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, 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.
The Holocene extinction includes the disappearance of large land mammals known as megafauna, starting between 9,000 and 13,000 years ago, the end of the last Ice Age. This may have been due to the extinction of the mammoths whose habits had maintained grasslands which became birch forests without them. The new forest and the resulting forest fires may have induced climate change. Such disappearances might be the result of the proliferation of modern humans. These extinctions, occurring near the Pleistocene–Holocene 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. 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 certainly coincides with megafaunal extinction in Australia, New Zealand and Madagascar, in a similar way that any large, adaptable predator moving into a new ecosystem would. Only during the most recent parts of the extinction have plants also suffered large losses.
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.
- 1 Human influence on extinction
- 2 Prehistoric extinctions
- 3 Extinctions into the Common Era
- 4 Recent extinctions - 1500 onwards
- 5 Contemporary crises
- 6 See also
- 7 References
- 8 Further reading
- 9 External links
Human influence on extinction
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. 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. 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. Other, related human causes of the extinction event include deforestation, hunting, pollution, 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) 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.
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". 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. 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. Numerous scientific studies—such as a 2004 report published in Nature, 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.
Recent extinctions described are well-documented, but the nomenclature used varies. The term Anthropocene is a term that is used by few scientists, and some commentators may refer to the current and projected future extinctions as part of a longer Holocene extinction. 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. 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."
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, a study by Lyons et al., demonstrated conclusively that a hyperdisease was unlikely to have caused the extinction. 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. 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. 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 disappearing from the fossil record around the same time as the earliest human settlers. It has been suggested that the African megafauna survived as they evolved alongside humans. 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.
Australia was once home to a large assemblage of megafauna, with many parallels to those found on the African continent today. Humans arrived on the continent very early, and evidence suggests the large marsupials began dying out immediately after this, 50,000 years ago. Scientific argument continues as to the exact date range. Examples of organisms thought to have been wiped out by humans are:
- Diprotodon (giant relatives of the wombats)
- Zygomaturus (a "marsupial rhino")
- Hulitherium (a large marsupial herbivore)
- Phascolonus (a giant wombat)
- Palorchestes azael (a marsupial "tapir")
- Macropus titan (a giant kangaroo)
- Procoptodon goliah (a hoof-toed giant short-faced kangaroo)
- Sthenurus (a giant kangaroo)
- Simosthenurus (a giant kangaroo)
- Protemnodon (a giant kangaroo)
- Propleopus oscillans (an omnivorous kangaroo)
- Wonambi (a five-to-six-metre-long Australian constrictor snake)
- Thylacoleo carnifex (a lioness-sized marsupial carnivore)
- Varanus priscus (or Megalania prisca) (a giant predatory monitor lizard)
- Quinkana (a five-to-six metre-long predatory terrestrial crocodile)
- Dromornithidae (an entire family of giant birds. Though around the size of emus, they are actually more related to ducks and other waterfowl.)
North and South America
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 of prey populations. Discoveries at Monte Verde in South America and at Meadowcroft Rock Shelter in Pennsylvania have caused a controversy 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. 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) 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.
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 but it is not known if this hunting was the cause of the subsequent massive ecological changes, widespread extinctions and climate changes.
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.
The following species, among many others, became extinct in this period.
- Doedicurus, a giant armadillo
- American cheetah
- American lion
- Holochilus primigenus
- Pygmy mammoth and woolly mammoth
- Smilodon, sabre toothed cats
Pacific, including Hawaii
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. It is currently estimated that among the bird species of the Pacific some 2000 species have gone extinct since the arrival of humans. Among the extinctions were:
- The moa-nalos, grazing ducks from Hawaii.
- The nēnē-nui, or woodwalking goose, a large species of goose that once inhabited the island of Maui.
- Sylviornis, a giant galliform bird from New Caledonia
- Mekosuchine crocodiles from New Caledonia, Fiji and Samoa.
- Meiolaniid turtles on Lord Howe Island and New Caledonia
- "Gavialis" papuensis, a marine crocodilian from the Solomon Islands.
- Pygmy hippos of Cyprus (Phanourios minutus), Crete (Hippopotamus creutzburgi), Malta (H. melitensis) and Sicily (H. pentlandi)
- Balearic Islands cave goat (Myotragus balearicus) of Majorca and Minorca
- Dwarf elephants of Cyprus (Elephas cypriotes), Sicily, Malta (E. falconeri) and many other islands
- Giant swan (Cygnus falconeri) of Malta
- Giant dormice: Minorcan giant dormouse, Majorcan giant dormouse
- Sardinian pika of Sardinia and Corsica
Extinctions into the Common Era
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.
Starting with the arrival of humans around 2,000 years ago, nearly all of the island's megafauna became extinct, including:
- Eight or more species of elephant birds, giant flightless ratites in the genera Aepyornis and Mullerornis.
- 17 species of lemur, known as giant, subfossil lemurs, including:
- Giant aye-aye (Daubentonia robusta)
- Sloth lemurs, including chimpanzee-sized Palaeopropithecus and gorilla-sized Archaeoindris
- Koala lemurs (Megaladapis), a koala-like, orangutan-sized arboreal lemur
- Monkey lemurs, most terrestrial of lemurs, often compared to baboons or macaques.
- Pachylemur, a larger, more robust genus of ruffed lemurs
- Giant fossa
- Voay, a terrestrial giant dwarf crocodile
- Plesiorycteropus, a genus containing two species of superficially Aardvark-like digging mammal unlike anything alive today
- Three species of Malagasy hippopotamus
- Malagasy crowned eagle (Stephanoaetus mahery, a giant bird of prey)
- Local giant tortoises
Recent extinctions - 1500 onwards
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. Stuart Pimm stated "the current rate of species extinction is about 100 times the natural rate" for plants. Mass extinctions are characterized by the loss of at least 75% of species within a geologically short period of time.
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.
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 (e.g. Père David's deer, Hawaiian crow) and by marginal survivals of highly publicized megafauna that are ecologically extinct (e.g. 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 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 organisms listed below are not comprehensive, but are some of the most famous examples.
Mass extinctions following recent human colonisation
New Zealand is characterised by its geographic isolation and island biogeography, and had been isolated from mainland Australia for 80 million years. The arrival of Polynesian settlers circa around the 13th century resulted in the extinction of all of the islands megafaunal birds, many of which had adapted flightlessness and ground nesting habits in response to having no extant endemic mammalian predators. Pressure from human hunting and introduced species such as possums and rats had a devastating effect on many of the bird radiations on the islands.
- 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.
- The giant Haast's eagle, Harpagornis
- Two species of giant flightless predatory adzebills.
- Finsch's duck, a large, flightless duck
Indian Ocean Islands
Starting circa 1500 years ago, a number of species became extinct upon human settlement of the islands, including:
- Several species of giant tortoise on the Seychelles and Mascarene Islands, including Cylindraspis and Meiolania platyceps
Other notable recent extinctions
Some examples of modern extinctions of "charismatic" mammal fauna include:
- Aurochs (1627), a wild cow that was domesticated to form modern cattle.
- Steller's sea cow (1768), a large sirenian driven to extinction across its prehistoric range across the North Pacific by hunting. The last populations, reduced to the Commander Islands, were driven to extinction 27 years after their discovery by Europeans in 1741 (where they may have numbered at 2000) after heavy hunting.
- Sardinian pika (1774) primitive lagomorph native to the Mediterranean islands of Sardinia and Corsica
- Bluebuck, a species of antelope. Despite already being uncommon by the time of European discovery, habitat conversion for agriculture and hunting resulted in its extinction by 1800.
- Atlas bear (1870s)
- Falkland Islands wolf (1876)
- Quagga (1883), zebra subspecies, Southeast Africa
- Tarpan, a European wild horse. Although extinct in the wild by 1890, the last captive animal died in 1909.
- Thylacine (1936) or Tasmanian tiger, Thylacinus cynocephalus, a marsupial carnivore prosecuted by farmers and the government for predating on livestock
- Caribbean monk seal (1950s)
- Pyrenean ibex (2000). Attempts were made to clone the species shortly after its extinction, and despite brief success the only surviving foetus died after 7 minutes.
- Yangtze dolphin, extinct 2006
- The closely related Bali tiger (1937) and Javan tiger (1970s)
- Eastern cougar (2011) a subspecies of North American cougar
- Western black rhinoceros (2011)
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:
- Many species of birds on the Mascarene Islands, including the giant flightless pigeons known as the dodo of Mauritius (1662) and the Rodrigues solitaire (1778) and the unrelated Réunion solitaire, which was a flightless ibis (early 1800s)
- Aldabrachampsus on Aldabra Atoll
- Great auk (1852) once highly abundant flightless auk around the North Atlantic
- Passenger pigeons (1914) of North America, once so abundant that accounts suggest their flocks took days to pass
- The Carolina parakeet (1918) of the American southeast
- Ten species or subspecies of birds have disappeared from the Hawaiian islands since the 1980s. These include:
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: "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."
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.
The decline of amphibian populations has also been identified as an indicator of environmental degradation. As well as habitat loss and pollution, Chytridiomycosis, a fungal infection thought to have been accidentally spread by human travel, has caused severe population drops of several species of frogs, including the extinction of the golden toad, the reduction of Rabb's fringe-limbed treefrog to an endling, and the extinction of the Panamanian golden frog in the wild, among many other less publicised declines. Amphibians are now the most endangered vertebrate group, having existed for more than 300 million years through three other mass extinctions.
Mass bat deaths
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 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
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.
Overfishing has had a devastating effect on marine organism populations for several centuries, and will continue to do so into the future.
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