Extinction and Extirpation
Extinction and Extirpation
Extinction is the complete disappearance of a species. Extirpation, also called local extinction, is the disappearance of a species only from a given area. Extirpation tends to make the extinction of a given species more likely by reducing that species’ population, genetic diversity, and geographic range. Both extinction and extirpation can occur naturally; most species that have ever existed are now extinct, and all species that exist today will someday be extinct. When many species become extinct in a short time, the event is termed a mass extinction.
Historical Background and Scientific Foundations
Extinction and Extirpation as Natural Processes
Single-celled life first appeared on Earth 3 to 4 billion years ago. Complex animals first became common and diverse during the early Cambrian period, which began about 540 million years ago. Since then, over 99% of all species of plant and animal that have evolved have become extinct.
In the fossil record, extinction occurs at a more or less steady rate of about one species per million per year, that is, 10 to 100 extinctions per year, counting microorganisms as well as multicellular plants and animals. In addition, the history of life has been punctuated by at least six major mass extinctions, and many smaller ones—events during which a significant fraction of Earth’s species become extinct in a short period of time. For example, about 65 million years ago, at the boundary between the Cretaceous and Triassic periods, about half of all species went extinct, including all dinosaur species then alive. Scientists continue to debate the causes of this event, but most agree that massive volcanic eruptions in Asia caused many extinctions and were followed by the coincidental impact of an asteroid several kilometers across that excavated the Mexico’s Chicxulub crater, which is 112 mi (180 km) across.
Over geological time, extinction is not a purely destructive process. Without it, human beings and other modern species would not exist, since they could not evolve until the extinction of dinosaurs and other ancient species opened up ecological niches for mammals and birds.
Extinction may be caused by the direct killing of all the individuals of a species at once or during a short time, as with volcanic eruptions, asteroid impacts, and other catastrophes. More commonly, species become extinct because the area that they inhabit shrinks. A species’ population must become smaller as its habitat shrinks, making it more vulnerable to diseases and other challenges. A species may also prove unable to compete in a shrinking ecosystem crowded with other species. The relationship between ecosystem area and the number of species of plants and animals that can live in that area is well known: The smaller the area, the fewer the species. This fact is often expressed as a mathematical equation called the species-area relationship. Writing S for the number of species, A for area, and c and z for certain fixed numbers or constants that depend on the ecological setting (desert, reef, temperate forest, or other), the species-area relationship is written S = cAz. For example, if c = 1 and z = 2, then S = A2. Halving the area of a given ecosystem, A will in this case reduce the number of species, S, to one-fourth its previous value. This rule is often a good predictor of how many species will disappear from a given habitat when it shrinks. If these species still exist in other habitats, then their disappearance from the shrinking habitat is a case of extirpation. If they are the last of their kind, then their disappearance is extinction.
The species-area relationship does not work in reverse—that is, quickly increasing ecosystem area does not bring more biodiversity (more species) into being. This is because extinction can happen overnight, but the evolution of new species is relatively slow. In general, many thousands of years are required for the evolution of a new species.
Extinction and Extirpation by Human Beings
Human beings cause habitat loss by converting natural landscapes to urban or agricultural landscapes and through pollution, mining, draining, and deforestation. Hunting can also drive species to extinction, as has occurred with several famous extinct species, including the passenger pigeon (which became extinct in 1914).
Today, most biologists agree that we are in the midst of a new, human-caused extinction event, the Holocene extinction. (The Holocene is the most recent geological epoch, stretching from about 10,000 years ago to the present.) This new wave of extinctions has been caused
WORDS TO KNOW
BIODIVERSITY: Literally, “life diversity”: the wide range of plants and animals that exist within any given geographical region.
GEOLOGICAL TIME: The period of time extending from the formation of Earth to the present.
FOSSIL RECORD: The time-ordered mass of fossils that is found in the sedimentary rocks of Earth. The fossil record is one of the primary sources of knowledge about evolution and is also used to date rock layers.
MASS EXTINCTION: An extinction event characterized by high levels (or rates) of species extinction in a geologically short period of time.
SPECIES: A biological classification group ranked below genus, consisting of related organisms capable of interbreeding.
by over-hunting, over-fishing, agriculture, pollution, the introduction of invasive species, and habitat destruction. Because evolution of new species is slow, the loss of biodiversity now being caused by human beings cannot be undone by speciation until hundreds of thousands of years have passed. Moreover, distinctive species such as the blue whale, once driven to extinction, will never be re-created by evolution.
As of 2007, according to the World Conservation Union, 16,306 species were known to be threatened with extinction. Many more were probably threatened, with thousands of unknown species destroyed in recent years, since many species in rain forests, which are being rapidly cleared throughout the tropics, have never been identified by biologists.
Impacts and Issues
A century ago, scientists advanced the theory that the mass extinction of large mammals in North America—giant ground sloth, saber-toothed cats, giant bears, mammoths, camels, and dozens of other creatures—about 10,500 years ago was caused by hunting by early Americans. This view, called the overkill hypothesis, is supported by the fact that the extinction of animals on islands such as Madagascar and New Zealand often occurred after colonization by prehistoric humans. However, the overkill hypothesis has been recently challenged by scientists who point to climate change at the end of the Ice Age as the likely cause of at least some of these extinctions. It is possible that both causes—natural climate change and hunting—were operative, that is, that climate change caused a number of large animal
species to become extinct in the early Holocene while humans finished off others.
Human-caused (anthropogenic) climate change will probably become a major cause of extinctions during the coming century. The United Nations’ Intergovernmental Panel on Climate Change (IPCC) stated in 2007 that 20 to 30% of all plant and animal species are likely to be at increasingly high extinction risk as the average global temperature warms to 3.6–5.4°F (2–3°C) above the level it held before the Industrial Revolution, when human beings first began to burn large amounts of fossil fuels. This much warming by 2100 is likely even under conservative scenarios for global warming.
A 2004 study by Chris Thomas and colleagues calculated that, out of a sample 1,103 animal and plant species, 15 to 37% would be committed to extinction by 2050 as a result of climate change—that is, that the number of living individuals of each species would be so small that the species’ extinction some time after 2050 would become highly likely. Thomas and his colleagues estimated that in many if not most ecological regions, climate change would become the greatest threat to biodiversity by 2050. There are 5 to 15 million species of creatures on Earth. If 20% of these were committed to extinction by climate change, then from 1 million to 3 million species would eventually become extinct as a result of global climate change. (Most of these would be species of insects.)
Climate-related extinctions and extirpations may have already begun. A 2006 study found that global warming almost certainly played a key role in the recent extinction of about 67% of the 110 or so species of the Monteverde harlequin tree frog of the mountains of Costa Rica, making the frogs more vulnerable to a fungus infection. However, in early 2008 another team of biologists challenged the conclusion that climate change was a key factor in the disappearance of the frogs.
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