Holocene extinction

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"Sixth extinction" redirects here. For other uses, see Sixth extinction (disambiguation).
The dodo, a flightless bird of Mauritius, became extinct during the mid-late seventeenth century after humans destroyed the forests where the birds made their homes and introduced mammals that ate their eggs.
The quagga became extinct in 1883.
The passenger pigeon became extinct in 1914.
The Mexican grizzly bear became extinct in 1964.
The Caribbean monk seal was officially declared extinct in 2008.

The Holocene extinction, otherwise referred to as the Sixth extinction or Anthropocene extinction, is a name proposed to describe the ongoing extinction event of species during the present Holocene epoch (since around 10,000 BCE) mainly due to human activity. The large number of extinctions span numerous families of plants and animals including mammals, birds, amphibians, reptiles and arthropods. Although 875 extinctions occurring between 1500 and 2009 have been documented by the International Union for Conservation of Nature and Natural Resources,[1] with widespread degradation of highly biodiverse habitats such as coral reefs and rainforest, as well as other areas, the vast majority are thought to be undocumented. According to the species-area theory and based on upper-bound estimating, the present rate of extinction may be up to 140,000 species per year,[2] making it the greatest loss of biodiversity since the Cretaceous–Paleogene extinction event.

The Holocene extinction includes the disappearance of large land animals known as megafauna, starting between 9,000 and 13,000 years ago, the end of the last Ice Age. Megafauna outside of the African continent, that did not evolve alongside humans, proved highly sensitive to the introduction of new predation, and many died out shortly after early humans began spreading and hunting across the Earth (additionally, many African species have also gone extinct in the Holocene). The extinction of the mammoths, whose habits had maintained grasslands became birch forests without them.[3] The new forest and the resulting forest fires may have induced climate change.[3] Such disappearances might be the result of the proliferation of modern humans. These extinctions, occurring near the PleistoceneHolocene boundary, are sometimes referred to as the Quaternary extinction event.

There is no general agreement on where the Holocene, or anthropogenic, extinction begins, and the Quaternary extinction event which includes climate change resulting in the end of the last ice age ends, or if they should be considered separate events at all.[3][4] Some have suggested that anthropogenic extinctions may have begun as early as when the first modern humans spread out of Africa between 100,000 and 200,000 years ago, which is supported by rapid megafaunal extinction following recent human colonisation in Australia, New Zealand and Madagascar,[5] in a similar way that any large, adaptable predator moving into a new ecosystem would. In many cases, it is suggested even minimal hunting pressure was enough to wipe out large fauna, particularly on geographically isolated islands.[6][7] Only during the most recent parts of the extinction have plants also suffered large losses.[8]

The ecology of Homo sapiens has been noted as being that of an unprecedented 'global superpredator' that regularly preys on the adults of other apex predators and has worldwide effects on food webs.[9] Extinctions of species have occurred on every land mass and ocean, with many famous examples within Africa, Asia, Europe, Australia, North and South America, and on smaller islands. Overall, the Holocene extinction can be characterized by the human impact on the environment. The Holocene extinction continues into the 21st century, with overfishing, ocean acidification and the amphibian crisis being a few broader examples of an almost universal, cosmopolitan decline of biodiversity.

It has been suggested human activity has made the period following the mid-20th century different enough from the Holocene to consider it a new geological epoch, known as the Anthropocene,[10] which will be considered for implementation into the timeline of Earth's history by the International Commission on Stratigraphy in 2016.[11]

Human influence on extinction[edit]

The % of megafauna on different land masses over time, with the arrival of humans indicated.

Extinction of animals, plants, and other organisms caused by human actions may go as far back as the late Pleistocene, over 12,000 years ago. There is evidence that abrupt climate change has especially played an enormous role in the extinction of larger mammals.[12] However, while previous mass extinctions were due to natural environmental causes, research shows that wherever on Earth humans have migrated, other species have gone extinct, and human population growth, most prominently in the past two centuries, is regarded as one of the underlying causes of extinction.[13] In terms of how humans have contributed, three major factors include the increased global concentration of greenhouse gases, affecting the global climate; oceanic devastation, such as through overfishing and contamination; and the modification and destruction of vast tracts of land and river systems around the world to meet solely human-centered ends (with 10 to 15 percent of Earth's land surface now used as urban-industrial or row-crop agricultural sites and 6 to 8 percent used as pastures), thus ruining the local ecosystems.[14][15] Other, related human causes of the extinction event include deforestation, hunting, pollution,[16] the introduction in various regions of non-native species, and the widespread transmission of infectious diseases. At present, the rate of extinction of species is estimated at 100 to 1,000 times higher than the "base" or historically typical rate of extinction (in terms of the natural evolution of the planet)[17] and also the current rate of extinction is, therefore, 10 to 100 times higher than any of the previous mass extinctions in the history of Earth. It is also the only known mass extinction of plants.[citation needed]

The abundance of species extinctions considered anthropogenic, or due to human activity, have sometimes (especially when referring to hypothesized future events) been collectively called the "Anthropocene extinction".[18][19] The Anthropocene is a term introduced in 2000. It is now widely accepted that a new geological epoch has begun, characterised by the most abrupt and widespread extinction of species since the Cretaceous–Paleogene extinction event 66 million years ago.[5] In The Future of Life (2002), E.O. Wilson of Harvard calculated that, if the current rate of human disruption of the biosphere continues, one-half of Earth's higher lifeforms will be extinct by 2100. A 1998 poll conducted by the American Museum of Natural History found that seventy percent of biologists believe that we are in the midst of an anthropogenic extinction.[20] Numerous scientific studies—such as a 2004 report published in Nature,[21] and papers authored by the 10,000 scientists who contribute to the IUCN's annual Red List of threatened species—have since reinforced this conviction.

The evidence of all previous extinctions is geological in nature, and shorter geological time scale is of the order of several hundred thousand to several million years. Even extinctions caused by instantaneous events such as the impact of the asteroid in Chicxulub, which is currently the best example, extend the equivalent of many human lives, due to complex ecological interactions that were triggered by the event.[citation needed]

Recent extinctions described are well-documented,[12] but the nomenclature used varies. The term Anthropocene is a term that is used by few scientists,[12] and some commentators may refer to the current and projected future extinctions as part of a longer Holocene extinction.[22] The Holocene–Anthropocene boundary is contested, with some commentators asserting significant human influence on climate for much of what is normally regarded as the Holocene Epoch.[23] Other commentators place the Holocene–Anthropocene boundary at the industrial revolution while also saying that "Formal adoption of this term in the near future will largely depend on its utility, particularly to earth scientists working on late Holocene successions."[12]

Three hypotheses have been proposed to explain the extinction of megafauna in the late Pleistocene. Of these, only two have much scientific credibility. Although Ross McPhee proposed that a hyper-disease may have been the cause of the extinction,[24] a study by Lyons et al., demonstrated conclusively that a hyperdisease was unlikely to have caused the extinction.[25] The two main theories to the extinction are climate change and human hunting. The climate change theory has suggested that a change in climate near the end of the late Pleistocene stressed the megafauna to the point of extinction.[26] Some scientists favor abrupt climate change as the catalyst for the extinction of the mega-fauna at the end of the Pleistocene, but there are many who believe increased hunting from early modern humans also played a part, with others even suggesting that the two interacted.[5][27][28] In the Americas, a controversial explanation for the shift in climate is presented under the Younger Dryas impact hypothesis.

Megafauna was once found on every continent of the world and large islands such as New Zealand and Madagascar, but is now almost exclusively found on the continent of Africa, with notable comparisons on Australia and the islands previously mentioned experiences population crashes and trophic cascades shortly after the earliest human settlers.[6][7] It has been suggested that the African megafauna survived as they evolved alongside humans.[5] The timing of South American megafaunal extinction does not appear to correspond to human arrival, although the possibility of whether human activity at the time may have impacted the global climate enough to cause such an extinction has been suggested.[5]

It has been noted, in the face of such evidence, Homo sapiens is unique in its ecology as an unprecedented 'global superpredator', regularly preying on large numbers of fully grown terrestrial and marine apex predators, and with a great deal of influence over food webs and climatic systems worldwide.[9]

Prehistoric extinctions[edit]

Although significant debate exists as to how much human predation and indirect effects contributed to prehistoric extinctions, certain population crashes have been directly correlated with human arrival.[4][5]

Australia[edit]

Australia was once home to a large assemblage of megafauna, with many parallels to those found on the African continent today. Australia's fauna is characterised by primarily marsupial mammals, and many reptiles and birds, all existing as giant forms until recently. Humans arrived on the continent very early, about 50,000 years ago.[5] The extent human arrival contributed is controversial; climatic drying of Australia 40,000-60,000 years ago was an unlikely cause, as it was less severe in speed or magnitude than previous regional climate change which failed to kill off megafauna.

Due to the older timeframe and the soil chemistry on the continent, very little subfossil preservation evidence exists than elsewhere.[29] However, continent-wide extinction of all genera weighing over 100 kilograms, and six of seven genera weighing between 45 and 100 kilograms occurred around 46,400 years ago (4,000 years after human arrival)[30] and the fact that megafauna survived until a later date on the island of Tasmania following the establishment of a land bridge[31] suggest direct hunting or anthropogenic ecosystem disruption such as fire-stick farming as likely causes. The first evidence of direct human predation leading to extinction in Australia was published in 2016.[32]

Genyornis newtoni, a 7 foot tall flightless bird. Evidence of egg cooking in this species is the first evidence of megafaunal hunting by humans on Australia.[32]

Extinctions in Australia continued from original settlement until today in both plants and animals, whilst many more animals and plants have declined or are endangered.

North and South America[edit]

Reconstructed woolly mammoth bone hut, based on finds in Mezhyrich.

There has been a debate as to the extent to which the disappearance of megafauna at the end of the last glacial period can be attributed to human activities by hunting, or even by slaughter[33] of prey populations. Discoveries at Monte Verde in South America and at Meadowcroft Rock Shelter in Pennsylvania have caused a controversy[34] regarding the Clovis culture. There likely would have been human settlements prior to the Clovis Culture, and the history of humans in the Americas may extend back many thousands of years before the Clovis culture.[34] The amount of correlation between human arrival and megafauna extinction is still being debated: for example, in Wrangel Island in Siberia the extinction of dwarf woolly mammoths (approximately 2000 BCE)[35] did not coincide with the arrival of humans, nor did megafaunal mass extinction on the South American continent, although it has been suggested climate changed induced by anthropogenic effects elsewhere in the world may have contributed.[5]

Comparisons are sometimes made between recent extinctions (approximately since the industrial revolution) and the Pleistocene extinction near the end of the last glacial period. The latter is exemplified by the extinction of large herbivores such as the woolly mammoth and the carnivores that preyed on them. We know that humans of this era actively hunted the mammoth and the mastodon[36] but it is not known if this hunting was the cause of the subsequent massive ecological changes, widespread extinctions and climate changes.[3][4]

The ecosystems encountered by the first Americans had not been exposed to human interaction, and may have been far less resilient to human made changes than the ecosystems encountered by industrial era humans. Therefore, the actions of the Clovis people, despite seeming insignificant by today's standards could indeed have had a profound effect on the ecosystems and wild life which was entirely unused to human influence.[5]

The following species, among many others, became extinct in this period.

Caribbean[edit]

Human arrival around 6,000 years ago is correlated with the extinction of many species.[37] Examples include:

Pacific islands[edit]

Recent research, based on archaeological and paleontological digs on 70 different islands, has shown that numerous species went extinct as people moved across the Pacific, starting 30,000 years ago in the Bismarck Archipelago and Solomon Islands.[38] It is currently estimated that among the bird species of the Pacific some 2000 species have gone extinct since the arrival of humans, representing a 20% drop in the biodiversity of birds worldwide.[39] Among the extinctions were:

Mammuthus creticus,a dwarf elephant once endemic to Crete. It was one of many species of small elephants found on islands in the Mediterranean.

Mediterranean Islands[edit]

Extinctions into the Common Era[edit]

More recent settlement of isolated land masses where megafauna continued to survive almost immediately resulted in their extinction. Calculations suggest this occurred even if only a small number of animals were hunted. It has been suggested this contemporary evidence supports the theory that humans were capable of causing or at least contributing to the extinctions of the Quaternary extinction event.[5]

Hawaiian islands[edit]

The first settlers are thought to have arrived in the islands between 300 and 800 CE, with European arrival in the 16th century. Hawaii is notable for its endemism of plants, birds, insects, mollusks and fish; 30% of its organisms are endemic. Many of its species are endangered or have gone extinct, primarily due to accidentally introduced species and livestock grazing. Over 40% of its bird species have gone extinct, and it is the location of 75% of extinctions in the United States.[40]

Extinction has increased in Hawaii over the last 200 years and is relatively well documented, with extinctions among native snails used as estimates for global extinction rates.[41]

Indian Ocean Islands[edit]

Starting circa 1500 years ago, a number of species became extinct upon human settlement of the islands, including:

Madagascar[edit]

Further information: Wildlife of Madagascar and Subfossil lemur
Megaladapis edwardsi, a large arboreal lemur, being one of the many of the diverse assemblage of megafauna recently wiped out by human activity on Madagascar.
Radiocarbon dating of multiple subfossil specimens shows that now extinct giant lemurs were present in Madagascar until after human arrival.

Within 500 years of the arrival of humans between 2,500-2,000 years ago, nearly all of Madagascar's distinct, endemic and geographically isolated megafauna became extinct.[42] The largest animals, of more than 150 kg, were extincted very shortly after the first human arrival, with large and medium-sized species dying out after prolonged hunting pressure from an expanding human population moving into more remote regions of the island around 1000 years ago. Smaller fauna experienced initial increases due to decreased competition, and then subsequent declines over the last 500 years.[7] All fauna weighing over 10 kg died out. The primary reasons for this are human hunting and habitat loss from early aridification, both of persist and threaten Madagascar's remaining taxa today.

Recent extinctions - 1500 onwards[edit]

One scientist estimates the current extinction rate may be 10,000 times the background extinction rate. Nevertheless, most scientists predict a much lower extinction rate than this outlying estimate.[45] Stuart Pimm stated "the current rate of species extinction is about 100 times the natural rate" for plants.[46] Mass extinctions are characterized by the loss of at least 75% of species within a geologically short period of time.[47][48][49]

In a pair of studies published in 2015, extrapolation from observed extinction of Hawaiian snails led to the conclusion that 7% of all species on Earth may have been lost already.[41]

Megafaunal extinctions continue into the 21st century. Modern extinctions are more directly attributable to human influences. Extinction rates are minimized in the popular imagination by the survival of captive populations of animals that are extinct in the wild (such as the Père David's deer, Hawaiian crow) and by marginal survivals of highly publicized megafauna that are ecologically extinct (such as the giant panda, Sumatran rhinoceros, North American black-footed ferret). However, the Holocene can also be characterisied by widespread extinctions among arthropods, widespread local extinctions of populations of species that still exist elsewhere (such as the extinction of gray whales in the Atlantic and of the leatherback sea turtle in Malaysia) and by universal declines in range and population of various animal and plant species throughout all of the world.

The IUCN characterises 'recent' extinction as those that have occurred past the cut-off point of 1500. The extinct species listed below are not comprehensive, but are some of the most famous examples.

New Zealand[edit]

A size comparison between 4 recently extinct New Zealand moa species and a human. 1. Dinornis novaezealandiae 2. Emeus crassus 3. Anomalopteryx didiformis 4. Dinornis robustus

New Zealand is characterised by its geographic isolation and island biogeography, and had been isolated from mainland Australia for 80 million years. It was one of the last large land masses to be colonised by humans. The arrival of Polynesian settlers circa 13th century resulted in the extinction of all of the islands megafaunal birds. Pressure from human hunting and introduced species such as possums and rats had a devastating effect on native birds, many of which had adapted flightlessness and ground nesting habits in response to having no extant endemic mammalian predators. Further extinctions have occurred since European settlement in the mid-19th century, and subsequent development.

Extinctions include, among many others:

  • Eleven species of moa, giant flightless ratites. The last moa went extinct around the 1500s, although accounts suggest some may have survived until more recently. It has been suggested even moderate hunting pressure was enough to drive moa across the islands to extinction in less than 200 years.[6]
  • The giant Haast's eagle, Harpagornis
  • Two species of giant flightless predatory adzebills.
  • Finsch's duck, a large, flightless duck

Other notable recent extinctions[edit]

Mammals[edit]

Javan tiger pictured 1938.

Some examples of modern extinctions of "charismatic" mammal fauna include:

Birds[edit]

The Poʻouli, a species of Hawaiian honeycreeper, and the only member of its genus. After severe habitat loss and predation from invasive species such as the small Asian mongoose and feral pigs and rats, the last individual died in 2004 after unsuccessful attempts at captive breeding.

Many birds have become extinct as a result of human activity, especially birds endemic to islands, including many flightless birds (see a more complete list under extinct birds). Notable extinct birds include:

Ten species or subspecies of birds have disappeared from the Hawaiian islands since the 1980s, primarily due to invasive species and habitat loss. These include:

Plants[edit]

The golden toad of Costa Rica, extinct since around 1989. Its disappearance has been attributed to a confluence of several factors, including El Niño warming, fungus, and the introduction of invasive species.

Contemporary crises[edit]

Peter Raven, past president of the American Association for the Advancement of Science (AAAS), states in the foreword to their publication AAAS Atlas of Population and Environment:[52] "We have driven the rate of biological extinction, the permanent loss of species, up several hundred times beyond its historical levels, and are threatened with the loss of a majority of all species by the end of the 21st century."[53]

189 countries which are signatory to the Convention on Biological Diversity (Rio Accord) have committed to preparing a Biodiversity Action Plan, a first step at identifying specific endangered species and habitats, country by country.

Various species are predicted to go extinct in the near future.[54][55][56][57]

Amphibian crisis[edit]

The decline of amphibian populations has also been identified as an indicator of environmental degradation. As well as habitat loss, introduced predators and pollution, Chytridiomycosis, a fungal infection thought to have been accidentally spread by human travel,[5] has caused severe population drops of several species of frogs, including (among many others) the extinction of the golden toad in Costa Rica and the Gastric-brooding frog in Australia. Many other amphibian species now face extinction, including the reduction of Rabb's fringe-limbed treefrog to an endling, and the extinction of the Panamanian golden frog in the wild. Chytrid fungus has spread across Australia, New Zealand, Central America and Africa, including countries with high amphibian diversity such as cloud forests in Honduras and Madagascar. Amphibians are now the most endangered vertebrate group, having existed for more than 300 million years through three other mass extinctions.

Mass bat deaths[edit]

Main article: White nose syndrome

Millions of bats in the US have been dying off since 2012 due to a fungal infection spread from European bats, which appear to be immune. Population drops have been as great as 90% within five years, and extinction of at least one bat species is predicted. There is currently no form of treatment, and such declines have been described as "unprecedented" in bat evolutionary history by Alan Hicks of the New York State Department of Environmental Conservation.

Global warming[edit]

Global warming is widely accepted as being a contributor to extinction worldwide, in a similar way that previous extinction events have generally included a rapid change in global climate and meteorology. It is also expected to disrupt sex ratios in many reptiles which have temperature-dependent sex determination.

Degradation of marine habitats[edit]

The collapse of Atlantic cod off the coast of Newfoundland in 1992 as a result of overfishing. The population never recovered, completely altering the ecosystem and rendering the species locally extinct in the area.

Rising levels of carbon dioxide are resulting in influx of this gas into the ocean, increasing its acidity. Marine organisms which possess Calcium Carbonate shells or exoskeletons experience physiological pressure as the carbonate reacts with acid. This is already resulting in coral bleaching on various coral reefs worldwide, which provide valuable habitat for very high biodiversity. Marine gastropods, bivalves and other invertebrates are also affected, as are any organisms that feed on them.

Fishing has had a devastating effect on marine organism populations for several centuries even before the explosion of destructive and highly effective fishing practices like trawling.[58] Humans are unique among predators in that they regularly predate on other adult apex predators, particularly in marine environments;[9] bluefin tuna and various sharks in particular are particularly vulnerable to predation pressure from human fishing.

Pollinator decline[edit]

The term pollinator decline refers to the reduction in abundance of insect and other animal pollinators in many ecosystems worldwide beginning at the end of the twentieth century, and continuing into the present day.[59]

Lists of affected species[edit]

See also[edit]

References[edit]

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