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8.3: When is a Species Extinct? - Biology

8.3: When is a Species Extinct? - Biology



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8.3: When is a Species Extinct?

Why It Matters When Species Go Extinct

Jennifer Bove is an award-winning writer and editor with a background in field biology.

We are surrounded by endangered species every day. Majestic tigers grace posters on bedroom walls, stuffed toy pandas stare blankly from shopping mall shelves with the click of a button, we can watch the elaborate courtship rituals of whooping cranes and the strategic hunting habits of the Amur leopard on the Discovery Channel. No matter where we look, images and information about the world's rarest animals are readily available, but do we ever stop to think about the effects endangered species have on their environments, what happens after they disappear?

Let's face it, few of us have crossed paths with a real, live endangered species today—one that is teetering on a tightrope of existence, like the Santa Barbara Song Sparrow or the Javan Rhino— much less consider the implications of their loss.

So, does it really matter if an animal goes extinct when we can still watch it on television, even after it's gone? A single species' disappearance can, in fact, make a huge difference on a global scale. Like pieces of yarn in a woven tapestry, the removal of one can start unraveling the whole system.


Contents

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. [20]

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. [22] 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". [23] 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. [14] [24]

Pseudoextinction Edit

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, [25] or split into more than one (cladogenesis). [26]

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.

Lazarus taxa Edit

The coelacanth, a fish related to lungfish and tetrapods, was considered to have been extinct since the end of the Cretaceous Period. In 1938, however, a living specimen was found off the Chalumna River (now Tyolomnqa) on the east coast of South Africa. [27] Museum curator Marjorie Courtenay-Latimer discovered the fish among the catch of a local trawler operated by Captain Hendrick Goosen, on December 23, 1938. [27] A local chemistry professor, JLB Smith, confirmed the fish's importance with a famous cable: "MOST IMPORTANT PRESERVE SKELETON AND GILLS = FISH DESCRIBED". [27]

Far more recent possible or presumed extinctions of species which may turn out still to exist include the thylacine, or Tasmanian tiger (Thylacinus cynocephalus), the last known example of which died in Hobart Zoo in Tasmania in 1936 the Japanese wolf (Canis lupus hodophilax), last sighted over 100 years ago the American ivory-billed woodpecker (Campephilus principalis), with the last universally accepted sighting in 1944 and the slender-billed curlew (Numenius tenuirostris), not seen since 2007. [28]

As long as species have been evolving, species have been going extinct. It is estimated that over 99.9% of all species that ever lived are extinct. The average lifespan of a species is 1–10 million years, [29] although this varies widely between taxa. There are a variety of causes that can contribute directly or indirectly to the extinction of a species or group of species. "Just as each species is unique", write Beverly and Stephen C. Stearns, "so is each extinction . the causes for each are varied—some subtle and complex, others obvious and simple". [30] Most simply, any species that cannot survive and reproduce in its environment and cannot move to a new environment where it can do so, dies out and becomes extinct. Extinction of a species may come suddenly when an otherwise healthy species is wiped out completely, as when toxic pollution renders its entire habitat unliveable or may occur gradually over thousands or millions of years, such as when a species gradually loses out in competition for food to better adapted competitors. Extinction may occur a long time after the events that set it in motion, a phenomenon known as extinction debt.

Assessing the relative importance of genetic factors compared to environmental ones as the causes of extinction has been compared to the debate on nature and nurture. [31] The question of whether more extinctions in the fossil record have been caused by evolution or by catastrophe is a subject of discussion Mark Newman, the author of Modeling Extinction, argues for a mathematical model that falls between the two positions. [5] By contrast, conservation biology uses the extinction vortex model to classify extinctions by cause. When concerns about human extinction have been raised, for example in Sir Martin Rees' 2003 book Our Final Hour, those concerns lie with the effects of climate change or technological disaster.

Currently, environmental groups and some governments are concerned with the extinction of species caused by humanity, and they try to prevent further extinctions through a variety of conservation programs. [9] Humans can cause extinction of a species through overharvesting, pollution, habitat destruction, introduction of invasive species (such as new predators and food competitors), overhunting, and other influences. Explosive, unsustainable human population growth and increasing per capita consumption are essential drivers of the extinction crisis. [32] [33] [34] [35] According to the International Union for Conservation of Nature (IUCN), 784 extinctions have been recorded since the year 1500, the arbitrary date selected to define "recent" extinctions, up to the year 2004 with many more likely to have gone unnoticed. Several species have also been listed as extinct since 2004. [36]

Genetics and demographic phenomena Edit

If adaptation increasing population fitness is slower than environmental degradation plus the accumulation of slightly deleterious mutations, then a population will go extinct. [37] Smaller populations have fewer beneficial mutations entering the population each generation, slowing adaptation. It is also easier for slightly deleterious mutations to fix in small populations the resulting positive feedback loop between small population size and low fitness can cause mutational meltdown.

Limited geographic range is the most important determinant of genus extinction at background rates but becomes increasingly irrelevant as mass extinction arises. [38] Limited geographic range is a cause both of small population size and of greater vulnerability to local environmental catastrophes.

Extinction rates can be affected not just by population size, but by any factor that affects evolvability, including balancing selection, cryptic genetic variation, phenotypic plasticity, and robustness. A diverse or deep gene pool gives a population a higher chance in the short term of surviving an adverse change in conditions. Effects that cause or reward a loss in genetic diversity can increase the chances of extinction of a species. Population bottlenecks can dramatically reduce genetic diversity by severely limiting the number of reproducing individuals and make inbreeding more frequent.

Genetic pollution Edit

Extinction sometimes results for species evolved to specific ecologies [39] that are subjected to genetic pollution—i.e., uncontrolled hybridization, introgression and genetic swamping that lead to homogenization or out-competition from the introduced (or hybrid) species. [40] Endemic populations can face such extinctions when new populations are imported or selectively bred by people, or when habitat modification brings previously isolated species into contact. Extinction is likeliest for rare species coming into contact with more abundant ones [41] interbreeding can swamp the rarer gene pool and create hybrids, depleting the purebred gene pool (for example, the endangered wild water buffalo is most threatened with extinction by genetic pollution from the abundant domestic water buffalo). Such extinctions are not always apparent from morphological (non-genetic) observations. Some degree of gene flow is a normal evolutionary process nevertheless, hybridization (with or without introgression) threatens rare species' existence. [42] [43]

The gene pool of a species or a population is the variety of genetic information in its living members. A large gene pool (extensive genetic diversity) is associated with robust populations that can survive bouts of intense selection. Meanwhile, low genetic diversity (see inbreeding and population bottlenecks) reduces the range of adaptions possible. [44] Replacing native with alien genes narrows genetic diversity within the original population, [41] [45] thereby increasing the chance of extinction.

Habitat degradation Edit

Habitat degradation is currently the main anthropogenic cause of species extinctions. The main cause of habitat degradation worldwide is agriculture, with urban sprawl, logging, mining and some fishing practices close behind. The degradation of a species' habitat may alter the fitness landscape to such an extent that the species is no longer able to survive and becomes extinct. This may occur by direct effects, such as the environment becoming toxic, or indirectly, by limiting a species' ability to compete effectively for diminished resources or against new competitor species.

Habitat degradation through toxicity can kill off a species very rapidly, by killing all living members through contamination or sterilizing them. It can also occur over longer periods at lower toxicity levels by affecting life span, reproductive capacity, or competitiveness.

Habitat degradation can also take the form of a physical destruction of niche habitats. The widespread destruction of tropical rainforests and replacement with open pastureland is widely cited as an example of this [13] elimination of the dense forest eliminated the infrastructure needed by many species to survive. For example, a fern that depends on dense shade for protection from direct sunlight can no longer survive without forest to shelter it. Another example is the destruction of ocean floors by bottom trawling. [46]

Diminished resources or introduction of new competitor species also often accompany habitat degradation. Global warming has allowed some species to expand their range, bringing unwelcome [ according to whom? ] competition to other species that previously occupied that area. Sometimes these new competitors are predators and directly affect prey species, while at other times they may merely outcompete vulnerable species for limited resources. Vital resources including water and food can also be limited during habitat degradation, leading to extinction.

Predation, competition, and disease Edit

In the natural course of events, species become extinct for a number of reasons, including but not limited to: extinction of a necessary host, prey or pollinator, inter-species competition, inability to deal with evolving diseases and changing environmental conditions (particularly sudden changes) which can act to introduce novel predators, or to remove prey. Recently in geological time, humans have become an additional cause of extinction (some people would say premature extinction [ citation needed ] ) of some species, either as a new mega-predator or by transporting animals and plants from one part of the world to another. Such introductions have been occurring for thousands of years, sometimes intentionally (e.g. livestock released by sailors on islands as a future source of food) and sometimes accidentally (e.g. rats escaping from boats). In most cases, the introductions are unsuccessful, but when an invasive alien species does become established, the consequences can be catastrophic. Invasive alien species can affect native species directly by eating them, competing with them, and introducing pathogens or parasites that sicken or kill them or indirectly by destroying or degrading their habitat. Human populations may themselves act as invasive predators. According to the "overkill hypothesis", the swift extinction of the megafauna in areas such as Australia (40,000 years before present), North and South America (12,000 years before present), Madagascar, Hawaii (AD 300–1000), and New Zealand (AD 1300–1500), resulted from the sudden introduction of human beings to environments full of animals that had never seen them before and were therefore completely unadapted to their predation techniques. [47]

Coextinction Edit

Coextinction refers to the loss of a species due to the extinction of another for example, the extinction of parasitic insects following the loss of their hosts. Coextinction can also occur when a species loses its pollinator, or to predators in a food chain who lose their prey. "Species coextinction is a manifestation of one of the interconnectednesses of organisms in complex ecosystems . While coextinction may not be the most important cause of species extinctions, it is certainly an insidious one." [48] Coextinction is especially common when a keystone species goes extinct. Models suggest that coextinction is the most common form of biodiversity loss. There may be a cascade of coextinction across the trophic levels. Such effects are most severe in mutualistic and parasitic relationships. An example of coextinction is the Haast's eagle and the moa: the Haast's eagle was a predator that became extinct because its food source became extinct. The moa were several species of flightless birds that were a food source for the Haast's eagle. [49]

Climate change Edit

Extinction as a result of climate change has been confirmed by fossil studies. [50] Particularly, the extinction of amphibians during the Carboniferous Rainforest Collapse, 305 million years ago. [50] A 2003 review across 14 biodiversity research centers predicted that, because of climate change, 15–37% of land species would be "committed to extinction" by 2050. [51] [52] The ecologically rich areas that would potentially suffer the heaviest losses include the Cape Floristic Region and the Caribbean Basin. These areas might see a doubling of present carbon dioxide levels and rising temperatures that could eliminate 56,000 plant and 3,700 animal species. [53] Climate change has also been found to be a factor in habitat loss and desertification. [54]

There have been at least five mass extinctions in the history of life on earth, and four in the last 350 million years in which many species have disappeared in a relatively short period of geological time. A massive eruptive event that released large quantities of tephra particles into the atmosphere is considered to be one likely cause of the "Permian–Triassic extinction event" about 250 million years ago, [55] which is estimated to have killed 90% of species then existing. [56] There is also evidence to suggest that this event was preceded by another mass extinction, known as Olson's Extinction. [55] The Cretaceous–Paleogene extinction event (K–Pg) occurred 66 million years ago, at the end of the Cretaceous period it is best known for having wiped out non-avian dinosaurs, among many other species.

Modern extinctions Edit

According to a 1998 survey of 400 biologists conducted by New York's American Museum of Natural History, nearly 70% believed that the Earth is currently in the early stages of a human-caused mass extinction, [57] known as the Holocene extinction. In that survey, the same proportion of respondents agreed with the prediction that up to 20% of all living populations could become extinct within 30 years (by 2028). A 2014 special edition of Science declared there is widespread consensus on the issue of human-driven mass species extinctions. [58] A 2020 study published in PNAS stated that the contemporary extinction crisis "may be the most serious environmental threat to the persistence of civilization, because it is irreversible." [59]

Biologist E. O. Wilson estimated [13] in 2002 that if current rates of human destruction of the biosphere continue, one-half of all plant and animal species of life on earth will be extinct in 100 years. [60] More significantly, the current rate of global species extinctions is estimated as 100 to 1,000 times "background" rates (the average extinction rates in the evolutionary time scale of planet Earth), [61] [62] while future rates are likely 10,000 times higher. [62] However, some groups are going extinct much faster. Biologists Paul R. Ehrlich and Stuart Pimm, among others, contend that human population growth and overconsumption are the main drivers of the modern extinction crisis. [63] [64] [32] [65]

In January 2020, the UN's Convention on Biological Diversity drafted a plan to mitigate the contemporary extinction crisis by establishing a deadline of 2030 to protect 30% of the earth's land and oceans and reduce pollution by 50%, with the goal of allowing for the restoration of ecosystems by 2050. [66] [67] The 2020 United Nations' Global Biodiversity Outlook report stated that of the 20 biodiversity goals laid out by the Aichi Biodiversity Targets in 2010, only 6 were "partially achieved" by the deadline of 2020. [68] The report warned that biodiversity will continue to decline if the status quo is not changed, in particular the "currently unsustainable patterns of production and consumption, population growth and technological developments". [69] In a 2021 report published in the journal Frontiers in Conservation Science, some top scientists asserted that even if the Aichi Biodiversity Targets set for 2020 had been achieved, it would not have resulted in a significant mitigation of biodiversity loss. They added that failure of the global community to reach these targets is hardly surprising given that biodiversity loss is "nowhere close to the top of any country's priorities, trailing far behind other concerns such as employment, healthcare, economic growth, or currency stability." [70] [71]

For much of history, the modern understanding of extinction as the end of a species was incompatible with the prevailing worldview. Prior to the 19th century, much of Western society adhered to the belief that the world was created by God and as such was complete and perfect. [73] This concept reached its heyday in the 1700s with the peak popularity of a theological concept called the great chain of being, in which all life on earth, from the tiniest microorganism to God, is linked in a continuous chain. [74] The extinction of a species was impossible under this model, as it would create gaps or missing links in the chain and destroy the natural order. [73] [74] Thomas Jefferson was a firm supporter of the great chain of being and an opponent of extinction, [73] [75] famously denying the extinction of the woolly mammoth on the grounds that nature never allows a race of animals to become extinct. [76]

A series of fossils were discovered in the late 17th century that appeared unlike any living species. As a result, the scientific community embarked on a voyage of creative rationalization, seeking to understand what had happened to these species within a framework that did not account for total extinction. In October 1686, Robert Hooke presented an impression of a nautilus to the Royal Society that was more than two feet in diameter, [77] and morphologically distinct from any known living species. Hooke theorized that this was simply because the species lived in the deep ocean and no one had discovered them yet. [74] While he contended that it was possible a species could be "lost", he thought this highly unlikely. [74] Similarly, in 1695, Sir Thomas Molyneux published an account of enormous antlers found in Ireland that did not belong to any extant taxa in that area. [75] [78] Molyneux reasoned that they came from the North American moose and that the animal had once been common on the British Isles. [75] [78] Rather than suggest that this indicated the possibility of species going extinct, he argued that although organisms could become locally extinct, they could never be entirely lost and would continue to exist in some unknown region of the globe. [78] The antlers were later confirmed to be from the extinct deer Megaloceros. [75] Hooke and Molyneux's line of thinking was difficult to disprove. When parts of the world had not been thoroughly examined and charted, scientists could not rule out that animals found only in the fossil record were not simply "hiding" in unexplored regions of the Earth. [79]

Georges Cuvier is credited with establishing the modern conception of extinction in a 1796 lecture to the French Institute, [72] [76] though he would spend most of his career trying to convince the wider scientific community of his theory. [80] Cuvier was a well-regarded geologist, lauded for his ability to reconstruct the anatomy of an unknown species from a few fragments of bone. [72] His primary evidence for extinction came from mammoth skulls found in the Paris basin. [72] Cuvier recognized them as distinct from any known living species of elephant, and argued that it was highly unlikely such an enormous animal would go undiscovered. [72] In 1812, Cuvier, along with Alexandre Brongniart and Geoffroy Saint-Hilaire, mapped the strata of the Paris basin. [74] They saw alternating saltwater and freshwater deposits, as well as patterns of the appearance and disappearance of fossils throughout the record. [75] [80] From these patterns, Cuvier inferred historic cycles of catastrophic flooding, extinction, and repopulation of the earth with new species. [75] [80]

Cuvier's fossil evidence showed that very different life forms existed in the past than those that exist today, a fact that was accepted by most scientists. [73] The primary debate focused on whether this turnover caused by extinction was gradual or abrupt in nature. [80] Cuvier understood extinction to be the result of cataclysmic events that wipe out huge numbers of species, as opposed to the gradual decline of a species over time. [81] His catastrophic view of the nature of extinction garnered him many opponents in the newly emerging school of uniformitarianism. [81]

Jean-Baptiste Lamarck, a gradualist and colleague of Cuvier, saw the fossils of different life forms as evidence of the mutable character of species. [80] While Lamarck did not deny the possibility of extinction, he believed that it was exceptional and rare and that most of the change in species over time was due to gradual change. [80] Unlike Cuvier, Lamarck was skeptical that catastrophic events of a scale large enough to cause total extinction were possible. In his geological history of the earth titled Hydrogeologie, Lamarck instead argued that the surface of the earth was shaped by gradual erosion and deposition by water, and that species changed over time in response to the changing environment. [80] [82]

Charles Lyell, a noted geologist and founder of uniformitarianism, believed that past processes should be understood using present day processes. Like Lamarck, Lyell acknowledged that extinction could occur, noting the total extinction of the dodo and the extirpation of indigenous horses to the British Isles. [74] He similarly argued against mass extinctions, believing that any extinction must be a gradual process. [72] [76] Lyell also showed that Cuvier's original interpretation of the Parisian strata was incorrect. Instead of the catastrophic floods inferred by Cuvier, Lyell demonstrated that patterns of saltwater and freshwater deposits, like those seen in the Paris basin, could be formed by a slow rise and fall of sea levels. [75]

The concept of extinction was integral to Charles Darwin's On the Origin of Species, with less fit lineages disappearing over time. For Darwin, extinction was a constant side effect of competition. [83] Because of the wide reach of On the Origin of Species, it was widely accepted that extinction occurred gradually and evenly (a concept now referred to as background extinction). [76] It was not until 1982, when David Raup and Jack Sepkoski published their seminal paper on mass extinctions, that Cuvier was vindicated and catastrophic extinction was accepted as an important mechanism. The current understanding of extinction is a synthesis of the cataclysmic extinction events proposed by Cuvier, and the background extinction events proposed by Lyell and Darwin.

Extinction is an important research topic in the field of zoology, and biology in general, and has also become an area of concern outside the scientific community. A number of organizations, such as the Worldwide Fund for Nature, have been created with the goal of preserving species from extinction. Governments have attempted, through enacting laws, to avoid habitat destruction, agricultural over-harvesting, and pollution. While many human-caused extinctions have been accidental, humans have also engaged in the deliberate destruction of some species, such as dangerous viruses, and the total destruction of other problematic species has been suggested. Other species were deliberately driven to extinction, or nearly so, due to poaching or because they were "undesirable", or to push for other human agendas. One example was the near extinction of the American bison, which was nearly wiped out by mass hunts sanctioned by the United States government, to force the removal of Native Americans, many of whom relied on the bison for food. [86]

Biologist Bruce Walsh states three reasons for scientific interest in the preservation of species: genetic resources, ecosystem stability, and ethics and today the scientific community "stress[es] the importance" of maintaining biodiversity. [87] [88]

In modern times, commercial and industrial interests often have to contend with the effects of production on plant and animal life. However, some technologies with minimal, or no, proven harmful effects on Homo sapiens can be devastating to wildlife (for example, DDT). [89] [90] Biogeographer Jared Diamond notes that while big business may label environmental concerns as "exaggerated", and often cause "devastating damage", some corporations find it in their interest to adopt good conservation practices, and even engage in preservation efforts that surpass those taken by national parks. [91]

Governments sometimes see the loss of native species as a loss to ecotourism, [92] and can enact laws with severe punishment against the trade in native species in an effort to prevent extinction in the wild. Nature preserves are created by governments as a means to provide continuing habitats to species crowded by human expansion. The 1992 Convention on Biological Diversity has resulted in international Biodiversity Action Plan programmes, which attempt to provide comprehensive guidelines for government biodiversity conservation. Advocacy groups, such as The Wildlands Project [93] and the Alliance for Zero Extinctions, [94] work to educate the public and pressure governments into action.

People who live close to nature can be dependent on the survival of all the species in their environment, leaving them highly exposed to extinction risks. However, people prioritize day-to-day survival over species conservation with human overpopulation in tropical developing countries, there has been enormous pressure on forests due to subsistence agriculture, including slash-and-burn agricultural techniques that can reduce endangered species's habitats. [95]

Antinatalist philosopher David Benatar concludes that any popular concern about non-human species extinction usually arises out of concern about how the loss of a species will impact human wants and needs, that "we shall live in a world impoverished by the loss of one aspect of faunal diversity, that we shall no longer be able to behold or use that species of animal." He notes that typical concerns about possible human extinction, such as the loss of individual members, are not considered in regards to non-human species extinction. [96]

Planned extinction Edit

Completed Edit

  • The smallpox virus is now extinct in the wild, [97] although samples are retained in laboratory settings.
  • The rinderpest virus, which infected domestic cattle, is now extinct in the wild. [98]

Proposed Edit

The poliovirus is now confined to small parts of the world due to extermination efforts. [99]

Dracunculus medinensis, a parasitic worm which causes the disease dracunculiasis, is now close to eradication thanks to efforts led by the Carter Center. [100]

Treponema pallidum pertenue, a bacterium which causes the disease yaws, is in the process of being eradicated.

Biologist Olivia Judson has advocated the deliberate extinction of certain disease-carrying mosquito species. In a September 25, 2003 article in The New York Times, she advocated "specicide" of thirty mosquito species by introducing a genetic element that can insert itself into another crucial gene, to create recessive "knockout genes". [101] She says that the Anopheles mosquitoes (which spread malaria) and Aedes mosquitoes (which spread dengue fever, yellow fever, elephantiasis, and other diseases) represent only 30 of around 3,500 mosquito species eradicating these would save at least one million human lives per annum, at a cost of reducing the genetic diversity of the family Culicidae by only 1%. She further argues that since species become extinct "all the time" the disappearance of a few more will not destroy the ecosystem: "We're not left with a wasteland every time a species vanishes. Removing one species sometimes causes shifts in the populations of other species—but different need not mean worse." In addition, anti-malarial and mosquito control programs offer little realistic hope to the 300 million people in developing nations who will be infected with acute illnesses this year. Although trials are ongoing, she writes that if they fail "we should consider the ultimate swatting." [101]

Biologist E. O. Wilson has advocated the eradication of several species of mosquito, including malaria vector Anopheles gambiae. Wilson stated, "I'm talking about a very small number of species that have co-evolved with us and are preying on humans, so it would certainly be acceptable to remove them. I believe it's just common sense." [102]

There have been many campaigns - some successful - to locally eradicate tsetse flies and their trypanosomes in areas, countries, and islands of Africa (including Príncipe). [103] [104] There are currently serious efforts to do away with them all across Africa, and this is generally viewed as beneficial and morally necessary, [105] although not always. [106]

Cloning Edit

Some, such as Harvard geneticist George M. Church, believe that ongoing technological advances will let us "bring back to life" an extinct species by cloning, using DNA from the remains of that species. Proposed targets for cloning include the mammoth, the thylacine, and the Pyrenean ibex. For this to succeed, enough individuals would have to be cloned, from the DNA of different individuals (in the case of sexually reproducing organisms) to create a viable population. Though bioethical and philosophical objections have been raised, [107] the cloning of extinct creatures seems theoretically possible. [108]

In 2003, scientists tried to clone the extinct Pyrenean ibex (C. p. pyrenaica). This attempt failed: of the 285 embryos reconstructed, 54 were transferred to 12 mountain goats and mountain goat-domestic goat hybrids, but only two survived the initial two months of gestation before they too died. [109] In 2009, a second attempt was made to clone the Pyrenean ibex: one clone was born alive, but died seven minutes later, due to physical defects in the lungs. [110]


Local extinction of species can occur with a substantial delay following habitat loss or degradation. Accumulating evidence suggests that such extinction debts pose a significant but often unrecognized challenge for biodiversity conservation across a wide range of taxa and ecosystems. Species with long generation times and populations near their extinction threshold are most likely to have an extinction debt. However, as long as a species that is predicted to become extinct still persists, there is time for conservation measures such as habitat restoration and landscape management. Standardized long-term monitoring, more high-quality empirical studies on different taxa and ecosystems and further development of analytical methods will help to better quantify extinction debt and protect biodiversity.

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What Caused Mass Extinctions ? | Species | Biosphere | Biology

It becomes clear that there is an intense amount of interconnectivity between the aspects of the biosphere. At the cellular level, there is a symbiotic effect and it continues up throughout the chain of organisms all the way to the biosphere levels.

This interconnectivity can have positive effects, but it can also lead to massive upheavals within the species. The result of these upheavals can be minor or they can be very dramatic. The worst case scenarios often result in the end of a species. When dramatic changes occur to an environment very fast, the organisms may not always be able to adjust quickly enough to the changes.

What is often the result? The mass extinction of one or more species may be the final consequence. Tampering with the environment can have long range consequences, both for the organisms, as well as the plants.

Often, these extinctions happen fairly quickly, but the recovery from them can take millions or billions of years. Additionally, the recovery does not mean that the lost species are ever returned. These extinctions provide a purging of the biosphere.

Scientists have concluded that there have been at least five of these dramatic changes that resulted in mass extinctions. Most have been a result of changes in the environment, primarily cooling. One of these is the purging of dinosaurs. Yet a majority of these were the result of natural corrections within the biosphere itself. So when did humans start having a greater effect on the biosphere?

To put it simply, the human population began to have the greatest effect when they became more organized as a people. For example, hunting alone results in the extinction of 15,000 to 30,000 species every year.

Unsustainable use of resources alters the biodiversity of the planet and can reduce or eliminate a species habitats. When habitats disappear, the extinction of a species can happen very quickly. No one would disagree that when someone’s home is destroyed, it can reduce their chances of survival.

Let us think of it in terms of a domino effect. When one domino is pushed over, the chain reaction begins. In the same way, pushing one domino, such as deforestation, has also begun a chain reaction with much larger consequences.

Most species struggle because their homes are being destroyed far too quickly, giving the species no time to adapt to a new habitat. The overarching environment is attempting to adjust to all the changes being made by humans and as a result of human activity.

Yet the adjustments are very dramatic, causing various conflicts between nations fighting for the most basic of resources, such as water and food. The resources that they do have available are often quickly depleted, without thought of sustainability or long term effects.

The nations that are not as developed find themselves trying to support growing populations without the access to the necessary assets. They are also ill-prepared to deal with the changes in the climate and the resulting significant weather events.

Many would argue that humans did not create all of this change, that some of it is part of the natural progression of the biosphere. Yet even if that is true, humanity does create a footprint on the biosphere, for good or for evil. Our actions do have consequences.

Some of these consequences are being felt now but will also be felt by future generations if corrections are not made to the course humanity is currently on. Symbiotic relationships are in jeopardy, not just for individual species, but for life on the planet as a whole.

Throughout the last several decades, humans have begun to change how they interact with their home. Finding sustainable ways to fish, grow food and maintain the forests are just a few of the ways people are changing their course to benefit the biosphere. Does this mean that more cannot be done?

No, in fact governments and individuals are producing energy in a more sustainable way. Most importantly, these official channels are changing their viewpoint about how the earth should be treated. It does not mean that the extinctions will stop overnight. But by creating the change we wish to see, we as individuals can have a greater impact on how the world and its organisms are cared for.


Extinction

Extinction is the dying out of a species. Extinction plays an important role in the evolution of life because it opens up opportunities for new species to emerge.

Biology, Ecology, Earth Science, Geology, Geography, Physical Geography

Dinogorgon Skull

Many species have gone extinct throughout history and all that marks their presence on Earth are fossils, such as this one of a dinogorgon.

Photograph by Jonathan Blair

When a species disappears, biologists say that the species has become extinct. By making room for new species, extinction helps drive the evolution of life. Over long periods of time, the number of species becoming extinct can remain fairly constant, meaning that an average number of species go extinct each year, century, or millennium. However, during the history of life on Earth, there have been periods of mass extinction, when large percentages of the planet&rsquos species became extinct in a relatively short amount of time. These extinctions have had widely different causes.

About 541 million years ago, a great expansion occurred in the diversity of multicellular organisms. Paleobiologists, scientists who study the fossils of plants and animals to learn how life evolved, call this event the Cambrian Explosion. Since the Cambrian Explosion, there have been five mass extinctions, each of which is named for the geological period in which it occurred, or for the periods that immediately preceded and followed it.

The first mass extinction is called the Ordovician-Silurian Extinction. It occurred about 440 million years ago, at the end of the period that paleontologists and geologists call the Ordovician, and followed by the start of the Silurian period. In this extinction event, many small organisms of the sea became extinct. The next mass extinction is called Devonian extinction, occurring 365 million years ago during the Devonian period. This extinction also saw the end of numerous sea organisms.

The largest extinction took place around 250 million years ago. Known as the Permian-Triassic extinction, or the Great Dying, this event saw the end of more than 90 percent of the Earth&rsquos species. Although life on Earth was nearly wiped out, the Great Dying made room for new organisms, including the first dinosaurs. About 210 million years ago, between the Triassic and Jurassic periods, came another mass extinction. By eliminating many large animals, this extinction event cleared the way for dinosaurs to flourish. Finally, about 65.5 million years ago, at the end of the Cretaceous period came the fifth mass extinction. This is the famous extinction event that brought the age of the dinosaurs to an end.

In each of these cases, the mass extinction created niches or openings in the Earth&rsquos ecosystems. Those niches allowed for new groups of organisms to thrive and diversify, which produced a range of new species. In the case of the Cretaceous extinction, the demise of the dinosaurs allowed mammals to thrive and grow larger.

Scientists refer to the current time as the Anthropocene period, meaning the period of humanity. They warn that, because of human activities such as pollution, overfishing, and the cutting down of forests, the Earth might be on the verge of&mdashor already in&mdasha sixth mass extinction. If that is true, what new life would rise up to fill the niche that we currently occupy?

Many species have gone extinct throughout history and all that marks their presence on Earth are fossils, such as this one of a dinogorgon.


How long do most species last before going extinct?

The majestic blue whale has plied the seas for about 4.5 million years, while the Neanderthals winked out of existence in a few hundred thousand years. But are those creatures representative of species overall? How long do species usually last before they go extinct?

It turns out the answer we find now could be very different than it usually is. Because of habitat destruction, climate change, and a range of other factors, plants and animals are disappearing from the planet faster than all but maybe five other points in history. Some experts say we're in the sixth mass extinction event. But even in calmer periods of Earth's history, the answer has varied depending on the type of species you're looking at. For mammals, the average species exists for 1 million to 2 million years, according to an article in the journal People & the planet.

However, this average doesn't hold during all geologic periods and for all mammals. The average for the Cenozoic era (65 million years ago to present) mammals is 3.21 million years, with larger mammals lasting longer than smaller mammals, according to a 2013 study in the journal Integrative Zoology. For invertebrate species, the duration is even more impressive they last between 5 million to 10 million years, on average.

These numbers, however, are contentious. Experts don't agree on the average amount of time that species in any category last before going extinct. The fossil record documents when a species shows up and when it disappears, but it leaves a wide margin of error because conditions must be perfect for fossils to form, and those conditions aren't always present when a species shows up and blinks out. And these longevity stats aren't that useful anyway. Stuart Pimm, a leading extinction expert and a conservation ecologist at Duke University's Nicholas School of the Environment, said he prefers to think about extinction in terms of how many species die out every day, or month, or year.

"It's easier to think of in terms of… death rates, largely because there are some species that live a really long time," Pimm said. "And then there are other species that are short-lived. And the average doesn't really help you as much as you might think."

This species death rate, called the background extinction rate, is also contentious. Pimm placed the historic number &mdash a figure that covers all time, excluding mass extinctions &mdash at around one species extinction per 1 million species per year. That means that if there were a million species on the planet, one would have gone extinct each year. (For comparison, there are about 8.7 million species on the planet today, according to a study in the journal PLOS Biology.) However, other experts estimate species typically die off at a rate of 0.1 species per million per year and still others at two species per million per year, according to a research article in the journal Science Advances.

The current extinction rate is much higher than any of these predictions about the past &mdash about 1,000 times more than Pimm's background extinction rate estimate, he said. However, not everyone agrees on how accelerated species extinction is now, said Tierra Curry, a senior scientist at the Center for Biological Diversity in Oregon. Some experts estimate that the current extinction rate is only 100 times faster or, at the other extreme, 10,000 times faster.

There are several reasons why estimates of the current extinction rate vary. "The extinction rate is based on how many species are on Earth and how rapidly they're going extinct," Curry said. "And no one actually knows the answer to either one of those questions." About 90% of living species &mdash largely insects &mdash are probably unnamed, Pimm added. And if researchers don't know that a species existed, they won't know it went extinct. Another complication is that it can be difficult to tell when species are dead. Just because researchers haven&rsquot seen them for several years doesn't mean they're gone for good. Calculations can get more difficult when species are extinct in the wild but live on in zoos.

One thing the experts do agree on is that the modern extinction rate is far too high. "Species are adapting as fast as they can," Pimm said. "But eventually the luck runs out and they don't adapt fast enough. And they go."


Related Biology Terms

  • Hybrid – An organism produced by the crossing of two distinct species.
  • Reproductive Barriers – Obstacles that prevent two animals from producing fertile offspring.
  • Binomial Nomenclature – The system of naming individual species with two Latin names, the first related to their genus, the second to their species.
  • Taxonomical Hierarchy – The system into which all organisms are placed for classification.

1. Domestic ferrets and wild black-footed ferrets look almost identical. The black-footed ferret, Mustela nigripes, is native to North America. The domestic ferret, Mustela putorius furo, is native to Europe and only exists in captivity in North America. Could these two populations be one species?
A. No
B. Yes
C. Maybe

2. If scientists tell species apart by how they look, why aren’t males and females that look differently considered different species?
A. They can interbreed
B. Sexual dimorphism is a type of speciation
C. They are considered different species

3. It was recently discovered that what where believed to be two species of trout, one in Russia and one in the US, have almost identical DNA and can reproduce if the eggs and sperm are artificially brought together. The populations are continually separated by saltwater ocean, which they cannot traverse. While some scientists have argued to make them one species, others have argued to keep them separate. What is the argument for keeping them as separate species?
A. They cannot successfully interbreed.
B. They are still separated by a geographical reproductive barrier.
C. There are no good arguments.


What is a species?

The most famous definition of a species comes from the 20th century German-born biologist Ernst Mayr, who emphasised the importance of interbreeding. The idea (roughly) is that two organisms are of the same species if they can breed with one another to produce fertile offspring. That is why a donkey and a horse aren’t the same species: they can breed and produce offspring, but not fertile offspring.

Mayr’s way of thinking about species has some amazing consequences. Recently, due to rising temperatures in the Arctic, polar bears and grizzly bears have been coming into increased contact, and have been producing fertile offspring. The offspring are (adorably) called grolar or pizzly bears. What this suggests is that polars and grizzlies may actually be the same species after all, despite radical differences in size, appearance, hibernation behaviours, diet and so on.

But it wasn’t long before the problems with Mayr’s approach became apparent. The definition makes use of the notion of interbreeding. This is all very well with horses and polar bears, but smaller organisms like bacteria do not interbreed at all. They reproduce entirely asexually, by simply splitting in two. So this definition of species can’t really apply to bacteria. Perhaps when we started thinking about species in terms of interbreeding, we were all just a bit too obsessed with sex.

Ernst Haeckel’s (1866) conception of the three kingdoms of life. Wikimedia Commons

So maybe we should forget about sex and look for a different approach to species. In the 1960s, another German biologist, Willi Hennig, suggested thinking about species in terms of their ancestry. In simple terms, he suggested that we should find an organism, and then group it together with its children, and its children’s children, and its children’s children’s children. Eventually, you will have the original organism (the ancestor) and all of its descendents. These groups are called clades. Hennig’s insight was to suggest that this is how we should be thinking about species.

But this approach faces its own problems. How far back should you go before you pick the ancestor in question? If you go back in history far enough, you’ll find that pretty much every animal on the planet shares an ancestor. But surely we don’t want to say that every single animal in the world, from the humble sea slug, to top-of-the-range apes like human beings, are all one big single species?


Causes of Extinction

Ultimate Causes

Ultimately, every species has three “choices”. They can adapt to a situation, somehow evolving a novel or more efficient way to live. They can migrate, in the hopes that other areas will provide the resources they need with less competition. Or, as is the case for many animals, they can die. Extinction, as has been demonstrated in the fossil record, far surpasses survival for most species. While this may be seen as a negative thing, remember that extinction not only leaves new niches open to colonize, but can also be caused by a species becoming more successful. While one species may take over for a while, they usually undergo speciation into a variety of forms.

Proximate Causes

There are many more proximate causes of extinction. In mathematical terms, extinction happens any time the rate of reproduction is lower than the rate individuals are dying. This situation inevitably leads to extinction, but there are a number of factors which can drive these rates.

Predation, for example, is a major cause of extinction for many animals. Many species of fish in the Caribbean are currently threatened by the emergence of a new species, the Lionfish. Lionfish are not native to the Caribbean, and have no natural predators of their own. As such, they have pretty much free reign on the fish of the Caribbean. Many of these endemic species are being wiped out by the lionfish, and extinction is the likely result. In a similar story, extinction is plaguing many species of birds and lizards which have been exposed the brown tree snake. The snake, transported on cargo ships during WWII, has no natural predators on the islands to which is was transported. As such, the snake population has exploded and driven its prey items towards extinction, if not into it.

Other causes, which are directly the result of human action, involve habitat destruction and fragmentation. As we destroy the resources animals need to survive, we decrease the capacity an area can hold. As we further divide these areas with roads, fences, and other boundaries, we decrease the ability of species to migrate and successfully reproduce. This phenomena, as well as hunting and exploitation of animals for meat and game, has cause the extinction of a massive amount of animals. Scientists now speculate that, due to human interactions with the rest of nature, the world is entering another mass extinction event.

1. How do we know an animal is really extinct?
A. We have no documented and confirmed sightings of the animal in recent times
B. We can never know
C. We find its fossils

2. When considering extinct organisms which do not leave good fossils, how can scientists claim to pinpoint their extinctions?
A. Voodoo Magic
B. Only organisms with fossils can be determined
C. Chemical evidence points to many extinction events

3. Scientists want to revive the Woolly Mammoth. To do so, they supposed that they could use the DNA found in a frozen male mammoth to impregnate a female elephant. Would this “reverse” extinction?
A. Yes
B. No
C. Only if the baby comes out a Mammoth


Watch the video: Endangered,endemic,extinct speciesBiodiversity (August 2022).