic (Tithonian) | 145 | ~18% | 20% | – | – | – |
| Cenomanian-Turonian | 94 | ~15% | 25% | – | – | – |
| Cretaceous–Paleogene (Maastrichtian) | 66 | ~39% | 40–47% | 40% | 2 | 38–40% |
| Eocene–Oligocene | 34 | ~11% | 15.6% | – | – | – |
The study of major extinction events
Breakthrough studies in the 1980s–1990s
For much of the 20th century, studying mass extinctions was difficult because there wasn’t enough data. Scientists thought these events were just rare exceptions to slow, gradual changes in life on Earth. This changed in 1980 when a team led by Luis Alvarez found evidence of an asteroid impact at the end of the Cretaceous period. Their Alvarez hypothesis for the end-Cretaceous extinction brought new attention to the idea that sudden, catastrophic events could cause mass extinctions.
Another important study came in 1982 when David M. Raup and Jack Sepkoski published work showing five major peaks in extinctions of marine animals. These peaks occurred at the ends of the Ashgillian, Late Permian, Norian, and Maastrichtian periods, as well as during the Devonian period. Their work helped scientists see mass extinctions as important parts of Earth’s history.
New data on genera: Sepkoski's compendium
After Sepkoski died in 1999, his collection of data on marine animals was published in 2002. This led to many new studies about how mass extinctions affected life on Earth. Scientists used this data to look at how new species appeared at the same time as extinctions. Some found that mass extinctions happened in quick bursts, which had a big effect on biodiversity. Others studied how different groups of animals responded to these events.
Tackling biases in the fossil record
As more data became available, scientists realized that the fossil record didn’t always show the true timing or severity of extinctions. For example, the last fossil of a species might appear before an extinction event, but the species could have died out because of that event. This is called the Signor-Lipps effect. To deal with such challenges, scientists developed new methods to estimate diversity more accurately. These methods help correct for problems like incomplete fossils or uneven sampling over time.
Uncertainty in the Proterozoic and earlier eons
Because most life on Earth is made of tiny microbial organisms, it is hard to find evidence of early extinction events in fossils. Most known extinction events happened during a time called the Phanerozoic eon, when animals with bones and shells lived. One exception is the Oxygen Catastrophe that happened around 2.45 billion years ago during the Proterozoic eon.
Scientists think the background rate of extinctions — how often species naturally disappear — is about two to five groups of ocean animals every million years, based on what we can see in the fossil record. The Oxygen Catastrophe may have been the first big extinction, but because we know so little about life from that time, it is hard to compare it with later extinctions.
Evolutionary importance
See also: Evolutionary radiation and Macroevolution
Mass extinctions have sometimes sped up the evolution of life on Earth. When one group of animals takes over from another after an extinction, it is usually not because the new group is better, but because the old group was removed. This allows the new group to grow and take over.
For example, mammaliaformes and then mammals lived during the time of the dinosaurs, but they could not take over because the dinosaurs were too strong. The end-Cretaceous mass extinction removed the dinosaurs, which allowed mammals to grow and take over those places. The dinosaurs themselves had benefited from an earlier mass extinction, the end-Triassic, which removed their rivals, the crurotarsans.
Patterns in frequency
Some scientists think that extinction events happen in a repeating pattern, roughly every 26 to 30 million years. They have many ideas about why this might happen, like the influence of space objects or movements in our solar system. However, other scientists believe there isn't strong evidence for these patterns and that mass extinctions don't follow a regular schedule.
Mass extinctions often happen when a long-term problem is made worse by a sudden event. Over time, some groups of animals have become better at surviving because of changes in their food chains and other factors. Even so, the rate at which new species appear and old ones disappear has slowly gone down over the past 500 million years.
Causes
There is still debate about the causes of all mass extinctions. In general, large extinctions may result when a biosphere under long-term stress undergoes a short-term shock. High diversity leads to a persistent increase in extinction rate; low diversity to a persistent increase in origination rate. These relationships likely amplify smaller perturbations to produce the global effects observed.
A good theory for a particular mass extinction should explain all of the losses, explain why particular groups of organisms died out and why others survived, provide mechanisms that are strong enough to cause a mass extinction but not a total extinction, and be based on events or processes that can be shown to have happened. It may be necessary to consider combinations of causes.
The most commonly suggested causes of mass extinctions include flood basalt events, sea-level falls, asteroid impacts, and various climatic changes. Flood basalt events, for example, could have produced dust and aerosols that inhibited photosynthesis, emitted sulfur oxides that caused acid rain, and emitted carbon dioxide leading to sustained global warming. Sea-level falls could reduce the area of continental shelves, disrupting marine life and weather patterns. Asteroid impacts could collapse food chains through dust and aerosols, and in some cases, cause acid rain and megatsunamis.
Other factors include global cooling or warming, anoxic events in the oceans, and hydrogen sulfide emissions. These events can stress ecosystems in different ways, often leading to significant losses of biodiversity. Human activities today are also causing concern, as they may be contributing to a potential sixth mass extinction through climate change, habitat loss, and overexploitation of resources.
Effects and recovery
The effects of mass extinction events varied widely. After a major extinction event, usually only weedy species survive due to their ability to live in diverse habitats. Later, species diversify and occupy empty niches. Generally, it takes millions of years for biodiversity to recover after extinction events.
The worst Phanerozoic event, the Permian–Triassic extinction, devastated life on Earth, killing over 90% of species. Life seemed to recover quickly after the P-T extinction, but this was mostly in the form of disaster taxa, such as the hardy Lystrosaurus. The most recent research indicates that the specialized animals that formed complex ecosystems took much longer to recover. It is thought that this long recovery was due to successive waves of extinction and prolonged environmental stress. Recent research indicates that recovery did not begin until the start of the mid-Triassic, four to six million years after the extinction.
In media
The term extinction-level event (ELE) has appeared in many stories. For example, the 1998 film Deep Impact shows what could happen if a comet hit Earth, calling it an ELE.
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