Species That Have Disappeared

[Kizzy Burnworth]

Extinctionis the death of all members of a species of plants, animals, or other organisms. One of the most dramatic examples of a modern extinction is the passenger pigeon. Until the early 1800s, billions of passenger pigeons darkened the skies of the United States in spectacular migratory flocks. Easy to trap or shoot, passenger pigeons became a popular, cheap food. Commercial hunters killed them in vast numbers, eventually decimating the population. The last passenger pigeon, named Martha, died in the Cincinnati Zoological Garden in 1914, and was donated to the Smithsonian Institution.

At five other times in the past, rates of extinction have soared. These are called mass extinctions, when huge numbers of species disappear in a relatively short period of time. Paleontologists know about these extinctions from remains of organisms with durable skeletons that fossilized.

1. End of the Cretaceous (66 million years ago): Extinction of many species in both marine and terrestrial habitats including pterosaurs, mosasaurs and other marine reptiles, many insects, and all non-Avian dinosaurs. The scientific consensus is that this mass extinction was caused by environmental consequences from the impact of a large asteroid hitting Earth in the vicinity of what is now Mexico.

2. Late Triassic (199 million years ago): Extinction of many marine sponges, gastropods, bivalves, cephalopods, brachiopods, as well as some terrestrial insects and vertebrates. The extinction coincides with massive volcanic eruptions along the margins of what is now the Atlantic Ocean.

5. Late Ordovician (447 million years ago): Extinction of marine organisms such as some bryozoans, reef-building brachiopods, trilobites, graptolites, and conodonts as a result of global cooling, glaciation, and lower sea levels.

Smithsonian Paleobiologists continue to study the role that past extinctions had on plants, animals, and other species. Dr. Gene Hunt studies how the relatedness and diversity of organisms relates to what happens to them in an extinction event. Dr. Richard Bambach conducts research on variation in marine biodiversity in relation to different extinction events. By studying the evolution and extinction of tiny organisms called foraminifera, Dr. Brian Huber assesses how Earth's conditions have changed over time.

At the end of the last ice age, 10,000 years ago, many North American animals went extinct, including mammoths, mastodons, and glyptodonts. While climate changes were a factor, paleontologists have evidence that overhunting by humans was also to blame. Early humans worked cooperatively to trap and slaughter large animals in pits. About the same time, humans began farming, settling down and making drastic changes in the habitats of other species.

Starting in the 1800s, industrialization drove up extinction rates and has continued to do so. For example, Chinese river dolphins, foothill yellow-legged frogs, and sockeye salmon are among the many species currently endangered by water pollution, dams, and other industrial pressures on rivers. Smithsonian Anthropologist Dr. Torben Rick leads an effort to understand how human activities affect biodiversity by studying interactions between humans and other species in the Channel Islands from ancient to modern times.

The value of a species may be judged by various criteria, depending on who is making decisions about what to conserve. For example, cultural value is important in efforts to conserve populations of Pacific salmon. A Smithsonian exhibit in the Sant Ocean Hall shows how salmon for centuries have shaped a way of life for Native Americans living in the Pacific Northwest.

Recent improvements in genetic engineering have raised questions about bringing extinct species back to life. Since Dolly the sheep was cloned in 1996, scientists know it is possible to create an organism from the DNA in a single cell. Stored in museum collections throughout the world are specimens of extinct animals containing DNA. The idea of using DNA to revive extinct species and repopulating them is controversial. How would we choose which ones? How would they impact species still on Earth?

Extinction is the termination of a taxon by the death of its last member. A taxon may become functionally extinct before the death of its last member if it loses the capacity to reproduce and recover. Because a species' potential range may be very large, determining this moment is difficult, and is usually done retrospectively. This difficulty leads to phenomena such as Lazarus taxa, where a species presumed extinct abruptly "reappears" (typically in the fossil record) after a period of apparent absence.

More than 99% of all species that ever lived on Earth, amounting to over five billion species,[1] are estimated to have died out.[2][3][4][5] It is estimated that there are currently around 8.7 million species of eukaryote globally,[6] and possibly many times more if microorganisms, like bacteria, are included.[7] Notable extinct animal species include non-avian dinosaurs, saber-toothed cats, dodos, mammoths, ground sloths, thylacines, trilobites, golden toads, and passenger pigeons.

Mass extinctions are relatively rare events; however, isolated extinctions of species and clades are quite common, and are a natural part of the evolutionary process.[9] Only recently have extinctions been recorded and scientists have become alarmed at the current high rate of extinctions.[10][11][12][13][14] Most species that become extinct are never scientifically documented. Some scientists estimate that up to half of presently existing plant and animal species may become extinct by 2100.[15] A 2018 report indicated that the phylogenetic diversity of 300 mammalian species erased during the human era since the Late Pleistocene would require 5 to 7 million years to recover.[16]

In June 2019, one million species of plants and animals were at risk of extinction. At least 571 plant species have been lost since 1750, but likely many more. The main cause of the extinctions is the destruction of natural habitats by human activities, such as cutting down forests and converting land into fields for farming.[21]

A species is extinct when the last existing member dies. Extinction therefore becomes a certainty when there are no surviving individuals that can reproduce and create a new generation. A species may become functionally extinct when only a handful of individuals survive, which cannot reproduce due to poor health, age, sparse distribution over a large range, a lack of individuals of both sexes (in sexually reproducing species), or other reasons.

Pinpointing the extinction (or pseudoextinction) of a species requires a clear definition of that species. If it is to be declared extinct, the species in question must be uniquely distinguishable from any ancestor or daughter species, and from any other closely related species. Extinction of a species (or replacement by a daughter species) plays a key role in the punctuated equilibrium hypothesis of Stephen Jay Gould and Niles Eldredge.[24]

In ecology, extinction is sometimes used informally to refer to local extinction, in which a species ceases to exist in the chosen area of study, despite still existing elsewhere. Local extinctions may be made good by the reintroduction of individuals of that species taken from other locations; wolf reintroduction is an example of this. Species that are not globally extinct are termed extant. Those species that are extant, yet are threatened with extinction, are referred to as threatened or endangered species.

Currently, an important aspect of extinction is human attempts to preserve critically endangered species. These are reflected by the creation of the conservation status "extinct in the wild" (EW). Species listed under this status by the International Union for Conservation of Nature (IUCN) are not known to have any living specimens in the wild and are maintained only in zoos or other artificial environments. Some of these species are functionally extinct, as they are no longer part of their natural habitat and it is unlikely the species will ever be restored to the wild.[26] When possible, modern zoological institutions try to maintain a viable population for species preservation and possible future reintroduction to the wild, through use of carefully planned breeding programs.

The extinction of one species' wild population can have knock-on effects, causing further extinctions. These are also called "chains of extinction".[27] This is especially common with extinction of keystone species.

A 2018 study indicated that the sixth mass extinction started in the Late Pleistocene could take up to 5 to 7 million years to restore 2.5 billion years of unique mammal diversity to what it was before the human era.[16][28]

Extinction of a parent species where daughter species or subspecies are still extant is called pseudoextinction or phyletic extinction. Effectively, the old taxon vanishes, transformed (anagenesis) into a successor,[29] or split into more than one (cladogenesis).[30]

Pseudoextinction is difficult to demonstrate unless one has a strong chain of evidence linking a living species to members of a pre-existing species. For example, it is sometimes claimed that the extinct Hyracotherium, which was an early horse that shares a common ancestor with the modern horse, is pseudoextinct, rather than extinct, because there are several extant species of Equus, including zebra and donkey; however, as fossil species typically leave no genetic material behind, one cannot say whether Hyracotherium evolved into more modern horse species or merely evolved from a common ancestor with modern horses. Pseudoextinction is much easier to demonstrate for larger taxonomic groups.

A Lazarus taxon or Lazarus species refers to instances where a species or taxon was thought to be extinct, but was later rediscovered. It can also refer to instances where large gaps in the fossil record of a taxon result in fossils reappearing much later, although the taxon may have ultimately become extinct at a later point.

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