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Extinction

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Extinction

Why do species die out? What happens after the extinction of organisms? Are humans causing mass extinction? In this article, we will discuss extinction: what it is, why it occurs, what impact human activities have on extinction, and some examples of extinction.

What is the definition of extinction?

Extinction occurs when all members of a species have died. Before a species becomes extinct, it can become locally extinct, extinct in the wild, or functionally extinct.

A species is considered locally extinct when it has died out in a particular geographical area.

The brown bear (Ursus arctos) is locally extinct in California. It no longer has an existing population in the state but exists in other parts of North America. The California population is believed to have consisted of a now-extinct subspecies, known as the California brown bear (U. a. californicus).

A species is considered extinct in the wild when they are no longer found in the areas they used to inhabit and are only found in captivity.

The northern white rhinoceros (Ceratotherium simum cottoni) (Fig. 1) is a subspecies of rhinoceros that is extinct in the wild, and the only surviving individuals are two females kept in the Ol Pejeta Conservancy in Kenya.

A species is considered functionally extinct if there are not enough members to fulfill its ecological niche and sustain a population.

Wild oysters are considered functionally extinct in certain parts of the world because overfishing has already destroyed over 85% of all the oyster reefs. Oysters sold in markets are most likely farmed, which means that they no longer fulfill their role in the ecosystem.

Extinction: Northern white rhinoceros | StudySmarter

Figure 1. This is an image of 'Sudan', the last male northern white rhinoceros. Sudan died in 2018. Source: Make it Kenya/Stuart Price, Public domain, via Wikimedia Commons.

Some events can cause the extinction of more than one species. Co-extinction is the disappearance of a species as a result of another disappearance. For example, the extinction of prey could also cause the extinction of the predatory species. On the other hand, mass extinction is the disappearance of over half the extant species worldwide. Paleontologists believe that there have been five mass extinctions, based on the sudden and dramatic disappearance of more than half of extant species in five strata in the fossil record.

Speciation and extinction have been taking place throughout the history of the world. These processes have shaped the evolutionary changes documented in the fossil record. While it takes thousands to millions of years to recover from mass extinction events, these create gaps in the Earth's ecosystems, where new groups of organisms can emerge.

Mass extinctions are often followed by adaptive radiation, the process in which a single species rapidly evolves into a large number of species adapted to different ecological niches. This happens because these organisms can colonize new regions with little competition. For example, the extinction of the non-avian dinosaurs allowed mammals to emerge and flourish.

What are the causes of extinction?

In this section, we will discuss a few possible environmental and genetic factors that can cause extinction.

Environmental factors

Natural disasters have led to many extinction events in the history of the world.

An example of this is the end-Cretaceous extinction event, which occurred around 66 million years ago. The end-Cretaceous extinction event was likely caused by the impact of a large meteor off the present-day Yucatan Peninsula. This event likely caused non-avian dinosaurs to become extinct.

While there is more evidence for the meteor impact, there are other theories behind the non-avian dinosaur extinction. One such theory points to a series of huge volcanic eruptions in western India called the Deccan Traps. The Deccan Traps unleashed over 264,000 cubic miles (or 1.1 million cubic kilometers) of lava in a span of 750,000 years.

Another example of how natural disasters can cause extinction can be observed in the case of the end-Permian mass extinction event. Scientists believe that it was caused by a massive volcanic activity that made the oceans anoxic (lacking in oxygen), causing about 96% of all marine species to suffocate and die out.

Did you know that not all dinosaurs are extinct? Some dinosaurs survived the end-Cretaceous extinction event. Non-avian dinosaurs died out, but many avian dinosaurs survived and eventually evolved into what we know as modern-day birds.

Genetic factors

When a population becomes very small, it is prone to losing genetic variation due to inbreeding and genetic drift, pulling the population into an extinction vortex, where the population shrinks until no more individuals survive.

Inbreeding is when closely-related individuals reproduce. Offspring produced through inbreeding have little genetic variation, making them more prone to diseases and defects. They are also more likely to suffer from genetic conditions caused by inheriting two recessive alleles.

Genetic drift is when chance events cause changes in allele frequencies. Unlike natural selection, where individuals with traits that help them survive in their environment can have more offspring because of those traits, genetic drift causes allele frequencies to change at random.

If conditions are stable, the population may survive. However, if conditions change –due to the emergence of infectious diseases, for instance– the population may be unable to cope. This is because with less genetic variation it is less likely that there are individuals with the traits needed to survive under the new conditions.

Populations of the greater prairie chicken (Tympanuchus cupido) that used to inhabit the western prairies of North America were greatly reduced as a result of habitat loss, due to land cultivation. In the 19th century, there were millions of greater prairie chickens in Illinois but by 1993, no more than 50 individuals survived. Ronald Westmeier and other scientists found that the population declined so quickly because of reduced genetic variation and fertility, which manifested in decreased egg viability. To address the loss of genetic variation, they exposed the Illinois population to prairie chicken populations from neighboring states. This led to an increase in the viability of their eggs and, eventually, an increase in the population.

Are humans causing extinction?

It is believed that the sixth mass extinction —referred to as the Anthropocene period— has already begun as a result of human activities. Habitat loss, overexploitation by hunting and harvesting, and the introduction of invasive species are just some of the human-induced factors leading to extinction. In fact, scientists believe that human-induced extinctions are about 1,000 times higher than background extinction rates.

Such extinction rate can be attributed to the explosive growth in the human population: from approximately 1 billion in 1850 to over 7.8 billion in 2020. The world's population is projected to grow to 10 billion by 2050. The growth of the human population has led to massive habitat loss.

Habitat loss has resulted in the extinction of many species (Fig. 2) as it reduces the survival rate of organisms. Without their habitat, the resources (food, territory, etc.) that they need to survive are lost as well. While habitat loss can be caused by nature, for instance, by natural disasters, human activities account for most habitat loss in modern times. An example of habitat loss is deforestation, which in most cases is the result of human land use (for agriculture, extraction of resources, land conversion, etc.).

Figure 2. Habitats like this old-growth forest in Armenia are destroyed for human activities. Source: Serouj (courtesy of Hrayr Savzyan), CC0, via Wikimedia Commons.

Furthermore, with the increase in levels of greenhouse gases, the world's climate has been changing. Rising surface temperatures are expected by the middle of the 21st century, forcing many species to seek habitats with similar climate conditions up mountain slopes and closer to the poles.

Background extinction refers to the extinction that occurs continually. It is distinguished from mass extinction where the decline in diversity is sudden and dramatic. Scientists estimate the background extinction rate to be around 1 per million species per year.

What are some examples of extinction caused by humans?

Let's discuss some examples of extinct species and the story behind their extinction.

The dodo bird

The dodo bird (Raphus cucullatus) used to inhabit the forests of the island of Mauritius (Fig. 3). The dodo bird was hunted down and consumed by sailors. It was easily hunted down because it approached people without fear. Pigs, rats, and dogs that the sailors brought with them also hunted down the young offspring of the dodo. The dodo became extinct in 1662.

Extinction: The Dodo Bird | StudySmarter

Figure 3. This is a plaster and wax model of the dodo bird. It was created by taxidermists of the Museum National d'Histoire Naturelle in Paris during the 19th century. Source: Jebulon, CC0, via Wikimedia Commons.

The passenger pigeon

The passenger pigeon (Ectopistes migratorius) used to inhabit the hardwood forests of the northern United States (Fig. 4). The species would migrate in flocks so huge that they would darken the sky. However, the species rapidly declined in the 19th Century due to overhunting and habitat loss.

When the forests that they inhabited were cleared for farming, they were forced to search for food in the farmlands, where they caused severe damage. In turn, the farmers hunted them down relentlessly. They were also hunted down and sold in the market for private consumption. By the 1890s, they were nearly wiped out in the wild. There were some conservation efforts made; for instance, a bill was introduced in Michigan which sought to illegalize hunting down passenger pigeons for ten years, but by then it was too late. In 1914, the last passenger pigeon died while in captivity.

Extinction: The Passenger Pigeon | StudySmarter

Figure 4. This is a model of the passenger pigeon found in the Pember Library and Museum in Granville, New York, USA. Source: Daderot, CC0, via Wikimedia Commons.

Where can I find the list of extinct species?

The International Union for Conservation of Nature and Natural Resources (IUCN) maintains a Red List of extinct and endangered species. The IUCN Red List is considered the most comprehensive data bank on the conservation status of various plant, animal, and fungi species.

Under the Red List, species are classified as:

  • Not Evaluated (NE)

  • Data Deficient (DD)

  • Least Concern (LC)

  • Near Threatened (NT)

  • Vulnerable (VU)

  • Endangered (EN)

  • Critically Endangered (CR)

  • Extinct in the Wild (EW)

  • Extinct (EX)

The list takes into consideration the pressures acting on the species and provides insight into the possible conservation measures that can be taken to prevent their extinction.

The IUCN lists the Sunda Pangolin (Manis javanica) as critically endangered and indicates that residential and commercial development, biological resource use, transportation and service corridors, and natural systems modification are the main threats to the species. It also assesses conservation actions. For instance, it shows that there are international management or trade controls in place, but there are no area based regional management plans tackling land/water protection.

While the list is not exhaustive, it describes over 380 vertebrate species that went extinct after 1500 AD. Notably, 86 of them were wiped out by overhunting or overfishing.

Extinction - Key takeaways

  • Extinction is the disappearance of a species throughout the world. It occurs when a species can no longer use its ecological niche for survival.
  • Speciation and extinction have been taking place throughout the history of the world. These processes have shaped the evolutionary changes documented in the fossil record.
  • Environmental and genetic factors can cause extinction. These include natural disasters and loss of genetic variation.
  • Human activities have also caused extinction through habitat loss, overexploitation, and the introduction of invasive species.
  • The IUCN maintains a list of extinct species and species threatened with extinction. It also indicates the conservation status of the species.

References

  1. Advanced Placement for AP Courses Textbook by Texas Education Agency
  2. The IUCN Redlist
  3. Campbell Biology Eleventh Edition by Person Higher Education
  4. Smithsonian on the passenger pigeon
  5. Columbia Climate School on wild oysters
  6. Britannica on extinction
  7. Princeton University on the Deccan Traps

Frequently Asked Questions about Extinction

Extinction occurs when all members of a species have died.

Environmental and genetic factors can cause extinction. These include natural disasters and loss of genetic variation.


Human activities have also caused extinction through habitat loss, overexploitation, and the introduction of invasive species.

Animals that are extinct include the dodo bird and the passenger pigeon.

Habitat loss, overexploitation by hunting and harvesting, and the introduction of invasive species are just some ways that humans are causing extinction.

Extinction has been occurring throughout evolutionary history, even before humans have existed. However, human activities have contributed to a dramatic increase in rates of extinction. We can prevent extinction by addressing activities that put tremendous pressure on species through conservation measures.

Final Extinction Quiz

Question

What does it mean for a species to be extinct in the wild?

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Answer

A species is considered extinct in the wild when they are no longer found in the areas they used to inhabit and are only found in captivity. 

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What does it mean for a species to be locally extinct?

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Answer

A species is considered locally extinct when it has died out in a particular geographical area. 

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What is co-extinction?

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Answer

Co-extinction is the disappearance of a species as a result of another disappearance.  

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What is mass extinction?

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Answer

Mass extinction is the disappearance of over half the extant species worldwide. 

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Why does adaptive radiation occur after mass extinction?

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Answer

While it takes millions of years to recover from mass extinction events, these create gaps in the Earth's ecosystems, allowing new groups of organisms to emerge through adaptive radiation.

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What is an extinction vortex?

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Answer

An extinction vortex is when a small population becomes even smaller due to inbreeding, genetic drift, and overall loss of genetic variation.

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How does loss of genetic variation cause extinction?

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Answer

With less genetic variation it is less likely that there are individuals with the traits needed to survive under changing environmental conditions.  

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What is considered the 6th mass extinction?

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Answer

Anthropocene period

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What is the Anthropocene period?

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Answer

It is the 6th mass extinction event which is caused by human activities

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Question

How does inbreeding lead to extinction?

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Answer

Offspring produced through inbreeding have little genetic variation, making them more prone to diseases and defects. They are also more likely to suffer from genetic conditions caused by inheriting two recessive alleles. These reduce their survival rate.

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Question

How does habitat loss lead to extinction?

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Answer

Habitat loss reduces the survival rate of organisms because the resources (food, territory, etc.) that they need to survive become limited.  

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Question

How did paleontologists identify the mass extinction events from the fossil record?

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Answer

Paleontologists believe that there have been five mass extinctions based on the sudden and dramatic disappearance of more than half of extant species in five strata in the fossil record.  

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Is diversity of life permanently lost after extinction?

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Answer

No

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Why is a small population prone to loss of genetic variation?

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Answer

A small population would be likely to undergo inbreeding where closely related individuals reproduce.

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How can genetic drift lead to extinction?

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Genetic drift would alter the allele frequency of a population at random. This means that the traits passed on to the succeeding generations will not give them the best chances of survival.

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This refers to the extinction that occurs continually as opposed to the sudden and dramatic extinction of many species.

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Background extinction

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What type of dinosaurs became extinct during the end-Cretaceous extinction event?

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Answer

Non-avian dinosaurs

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Not all dinosaurs became extinct after the end-Cretaceous extinction event. What are the surviving dinosaurs?

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Answer

Modern-day birds

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