Extinction
In biology and ecology, extinction is the disappearance of all members of a species or group of taxa. A species is considered extinct from the moment the last individual of it dies. Because its potential biogeographic area may be very large, determining that time can be difficult, so it is usually done in retrospect. These difficulties can lead to phenomena such as the lazarus taxon, in which a species thought to be extinct abruptly reappears after a period of apparent absence. In the case of species that reproduce sexually, extinction is generally inevitable when only one individual of the species remains, or only individuals of the same sex.
Through biological evolution, new species arise through speciation, as well as other species become extinct when they are no longer able to survive in changing conditions or against other competitors. Typically, a species becomes extinct within ten million years of first appearing, although some species, called living fossils, survive virtually unchanged for hundreds of millions of years. Extinction is historical and usually a natural phenomenon. It is estimated that about 99.9% of all species that ever existed are now extinct.
Prior to the spread of humans across the planet, extinction generally occurred at a continuously low rate, and mass extinctions were relatively rare events. But approximately one hundred thousand years ago, and in coincidence with the increase in the population and the geographic distribution of humans, extinctions have increased to levels not seen before since the Cretaceous-Tertiary mass extinction. This is known as the Holocene mass extinction, and it is estimated that by the year 2100 the number of extinct species could reach high levels, even half of all species that currently exist.
Definition
A species becomes extinct when its last individual dies. Therefore, extinction becomes a certainty when there is no member capable of reproducing and giving rise to a new generation. A species can also become functionally extinct when only a very small number of members survive, which are incapable of reproducing due to health problems, age, geographical distance due to a very large range of distribution, due to lack of individuals of both sexes (in species that reproduce sexually), or other reasons.
In ecology, the term extinction is also often used informally to refer to a local extinction, in which the species ceases to exist in a given area, although it continues to live elsewhere.. This phenomenon is also known as extirpation. Local extinctions may precede a replacement of the species, from specimens elsewhere; the reintroduction of the wolf is an example of this. Species that are not extinct are called extant, and extant species that are threatened with extinction fall into categories such as threatened species, endangered species, or critically endangered species.
An important aspect of extinction today is human attempts to preserve endangered species, reflected in the creation of the conservation status “extinct in the wild” (abbreviated as extinct in the wild). EW). Species listed under this status on the IUCN Red List are not known to be living in the wild or naturally, and the only extant specimens are kept in zoos or other artificial environments. Some of these species are functionally extinct, as they are no longer part of their natural habitat and are unlikely to be reintroduced to the wild. Some institutions attempt to maintain a viable population for species that can be reintroduced to the wild if possible. their natural state through carefully planned breeding programs.
The extinction of a species can cause a knock-on effect in its natural habitat, causing the extinction of other species within it. This is also called "extinction chains".
Currently, many environmental organizations and governments are concerned about the extinction of species due to human intervention and ensure its prevention. Among the artificial causes of extinction are hunting, pollution, the destruction of their habitat, the introduction of new predators, among others.
Types of extinctions
There are two types of extinctions:
Terminal
It is one in which the disappeared species does not leave any kind of offspring anywhere, neither with its own DNA nor another evolved one. Dinosaurs have been included in this group for a long time, but since the 1980s the idea has been suggested that these reptiles, or at least a part of them, could leave offspring in birds. Terminal extinctions in turn are divided into two:
Mass extinction
This extinction, the most interesting for geologists and paleontologists, can develop in two ways: ending with 10% of the species or more in less than a year, and the one that needs up to three and a half million years, but it kills more than half of the species.
Background extinction
It is the most common and consists of the progressive disappearance of one or several species over hundreds or thousands of years until leaving no descendants. It is usually due to changes in the environment to which the species does not adapt, due to factors such as its excessive specialization, the case of big cats such as Smilodon may be an example, or to the modification of the conditions environmental, when it is faster than the adaptive response capacity of the species. For a long time it was thought that that of the dinosaurs was a case, but currently it points more to an abrupt extinction.
Phyletic or pseudoextinction
There may or may not be descendants of an extinct species. The descendant or daughter species evolves from the parent species with most of the genetic information of the latter, and even if the parent species becomes extinct, its descendant continues to exist, this is the case of the so-called phyletic evolution. or anagenesis. This fact is also called pseudoextinction.
Demonstrating a pseudo-extinction is difficult, because compelling evidence is required to link members of a given species to a pre-existing one. For example, Hyracotherium, an ancient animal that shared a common ancestor with the modern-day horse, is sometimes mentioned as being pseudo-extinct, as there are several extant species of Equus (the genus of the horse), including the zebra and the donkey. However, because fossil species do not usually leave genetic material behind, it is not possible to say whether Hyracotherium evolved in modern horse species, or simply from a common ancestor with modern horses. Consequently, pseudoextinction is usually easier to demonstrate for large taxa.
To differentiate terminal extinction from phyletic extinction (or pseudoextinction) it is necessary that it be clearly defined. If ultimately declared extinct, the species in question must be clearly distinguished from any other ancestor, descendant, or related species. The extinction of a species (or its replacement by a descendant) plays a key role in the punctuated equilibrium theory of Stephen Jay Gould and Niles Eldredge.
Causes
There are a variety of causes that can directly or indirectly contribute to the extinction of a species or a group of species.
Just as every species is unique, it is every extinction. The causes for each are varied—some subtle and complex, others obvious and simple—In Watching, from the Edge of Extinction
In simple terms, any species that is unable to survive or reproduce in its environment, and cannot move to a new environment where it is capable of doing these things, dies and becomes extinct. The extinction of a species can happen suddenly (for example, when pollution renders an entire habitat uninhabitable), or it can occur gradually over even hundreds of millions of years, as can happen when the species in question gradually loses its existence. competition for food against other better adapted species.
Assessing the relative importance of genetic versus environmental factors in causing extinction has been pitted against the inborn versus acquired debate. The question of whether extinctions in the fossil record have been largely caused by catastrophes or by evolution has been subject to discussion; for example, Mark Newman, the author of Modeling Extinction, presents a mathematical model for mass extinctions between the two positions. In contrast, conservation biology uses the extinction vortex model to Classify extinctions by their causes. When human extinction is raised, these causes are found with the effects of climate change or a technological disaster, which is shown for example in the book Our Final Hour (2003) by Martin Rees.
Currently, environmental groups and some governments are concerned about human-caused extinctions, and attempt to combat them through a variety of conservation programs. Humans can cause a species to become extinct due to the overexploitation of an environment, pollution, habitat destruction, the introduction of new predators or competitors, exaggerated hunting, among other reasons. The 2008 edition of the IUCN Red List lists 717 animal taxa and 87 plant taxa under the category "extinct" (EX), all extinctions occurring after the year 1500, although the number may be an underestimate. of the actual number of extinctions.
Demographic and genetic phenomena
Population genetics and demographic phenomena affect evolution, and consequently the risk of extinction of a species. Species with small populations are often much more vulnerable to extinction and these effects.
Natural selection works by propagating beneficial genetic traits for a species and eliminating deficient ones. However, a deleterious mutation can spread to an entire population through the effect of genetic drift.
A diverse gene pool gives a population a higher chance of surviving adverse changes in its environment. Consequently, effects that highlight the loss of genetic variability can increase the chances of extinction of a species. Bottlenecks can dramatically reduce genetic diversity, severely limiting the number of individuals able to reproduce and making inbreeding a more frequent occurrence. The founder effect can cause rapid individual speciation, and is one of the examples of a bottleneck.
Genetic contamination
Species that have historically and naturally developed in a specific region may be highly threatened due to the process of genetic contamination (for example, homogenization or replacement of local genotypes caused by introgression) that could lead to the introduction of animals or plants with a numerical or fitness advantage. In this case, non-native species can lead to the extinction of native animals or plants due to their deliberate introduction or destruction of their habitat, bringing species into contact with each other. that were previously isolated. This phenomenon can be especially detrimental to rare species that come into contact with more abundant ones, causing them to interbreed and modifying their gene pool, creating hybrids and driving the original native species to complete extinction.
Extinctions like these are not always evident from morphological observations alone (ie, by outward appearance). A certain degree of gene flow may be normal and a constructive evolutionary process, since it is impossible to preserve all constellations of genotypes and genes. However, hybridization with or without introgression can be a threat to the existence of rare species.
Pervasive genetic pollution also leads to a weakening of the natural development of the region-specific gene pool, causing weaker hybrid animals and plants unable to live in changing natural environments in the long term, ultimately leading to extinction.
The gene pool of a species or population is the complete set of unique alleles found by inspecting the genetic material of every living member of the species or population. A large gene pool indicates high genetic diversity, which is associated with robust populations that can survive intensive bouts of selection. Conversely, low genetic diversity (see inbreeding and bottleneck) can cause reduced biological fitness and increase the possibility of extinction among a reduced population of individuals of a pure and gaseous species.
Habitat degradation
The degradation of a species' habitat can alter the adaptive landscape to the point that that species is no longer able to survive and becomes extinct. This can happen due to direct effects (such as the environment becoming toxic) or indirect effects (limiting the ability of the species to compete against new competitors or due to a decreased amount of resources).
Habitat degradation due to toxicity can kill one or several species quickly, through contamination or causing sterility. This can also happen over longer periods of time with a continued low level of toxicity, affecting their life expectancy, reproductive capacity, or competitiveness.
On the other hand, habitat degradation can also take the form of physical destruction of the habitat. The widespread destruction of rainforest and its replacement with grazing land is widely cited as an example of this; the removal of dense forests also destroys the infrastructure that many species depend on for survival. For example, a fern that depends on dense shade for protection from direct sunlight cannot survive without trees to provide shade. Another example is the destruction of the ocean floor caused by trawling.
Resource decline or the introduction of new competing species can also often accompany habitat degradation. Global warming has also allowed some species to expand their range, causing competition with other species that previously occupied those areas. Sometimes these new competitors are predators and directly affect species as prey, while other times they take advantage of more vulnerable species in obtaining resources. These vital resources include water and food, and are often limited due to habitat degradation itself. All this can ultimately lead to the extinction in the natural or complete state of these species.
Predation, competition and disease
Humans have moved animals and plants from one part of the world to another for hundreds of years, sometimes deliberately (for example, cattle brought to various islands by sailors as a source of food) and sometimes deliberately. accidental (for example, rats escaping from ships). In most cases, these introductions are not successful, but when they become established as an invasive species, the consequences can be catastrophic. Invasive species can affect endemics, for example by preying on them, competing with them, indirectly destroying or degrading their habitat, or by introducing pathogens or parasites capable of making them sick or killing them. The same human populations can act as invasive predators. According to the "overhunting hypothesis", the rapid extinction of megafauna in areas such as New Zealand, Australia, Madagascar or Hawaii is the result of the sudden arrival of humans in these environments, with animals that had never come into contact before. with the former and that they were completely unadapted for their predatory techniques.
Coextinction
Coextinction refers to the loss of one species due to the extinction of another. An example would be the disappearance of a parasite species in the event of the extinction of its host. Coextinction can also occur when a species loses its pollinators, or predators in a food chain in the event of the disappearance of its prey.
Global warming
There is a discussion about how long-term global warming affects the disappearance of different species. Some studies show that it could lead to the extinction of even a quarter of all animals and plants by the year 2050.
It has been said that Hemibelideus lemuroides, found only in the mountains of northern Queensland, Australia, would be the first mammal to go extinct due to global warming. The species has not been seen in three years and an expedition carried out in early 2009 failed to find any specimens.
The human being in extinction
When extinction was described in a lecture by Georges Cuvier in 1796, the idea seemed threatening to those who believed in the chain of beings or scala naturae, a notion establishing the connection of living beings with God and did not allow any "link" in this connection to disappear. Consequently, the idea was not widely accepted before the 19th century.
This was also supported by the fact that at the time, the world was still not fully explored and mapped, so scientists could not rule out that species found in the fossil record were "hidden" in unexplored parts of the world. planet. Later, with the irruption of the theories of evolution, extinction was not considered problematic because it only ended with maladapted animals, thus the dodo came to be described. It would be in the XX century when extinction began to be considered a problem for the entire environment, including humans, by promoting the proliferation of pests, reducing the genetic variety and losing organisms of great intrinsic value due to their qualities or behaviors.
The possible sixth mass extinction
According to a 1998 survey of four hundred biologists by the American Museum of Natural History, seven in ten believed it was in the early stages of a human-caused mass extinction, known as the mass extinction. of the Holocene.
In the same survey, 70% believed that within the next thirty years (that is, around 2028) one-fifth of all species would become extinct. Biologist E.O. Wilson estimated in 2002 that if the current rate of human destruction of the biosphere continues, half of all species living on the planet will become extinct within a hundred years.
However, the fact that this event is something current prevents us from seeing the problem with the proper historical perspective, so there is a difference of opinion about it.
Planned extinctions
Humans have worked aggressively to extinct many species of viruses and bacteria in order to eradicate various diseases. For example, the smallpox virus is essentially extinct in the wild, while the virus that causes polio is confined to small locations around the world as a result of efforts to cure the disease it causes.
Biologist Olivia Judson, among other scientists, have advocated the deliberate extinction of some specific species, such as the Anopheles (which transmit malaria) and Aedes (malaria) mosquitoes (that transmit dengue, yellow fever, elephantiasis, among other diseases), arguing that its eradication could save at least a million people. To achieve this, he proposes the introduction of a genetic element capable of introducing itself into another crucial gene, creating a recessive knockout gene. The consequent reduction in the genetic diversity of the Culicidae family would reach 1%.
The fight against extinction
Some technologies that have little or no proven harmful effects on Homo sapiens can be devastating to wildlife (for example, DDT). Loss of native species is sometimes viewed by governments as harm to their ecotourism, so they enact laws that sanction the trade or hunting of these species in an effort to prevent extinction in the wild. Nature reserves are also created in order to provide a continuous and stable habitat for species threatened by human expansion. The 1992 Convention on Biological Diversity resulted in several international action plans, which attempt to provide guidelines for the conservation of biodiversity by different governments. Groups like the Alliance for Zero Extinctions work to educate the public and pressure governments to take action against the threat of different species.
Currently, extinction is a major research topic in the fields of zoology and biology in general, and has also become a topic of concern outside the scientific community. Organizations like the World Wide Fund for Nature were created with the aim of preserving species from extinction. Some countries have also tried to prevent the destruction of habitats, the overexploitation of soils, and pollution, through the enactment of laws and decrees.
Recent technological advances have encouraged the hypothesis that using the DNA of an extinct species the latter could be brought "back to life" through the process of cloning. Some proposed targets for cloning include the mammoth, the thylacine, the quagga and the dodo.
The concept, popularized by the novel and subsequent film Jurassic Park, was put to the test when the first cloned bucardo goat, a subspecies of goat from the Huesca Pyrenees, was born in 2003 after it became extinct in the year 2000. The specimen died about seven minutes later due to congenital lung problems.
Some species or subspecies extinct since 1900
Includes all animals confirmed by the International Union for Conservation of Nature and also adds a couple that have been declared extinct by other credible organizations and individuals. They are ordered by date, from the most recent date to oldest dates:
- 2022 Yangtse spatula, Psephurus gladius
- 2013 Leopardo nublado de Formosa, Neofelis nebula brachyura
- 2012 Giant Turtle of Pinta, Chelonoidis abingdonii
- 2011 Rinoceronte de Java Vietnamita, Rhinoceros sondaicus annamiticus
- 2000 Bucardo, Pyrenaica Pyrenaica Capra
- 1998 Nukupuu, Hemignathus lucidus
- 1997 Western black rhinoceros, Bicornis longipes
- 1996 Leopardo de Zanzibar, Panthera pardus adersi
- 1989 Golden Sapo, Incilius periglenes
- 1986 Pato Poc, Podilymbus gigas
- 1985 Christmas Island Musa, Crocidura trichura
- 1983 Monarch of Guam, Myiagra freycineti
- 1977 Colombian Zampullin, Podiceps andinus
- 1976 Java Tiger, Panthera tigris sondaica
- 1976 Dwarf hippotamus of Madagascar, Hippopotamus lemerlei
- 1975 Boa excavator of Redonda Island, Bolyeria multocarinata
- 1970 Great White Butterfly of Madeira, Pieris brassicae wollastoni
- 1970 Persian Tiger, Panthera tigris virgata
- 1970 Blue Pike, Sander vitreus glaucus
- 1964 Mexican brown bear, Ursus arctos nelsoni
- 1963 Kakawajie, Paroreomyza flammea
- 1962 Marmosa grácil of red belly, Cryptonanus ignitus
- 1960 Ualabí de central hare, Lagorchestes asomatus
- 1956 Western rabbly kangaroo, Onychogalea lunata
- 1952 Foca monk of the Caribbean, Monachus tropicalis
- 1951 Gacela of the Queen of Sheba, Gazella bilkis
- 1951 Japanese sea lion, Zalophus japonicus
- 1950 Bandicut of pork feet, Chaeropus ecaudatus
- 1949 Pink-headed duck, Rhodonessa caryophyllacea
- 1942 Lion of Atlas, Panthera read
- 1941 Arabian oats, Struthio camelus syriacus
- 1940 Perlific oyster shaped like arch, Arkish Epioblasma
- 1939 Grey's kangaroo, Macropus greyi
- 1938 Schomburgk Deer, Rucervus schomburgki
- 1936 Tiger of Tasmania, Thylacinus cynocephalus
- 1928 Syrian wild ass, Equus hemionus hemippus
- 1927 Caucasian Bison, Bison bonasus caucasicus
- 1923 Common buffalo, Alcelaphus buselaphus buselaphus
- 1922 Red mustache dove, Ptilinopus mercierii
- 1914 Dove migratory, Ectopistes migratorius