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.

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.

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.

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

Figure 1. This is an image of 'Sudan', the last male northern white rhinoceros. Sudan died in 2018.

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.

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.

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.

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.

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.

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.

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.

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.

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.

Figure 4. This is a model of the passenger pigeon found in the Pember Library and Museum in Granville, New York, USA.

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.

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.