Holocene extinction

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The dodo became extinct during the mid-to-late 17th century due to habitat destruction and predation by introduced mammals.[1] It is an often-cited example of modern extinction.[2]

The Holocene extinction, otherwise referred to as the sixth mass extinction or Anthropocene extinction, is an ongoing extinction event of species during the present Holocene epoch (with the more recent time sometimes called Anthropocene) as a result of human activity.[3][4][5] The included extinctions span numerous families of plants[6] and animals, including mammals, birds, reptiles, amphibians, fishes and invertebrates. With widespread degradation of highly biodiverse habitats such as coral reefs and rainforests, as well as other areas, the vast majority of these extinctions are thought to be undocumented, as the species are undiscovered at the time of their extinction, or no one has yet discovered their extinction. The current rate of extinction of species is estimated at 100 to 1,000 times higher than natural background extinction rates.[4][7][8][9][10][11]

The Holocene extinction includes the disappearance of large land animals known as megafauna, starting at the end of the last glacial period. Megafauna outside of the African mainland (thus excluding Madagascar), which 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[12][13] (many African species have also gone extinct in the Holocene, but – with few exceptions – megafauna of the mainland was largely unaffected until a few hundred years ago).[14] These extinctions, occurring near the PleistoceneHolocene boundary, are sometimes referred to as the Quaternary extinction event.

The most popular theory is that human overhunting of species added to existing stress conditions as the extinction coincides with human emergence. Although there is debate regarding how much human predation affected their decline, certain population declines have been directly correlated with human activity, such as the extinction events of New Zealand and Hawaii. Aside from humans, climate change may have been a driving factor in the megafaunal extinctions, especially at the end of the Pleistocene.

Ecologically, humanity has been noted as an unprecedented "global superpredator"[15] that consistently preys on the adults of other apex predators, and has worldwide effects on food webs. There have been extinctions of species on every land mass and in every ocean: there are many famous examples within Africa, Asia, Europe, Australia, North and South America, and on smaller islands. Overall, the Holocene extinction can be linked to the human impact on the environment. The Holocene extinction continues into the 21st century, with meat consumption, overfishing, and ocean acidification and the decline in amphibian populations[16] being a few broader examples of a cosmopolitan decline in biodiversity. Human population growth and increasing per capita consumption are considered to be the primary drivers of this decline.[11][17][18][19]

The 2019 Global Assessment Report on Biodiversity and Ecosystem Services, published by the United Nations' Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services, posits that roughly one million species of plants and animals face extinction within decades as the result of human actions.[19][20][21][22]

Definitions[edit]

Extinction intensity.svgCambrianOrdovicianSilurianDevonianCarboniferousPermianTriassicJurassicCretaceousPaleogeneNeogene
Marine extinction intensity during the Phanerozoic
%
Millions of years ago
Extinction intensity.svgCambrianOrdovicianSilurianDevonianCarboniferousPermianTriassicJurassicCretaceousPaleogeneNeogene
The percentage of marine animal extinction at the genus level through the five mass extinctions

The Holocene extinction is also known as the "sixth extinction", as it is possibly the sixth mass extinction event, after the Ordovician–Silurian extinction events, the Late Devonian extinction, the Permian–Triassic extinction event, the Triassic–Jurassic extinction event, and the Cretaceous–Paleogene extinction event.[3][5][17][23][24][25] Mass extinctions are characterized by the loss of at least 75% of species within a geologically short period of time.[26][27] 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.[28][29] Some have suggested that anthropogenic extinctions may have begun as early as when the first modern humans spread out of Africa between 200,000 and 100,000 years ago; this is supported by rapid megafaunal extinction following recent human colonisation in Australia, New Zealand and Madagascar,[23] as might be expected when any large, adaptable predator (invasive species) moves into a new ecosystem. In many cases, it is suggested that even minimal hunting pressure was enough to wipe out large fauna, particularly on geographically isolated islands.[30][31] Only during the most recent parts of the extinction have plants also suffered large losses.[32]

In The Future of Life (2002), Edward Osborne 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 70% of biologists acknowledge an ongoing anthropogenic extinction event.[33] At present, the rate of extinction of species is estimated at 100 to 1,000 times higher than the background extinction rate, the historically typical rate of extinction (in terms of the natural evolution of the planet);[7][8][34] also, the current rate of extinction is 10 to 100 times higher than in any of the previous mass extinctions in the history of Earth. One scientist estimates the current extinction rate may be 10,000 times the background extinction rate, although most scientists predict a much lower extinction rate than this outlying estimate.[35] Theoretical ecologist Stuart Pimm stated that the extinction rate for plants is 100 times higher than normal.[36]

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.[37][38]

There is widespread consensus among scientists that human activity is accelerating the extinction of many animal species through the destruction of habitats, the consumption of animals as resources, and the elimination of species that humans view as threats or competitors.[39] That humans have become the primary driver of modern extinctions is undeniable, rapidly rising extinction trends impacting numerous animal groups including mammals, birds, reptiles, and amphibians has prompted scientists to declare a biodiversity crisis.[40] But some contend that this biotic destruction has yet to reach the level of the previous five mass extinctions.[41] Stuart Pimm, for example, asserts that the sixth mass extinction "is something that hasn't happened yet – we are on the edge of it."[42] In November 2017, a statement, titled "World Scientists’ Warning to Humanity: A Second Notice", led by eight authors and signed by 15,364 scientists from 184 countries asserted that, among other things, "we have unleashed a mass extinction event, the sixth in roughly 540 million years, wherein many current life forms could be annihilated or at least committed to extinction by the end of this century."[3] According to the World Wide Fund for Nature's 2020 Living Planet Report, wildlife populations have declined by 68% since 1970 as a result of overconsumption, population growth and intensive farming, which is further evidence that humans have unleashed a sixth mass extinction event.[43][44][45]

Anthropocene[edit]

A diagram showing the ecological processes of coral reefs before and during the Anthropocene

The abundance of species extinctions considered anthropogenic, or due to human activity, has sometimes (especially when referring to hypothesized future events) been collectively called the "Anthropocene extinction".[39][46][47] "Anthropocene" is a term introduced in 2000. Some now postulate that a new geological epoch has begun, with the most abrupt and widespread extinction of species since the Cretaceous–Paleogene extinction event 66 million years ago.[23]

The term "anthropocene" is being used more frequently by scientists, and some commentators may refer to the current and projected future extinctions as part of a longer Holocene extinction.[48][49] 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.[50] Other commentators place the Holocene–Anthropocene boundary at the industrial revolution and also say that "[f]ormal adoption of this term in the near future will largely depend on its utility, particularly to earth scientists working on late Holocene successions."

It has been suggested that human activity has made the period starting from the mid-20th century different enough from the rest of the Holocene to consider it a new geological epoch, known as the Anthropocene,[51] a term which was considered for inclusion in the timeline of Earth's history by the International Commission on Stratigraphy in 2016.[52][53] In order to constitute the Holocene as an extinction event, scientists must determine exactly when anthropogenic greenhouse gas emissions began to measurably alter natural atmospheric levels on a global scale, and when these alterations caused changes to global climate. Using chemical proxies from Antarctic ice cores, researchers have estimated the fluctuations of carbon dioxide (CO2) and methane (CH4) gases in the Earth's atmosphere during the late Pleistocene and Holocene epochs.[50] Estimates of the fluctuations of these two gases in the atmosphere, using chemical proxies from Antarctic ice cores, generally indicate that the peak of the Anthropocene occurred within the previous two centuries: typically beginning with the Industrial Revolution, when the highest greenhouse gas levels were recorded.[54][55]

Influences[edit]

Human activity[edit]

Activities contributing to extinctions[edit]

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

The Holocene extinction is mainly caused by human activities.[5][11][25][39] 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.[39] There is a correlation between megafaunal extinction and the arrival of humans,[56][57] and contemporary human population size and growth, along with per-capita consumption growth, prominently in the past two centuries, are regarded as the underlying causes of extinction.[11][17][19][24][40][58]

Biomass of mammals on Earth as of 2018[59][60]

  Livestock, mostly cattle and pigs (60%)
  Humans (36%)
  Wild animals (4%)

Human civilization was founded on and grew from agriculture.[61] The more land used for farming, the greater the population a civilization could sustain,[50][61] and subsequent popularization of farming led to habitat conversion.[11]

Habitat destruction by humans, including 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 13 percent of Earth's ice-free land surface now used as row-crop agricultural sites, 26 percent used as pastures, and 4 percent urban-industrial areas[62]), thus replacing the original local ecosystems.[63] The sustained conversion of biodiversity rich forests and wetlands into poorer fields and pastures (of lesser carrying capacity for wild species), over the last 10,000 years, has considerably reduced the Earth's carrying capacity for wild birds, among other organisms, in both population size and species count.[10][64][65]

Other, related human causes of the extinction event include deforestation, hunting, pollution,[66] the introduction in various regions of non-native species, and the widespread transmission of infectious diseases spread through livestock and crops.[8] Humans both create and destroy crop cultivar and domesticated animal varieties. Advances in transportation and industrial farming has led to monoculture and the extinction of many cultivars. The use of certain plants and animals for food has also resulted in their extinction, including silphium and the passenger pigeon.[67]

Some scholars assert that the emergence of capitalism as the dominant economic system has accelerated ecological exploitation and destruction, and has also exacerbated mass species extinction.[68] CUNY professor David Harvey, for example, posits that the neoliberal era "happens to be the era of the fastest mass extinction of species in the Earth's recent history".[69]

Apex predator[edit]

Megafauna were once found on every continent of the world and large islands such as New Zealand and Madagascar, but are now almost exclusively found on the continent of Africa, with notable comparisons on Australia and the islands previously mentioned experiencing population crashes and trophic cascades shortly after the earliest human settlers.[30][31] It has been suggested that the African megafauna survived because they evolved alongside humans.[23] The timing of South American megafaunal extinction appears to precede human arrival, although the possibility that human activity at the time impacted the global climate enough to cause such an extinction has been suggested.[23]

It has been noted, in the face of such evidence, that humans are unique in 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.[15] 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.[13][23][29][39] Human activity has been the main cause of mammalian extinctions since the Late Pleistocene.[70] A 2018 study published in PNAS found that since the dawn of human civilization, 83% of wild mammals, 80% of marine mammals, 50% of plants and 15% of fish have vanished. Currently, livestock make up 60% of the biomass of all mammals on earth, followed by humans (36%) and wild mammals (4%). As for birds, 70% are domesticated, such as poultry, whereas only 30% are wild.[59][60]

Agriculture and climate change[edit]

Recent investigations about hunter-gatherer landscape burning has a major implication for the current debate about the timing of the Anthropocene and the role that humans may have played in the production of greenhouse gases prior to the Industrial Revolution.[61] Studies on early hunter-gatherers raises questions about the current use of population size or density as a proxy for the amount of land clearance and anthropogenic burning that took place in pre-industrial times.[71][72] Scientists have questioned the correlation between population size and early territorial alterations.[72] Ruddiman and Ellis' research paper in 2009 makes the case that early farmers involved in systems of agriculture used more land per capita than growers later in the Holocene, who intensified their labor to produce more food per unit of area (thus, per laborer); arguing that agricultural involvement in rice production implemented thousands of years ago by relatively small populations have created significant environmental impacts through large-scale means of deforestation.[61]

While a number of human-derived factors are recognized as contributing to rising atmospheric concentrations of CH4 (methane) and CO2 (carbon dioxide), deforestation and territorial clearance practices associated with agricultural development may be contributing most to these concentrations globally.[54][61][73] Scientists that are employing a variance of archaeological and paleoecological data argue that the processes contributing to substantial human modification of the environment spanned many thousands of years ago on a global scale and thus, not originating as early as the Industrial Revolution. Gaining popularity on his uncommon hypothesis, palaeoclimatologist William Ruddiman in 2003, stipulated that in the early Holocene 11,000 years ago, atmospheric carbon dioxide and methane levels fluctuated in a pattern which was different from the Pleistocene epoch before it.[50][71][73] He argued that the patterns of the significant decline of CO2 levels during the last ice age of the Pleistocene inversely correlates to the Holocene where there have been dramatic increases of CO2 around 8000 years ago and CH4 levels 3000 years after that.[73] The correlation between the decrease of CO2 in the Pleistocene and the increase of it during the Holocene implies that the causation of this spark of greenhouse gases into the atmosphere was the growth of human agriculture during the Holocene such as the anthropogenic expansion of (human) land use and irrigation.[50][73]

Islands[edit]

Human arrival in the Caribbean around 6,000 years ago is correlated with the extinction of many species.[74] Examples include many different genera of ground and arboreal sloths across all islands. These sloths were generally smaller than those found on the South American continent. Megalocnus were the largest genus at up to 90 kilograms (200 lb), Acratocnus were medium-sized relatives of modern two-toed sloths endemic to Cuba, Imagocnus also of Cuba, Neocnus and many others.[75]

Recent research, based on archaeological and paleontological digs on 70 different Pacific islands has shown that numerous species became extinct as people moved across the Pacific, starting 30,000 years ago in the Bismarck Archipelago and Solomon Islands.[76] 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.[77]

Genyornis newtoni, a 2-metre (7 ft) tall flightless bird. Evidence of egg cooking in this species is the first evidence of megafaunal hunting by humans in Australia.[78]

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.[79] 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.[37]

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.[23] 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. 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.[80]

Due to the older timeframe and the soil chemistry on the continent, very little subfossil preservation evidence exists relative to elsewhere.[81] 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)[82] and the fact that megafauna survived until a later date on the island of Tasmania following the establishment of a land bridge[83] 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.[78]

Madagascar[edit]
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 and 2,000 years ago, nearly all of Madagascar's distinct, endemic and geographically isolated megafauna became extinct.[84] The largest animals, of more than 150 kilograms (330 lb), were extinct 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.[31] All fauna weighing over 10 kilograms (22 lb) died out. The primary reasons for this are human hunting and habitat loss from early aridification, both of which persist and threaten Madagascar's remaining taxa today.[citation needed]

The eight or more species of elephant birds, giant flightless ratites in the genera Aepyornis, Vorombe, and Mullerornis, are extinct from over-hunting,[85] as well as 17 species of lemur, known as giant, subfossil lemurs. Some of these lemurs typically weighed over 150 kilograms (330 lb), and fossils have provided evidence of human butchery on many species.[86]

New Zealand[edit]

New Zealand is characterised by its geographic isolation and island biogeography, and had been isolated from mainland Australia for 80 million years. It was the last large land mass to be colonised by humans. The arrival of Polynesian settlers circa 12th century resulted in the extinction of all of the islands' megafaunal birds within several hundred years.[87] The last moa, large flightless ratites, became extinct within 200 years of the arrival of human settlers.[30] The Polynesians also introduced the Polynesian rat. This may have put some pressure on other birds but at the time of early European contact (18th century) and colonisation (19th century) the bird life was prolific. With them, the Europeans brought ship rats, possums, cats and mustelids which decimated native bird life, some of which had adapted flightlessness and ground nesting habits and others had no defensive behavior as a result of having no extant endemic mammalian predators. The kakapo, the world's biggest parrot, which is flightless, now only exists in managed breeding sanctuaries. New Zealand's national emblem, the kiwi, is on the endangered bird list.[87]

Americas[edit]

Reconstructed woolly mammoth bone hut, based on finds in Mezhyrich.
The passenger pigeon was a species of pigeon endemic to North America. It experienced a rapid decline in the late 1800s due to habitat destruction and intense hunting after the arrival of Europeans. The last wild bird is thought to have been shot in 1901.

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[88] of prey populations. Discoveries at Monte Verde in South America and at Meadowcroft Rock Shelter in Pennsylvania have caused a controversy[89] 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.[89] 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)[90] did not coincide with the arrival of humans, nor did megafaunal mass extinction on the South American continent, although it has been suggested climate changes induced by anthropogenic effects elsewhere in the world may have contributed.[23]

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. Humans of this era actively hunted the mammoth and the mastodon,[91] but it is not known if this hunting was the cause of the subsequent massive ecological changes, widespread extinctions and climate changes.[28][29]

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.[23]

Afroeurasia[edit]

Africa experienced the smallest decline in megafauna compared to the other continents. This is presumably due to the idea that Afroeurasian megafauna evolved alongside humans, and thus developed a healthy fear of them, unlike the comparatively tame animals of other continents.[92] Unlike other continents, the megafauna of Eurasia went extinct over a relatively long period of time, possibly due to climate fluctuations fragmenting and decreasing populations, leaving them vulnerable to over-exploitation, as with the steppe bison (Bison priscus).[93] The warming of the arctic region caused the rapid decline of grasslands, which had a negative effect on the grazing megafauna of Eurasia. Most of what once was mammoth steppe has been converted to mire, rendering the environment incapable of supporting them, notably the woolly mammoth.[94]

Climate change[edit]

Top: Arid ice age climate
Middle: Atlantic Period, warm and wet
Bottom: Potential vegetation in climate now if not for human effects like agriculture.[95]
Bramble Cay melomys were declared extinct in June 2016. This is the first recorded mammalian extinction due to anthropogenic climate change.[96]

One of the main theories to the extinction is climate change. 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.[48][97] 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.[23][98][99] However, the annual mean temperature of the current interglacial period for the last 10,000 years is no higher than that of previous interglacial periods, yet some of the same megafauna survived similar temperature increases.[100][101][102][103][104][105] In the Americas, a controversial explanation for the shift in climate is presented under the Younger Dryas impact hypothesis, which states that the impact of comets cooled global temperatures.[106][107][108]

A 2020 study published in Science Advances found that human population size and/or specific human activities, not climate change, caused rapidly rising global mammal extinction rates during the past 126,000 years. Around 96% of all mammalian extinctions over this time period are attributable to human impacts. According to Tobias Andermann, lead author of the study, "these extinctions did not happen continuously and at constant pace. Instead, bursts of extinctions are detected across different continents at times when humans first reached them. More recently, the magnitude of human driven extinctions has picked up the pace again, this time on a global scale."[109][40]

Megafaunal extinction[edit]

Megafauna play a significant role in the lateral transport of mineral nutrients in an ecosystem, tending to translocate them from areas of high to those of lower abundance. They do so by their movement between the time they consume the nutrient and the time they release it through elimination (or, to a much lesser extent, through decomposition after death).[110] In South America's Amazon Basin, it is estimated that such lateral diffusion was reduced over 98% following the megafaunal extinctions that occurred roughly 12,500 years ago.[111][112] Given that phosphorus availability is thought to limit productivity in much of the region, the decrease in its transport from the western part of the basin and from floodplains (both of which derive their supply from the uplift of the Andes) to other areas is thought to have significantly impacted the region's ecology, and the effects may not yet have reached their limits.[112] The extinction of the mammoths allowed grasslands they had maintained through grazing habits to become birch forests.[28] The new forest and the resulting forest fires may have induced climate change.[28] Such disappearances might be the result of the proliferation of modern humans; some recent studies favor this theory.[39][113]

Large populations of megaherbivores have the potential to contribute greatly to the atmospheric concentration of methane, which is an important greenhouse gas. Modern ruminant herbivores produce methane as a byproduct of foregut fermentation in digestion, and release it through belching or flatulence. Today, around 20% of annual methane emissions come from livestock methane release. In the Mesozoic, it has been estimated that sauropods could have emitted 520 million tons of methane to the atmosphere annually,[114] contributing to the warmer climate of the time (up to 10 °C warmer than at present).[114][115] This large emission follows from the enormous estimated biomass of sauropods, and because methane production of individual herbivores is believed to be almost proportional to their mass.[114]

Recent studies have indicated that the extinction of megafaunal herbivores may have caused a reduction in atmospheric methane. This hypothesis is relatively new.[116] One study examined the methane emissions from the bison that occupied the Great Plains of North America before contact with European settlers. The study estimated that the removal of the bison caused a decrease of as much as 2.2 million tons per year.[117] Another study examined the change in the methane concentration in the atmosphere at the end of the Pleistocene epoch after the extinction of megafauna in the Americas. After early humans migrated to the Americas about 13,000 BP, their hunting and other associated ecological impacts led to the extinction of many megafaunal species there. Calculations suggest that this extinction decreased methane production by about 9.6 million tons per year. This suggests that the absence of megafaunal methane emissions may have contributed to the abrupt climatic cooling at the onset of the Younger Dryas.[116] The decrease in atmospheric methane that occurred at that time, as recorded in ice cores, was 2–4 times more rapid than any other decrease in the last half million years, suggesting that an unusual mechanism was at work.[116]

Disease[edit]

The hyperdisease hypothesis, proposed by Ross MacPhee in 1997, states that the megafaunal die-off was due to an indirect transmission of diseases by newly arriving aboriginal humans.[118][119][120] According to MacPhee, aboriginals or animals travelling with them, such as domestic dogs or livestock, introduced one or more highly virulent diseases into new environments whose native population had no immunity to them, eventually leading to their extinction. K-selection animals, such as the now-extinct megafauna, are especially vulnerable to diseases, as opposed to r-selection animals who have a shorter gestation period and a higher population size. Humans are thought to be the sole cause as other earlier migrations of animals into North America from Eurasia did not cause extinctions.[118]

There are many problems with this theory, as this disease would have to meet several criteria: it has to be able to sustain itself in an environment with no hosts; it has to have a high infection rate; and be extremely lethal, with a mortality rate of 50–75%. Disease has to be very virulent to kill off all the individuals in a genus or species, and even such a virulent disease as West Nile fever is unlikely to have caused extinction.[121]

However, diseases have been the cause for some extinctions. The introduction of avian malaria and avipoxvirus, for example, have had a negative impact on the endemic birds of Hawaii.[122]

Defaunation[edit]

History[edit]

There are roughly 880 mountain gorillas remaining. 60% of primate species face an anthropogenically driven extinction crisis and 75% have declining populations.[123]

The loss of species from ecological communities, defaunation, is primarily driven by human activity.[5] This has resulted in empty forests, ecological communities depleted of large vertebrates.[39][124] This is not to be confused with extinction, as it includes both the disappearance of species and declines in abundance.[125] Defaunation effects were first implied at the Symposium of Plant-Animal Interactions at the University of Campinas, Brazil in 1988 in the context of Neotropical forests.[126] Since then, the term has gained broader usage in conservation biology as a global phenomenon.[5][126]

Big cat populations have severely declined over the last half-century and could face extinction in the following decades. According to IUCN estimates: lions are down to 25,000, from 450,000; leopards are down to 50,000, from 750,000; cheetahs are down to 12,000, from 45,000; tigers are down to 3,000 in the wild, from 50,000.[127] A December 2016 study by the Zoological Society of London, Panthera Corporation and Wildlife Conservation Society showed that cheetahs are far closer to extinction than previously thought, with only 7,100 remaining in the wild, and crammed within only 9% of their historic range.[128] Human pressures are to blame for the cheetah population crash, including prey loss due to overhunting by people, retaliatory killing from farmers, habitat loss and the illegal wildlife trade.[129]

We are seeing the effects of 7 billion people on the planet. At present rates, we will lose the big cats in 10 to 15 years.

— Naturalist Dereck Joubert, co-founder of the National Geographic Big Cats Initiative[127]

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.[130] Pollinators, which are necessary for 75% of food crops, are declining globally in both abundance and diversity.[131] A 2017 study led by Radboud University's Hans de Kroon indicated that the biomass of insect life in Germany had declined by three-quarters in the previous 25 years. Participating researcher Dave Goulson of Sussex University stated that their study suggested that humans are making large parts of the planet uninhabitable for wildlife. Goulson characterized the situation as an approaching "ecological Armageddon", adding that "if we lose the insects then everything is going to collapse."[132] As of 2019, 40% of insect species are in decline, and a third are endangered.[133] The most significant drivers in the decline of insect populations are associated with intensive farming practices, along with pesticide use and climate change.[134]

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.

— Peter Raven, former president of the American Association for the Advancement of Science (AAAS), in the foreword to their publication AAAS Atlas of Population and Environment[135]
Angalifu, a male northern white rhinoceros at the San Diego Zoo Safari Park (died December 2014).[136] Sudan, the last male of the subspecies died on March 19, 2018.[137]

Various species are predicted to become extinct in the near future,[138] among them the rhinoceros,[139][140] nonhuman primates,[123] pangolins,[141] and giraffes.[142][143] Hunting alone threatens bird and mammalian populations around the world.[144][145][146] The direct killing of megafauna for meat and body parts is the primary driver of their destruction, with 70% of the 362 megafauna species in decline as of 2019.[147][148] Mammals in particular have suffered such severe losses as the result of human activity that it could take several million years for them to recover.[149][150] 189 countries, which are signatory to the Convention on Biological Diversity (Rio Accord),[151] have committed to preparing a Biodiversity Action Plan, a first step at identifying specific endangered species and habitats, country by country.[152]

For the first time since the demise of the dinosaurs 65 million years ago, we face a global mass extinction of wildlife. We ignore the decline of other species at our peril – for they are the barometer that reveals our impact on the world that sustains us.

— Mike Barrett, director of science and policy at WWF's UK branch[153]

A June 2020 study published in PNAS posits that the contemporary extinction crisis "may be the most serious environmental threat to the persistence of civilization, because it is irreversible" and that its acceleration "is certain because of the still fast growth in human numbers and consumption rates." The study found that more than 500 vertebrate species are poised to be lost in the next two decades.[154]

Recent extinction[edit]

Recent extinctions are more directly attributable to human influences, whereas prehistoric extinctions can be attributed to other factors, such as global climate change.[5][24] The International Union for Conservation of Nature (IUCN) characterises 'recent' extinction as those that have occurred past the cut-off point of 1500,[155] and at least 875 species have gone extinct since that time and 2012.[156] Some species, such as the Père David's deer[157] and the Hawaiian crow,[158] are extinct in the wild, and survive solely in captive populations. Other species, such as the Florida panther, are ecologically extinct, surviving in such low numbers that they essentially have no impact on the ecosystem.[159]:318 Other populations are only locally extinct (extirpated), still existence elsewhere, but reduced in distribution,[159]:75–77 as with the extinction of gray whales in the Atlantic,[160] and of the leatherback sea turtle in Malaysia.[161]

Most recently, insect populations have experienced rapid surprising declines. Insects have declined at an annual rate of 2.5% over the last 25–30 years. The most severe effects may include Puerto Rico, where insect ground fall has declined by 98% in the previous 35 years. Butterflies and moths are experiencing some of the most severe effect. Butterfly species have declined by 58% on farmland in England. In the last ten years, 40% of insect species and 22% of mammal species have disappeared. Germany is experiencing a 75% decline. Climate change and agriculture are believed to be the most significant contributors to the change.[162]

A 2019 study published in Nature Communications found that rapid biodiversity loss is impacting larger mammals and birds to a much greater extent than smaller ones, with the body mass of such animals expected to shrink by 25% over the next century. Over the past 125,000 years, the average body size of wildlife has fallen by 14% as human actions eradicated megafauna on all continents with the exception of Africa.[163] Another 2019 study published in Biology Letters found that extinction rates are perhaps much higher than previously estimated, in particular for bird species.[164]

The 2019 Global Assessment Report on Biodiversity and Ecosystem Services lists the primary causes of contemporary extinctions in descending order: (1) changes in land and sea use (primarily agriculture and overfishing respectively); (2) direct exploitation of organisms such as hunting; (3) anthropogenic climate change; (4) pollution and (5) invasive alien species spread by human trade.[19] This report, along with the 2020 Living Planet Report by the WWF, both project that climate change will be the leading cause in the next several decades.[19][45]

Habitat destruction[edit]

Satellite image of rainforest converted to oil palm plantations.[165]

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.

The removal of land to clear way for palm oil plantations releases carbon emissions held in the peatlands of Indonesia.[166][167] Palm oil mainly serves as a cheap cooking oil,[168] and also as a (controversial) biofuel. However, damage to peatland contributes to 4% of global greenhouse gas emissions, and 8% of those caused by burning fossil fuels.[169] Palm oil cultivation has also been criticized for other impacts to the environment,[170][171] including deforestation,[172] which has threatened critically endangered species such as the orangutan[173] and the tree-kangaroo.[174] The IUCN stated in 2016 that the species could go extinct within a decade if measures are not taken to preserve the rainforests in which they live.[175]

Some scientists and academics assert that industrial agriculture and the growing demand for meat is contributing to significant global biodiversity loss as this is a significant driver of deforestation and habitat destruction; species-rich habitats, such as significant portions of the Amazon region, are being converted to agriculture for meat production.[25][176][177][178][179] A 2017 study by the World Wildlife Fund (WWF) found that 60% of biodiversity loss can be attributed to the vast scale of feed crop cultivation required to rear tens of billions of farm animals.[180] Moreover, a 2006 report by the Food and Agriculture Organization (FAO) of the United Nations, Livestock's Long Shadow, also found that the livestock sector is a "leading player" in biodiversity loss.[181] More recently, in 2019, the IPBES Global Assessment Report on Biodiversity and Ecosystem Services attributed much of this ecological destruction to agriculture and fishing, with the meat and dairy industries having a very significant impact.[182] Since the 1970s food production has soared in order to feed a growing human population and bolster economic growth, but at a huge price to the environment and other species. The report says some 25% of the earth's ice-free land is used for cattle grazing.[183] A 2020 study published in Nature Communications warned that human impacts from housing, industrial agriculture and in particular meat consumption are wiping out a combined 50 billion years of earth's evolutionary history [defined as phylogenetic diversity (PD). PD is the sum of the phylogenetic branch lengths in years connecting a set of species to each other across their phylogenetic tree, and measures their collective contribution to the tree of life.] and driving to extinction some of the "most unique animals on the planet," among them the Aye-aye lemur, the Chinese crocodile lizard and the pangolin.[184][185] Said lead author Rikki Gumbs:

"We know from all the data we have for threatened species, that the biggest threats are agriculture expansion and the global demand for meat. Pasture land, and the clearing of rainforests for production of soy, for me, are the largest drivers -- and the direct consumption of animals."[184]

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. For example, this is already resulting in coral bleaching on various coral reefs worldwide, which provide valuable habitat and maintain a high biodiversity. Marine gastropods, bivalves and other invertebrates are also affected, as are the organisms that feed on them.[citation needed] According to a 2018 study published in Science, global Orca populations are poised to collapse due to toxic chemical and PCB pollution. PCBs are still leaking into the sea in spite of being banned for decades.[186]

Some researchers suggest that by 2050 there could be more plastic than fish in the oceans by weight,[187] with about 8,800,000 metric tons (9,700,000 short tons) of plastic being discharged into the oceans annually.[citation needed] Single-use plastics, such as plastic shopping bags, make up the bulk of this, and can often be ingested by marine life, such as with sea turtles.[188] These plastics can degrade into microplastics, smaller particles that can affect a larger array of species. Microplastics make up the bulk of the Great Pacific Garbage Patch, and their smaller size is detrimental to cleanup efforts.[189]

In March 2019, Nature Climate Change published a study by ecologists from Yale University, who found that over the next half century, human land use will reduce the habitats of 1,700 species by up to 50%, pushing them closer to extinction.[190][191] That same month PLOS Biology published a similar study drawing on work at the University of Queensland, which found that "more than 1,200 species globally face threats to their survival in more than 90% of their habitat and will almost certainly face extinction without conservation intervention".[192][193]

Since 1970, the populations of migratory freshwater fish have declined by 76%, according to research published by the Zoological Society of London in July 2020. Overall, around one in three freshwater fish species are threatened with extinction due to human-driven habitat degradation and overfishing.[194]

Overexploitation[edit]

The vaquita, the world's most endangered marine mammal, has been reduced to only 30 individuals as of February 2017. They are often killed by commercial fishing nets.[195] As of March 2019, only 10 remain, according to The International Committee for the Recovery of the Vaquita.[196]
The collapse of the Atlantic northwest cod fishery in 1992 as a result of overfishing, and subsequent recovery.

Overhunting can reduce the local population of game animals by more than half, as well as reducing population density, and may lead to extinction for some species.[197] Populations located nearer to villages are significantly more at risk of depletion.[198][199] Several conservationist organizations, among them IFAW and HSUS, assert that trophy hunters, particularly from the United States, are playing a significant role in the decline of giraffes, which they refer to as a "silent extinction".[200]

The surge in the mass killings by poachers involved in the illegal ivory trade along with habitat loss is threatening African elephant populations.[201][202] In 1979, their populations stood at 1.7 million; at present there are fewer than 400,000 remaining.[203] Prior to European colonization, scientists believe Africa was home to roughly 20 million elephants.[204] According to the Great Elephant Census, 30% of African elephants (or 144,000 individuals) disappeared over a seven-year period, 2007 to 2014.[202][205] African elephants could become extinct by 2035 if poaching rates continue.[143]

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.[206] Humans are unique among predators in that they regularly prey on other adult apex predators, particularly in marine environments;[15] bluefin tuna, blue whales, North Atlantic right whales[207] and over fifty species of sharks and rays are vulnerable to predation pressure from human fishing, in particular commercial fishing.[208] A 2016 study published in Science concludes that humans tend to hunt larger species, and this could disrupt ocean ecosystems for millions of years.[209] A 2020 study published in Science Advances found that around 18% of marine megafauna, including iconic species such as the Great white shark, are at risk of extinction from human pressures over the next century. In a worst-case scenario, 40% could go extinct over the same time period.[210]

If this pattern goes unchecked, the future oceans would lack many of the largest species in today’s oceans. Many large species play critical roles in ecosystems and so their extinctions could lead to ecological cascades that would influence the structure and function of future ecosystems beyond the simple fact of losing those species.

— Jonathan Payne, associate professor and chair of geological sciences at Stanford University[211]

Disease[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, habitat loss and the introduction of invasive species.[212]
Toughie, the last Rabbs' fringe-limbed treefrog, died in September 2016.[213] The species was killed off from the chytrid fungus Batrachochytrium dendrobatidis[214]

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 accidentally spread by human travel,[23] globalization and the wildlife trade, has caused severe population drops of over 500 amphibian species, and perhaps 90 extinctions,[215] 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. Batrachochytrium salamandrivorans is a similar infection currently threatening salamanders. Amphibians are now the most endangered vertebrate group, having existed for more than 300 million years through three other mass extinctions.[23]:17

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.[216]

Between 2007 and 2013, over ten million beehives were abandoned due to colony collapse disorder, which causes worker bees to abandon the queen.[217] Though no single cause has gained widespread acceptance by the scientific community, proposals include infections with Varroa and Acarapis mites; malnutrition; various pathogens; genetic factors; immunodeficiencies; loss of habitat; changing beekeeping practices; or a combination of factors.[218][219]

Mitigation[edit]

Some leading scientists have advocated for the global community to designate as protected areas 30 percent of the planet by 2030, and 50 percent by 2050, in order to mitigate the contemporary extinction crisis as the human population is projected to grow to 10 billion by the middle of the century. Human consumption of food and water resources is also projected to double by this time.[220]

In November 2018, the UN's biodiversity chief Cristiana Pașca Palmer urged people around the world to put pressure on governments to implement significant protections for wildlife by 2020, as rampant biodiversity loss is a "silent killer" as dangerous as global warming, but has received little attention by comparison. She says that "It’s different from climate change, where people feel the impact in everyday life. With biodiversity, it is not so clear but by the time you feel what is happening, it may be too late."[221] In January 2020, the UN Convention on Biological Diversity drafted a Paris-style plan to stop biodiversity and ecosystem collapse by setting 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. The world failed to meet similar targets for 2020 set by the convention during a summit in Japan in 2010.[222][223] Of the 20 biodiversity targets proposed, only six were "partially achieved" by the deadline.[224] It was called a global failure by Inger Andersen, head of the United Nations Environment Programme:

"From COVID-19 to massive wildfires, floods, melting glaciers and unprecedented heat, our failure to meet the Aichi (biodiversity) targets — protect our our home — has very real consequences. We can no longer afford to cast nature to the side."[225]

Some scientists have proposed keeping extinctions below 20 per year for the next century as a global target to reduce species loss, which is the biodiversity equivalent of the 2 °C climate target, although it is still much higher than the normal background rate of two per year prior to anthropogenic impacts on the natural world.[226][227]

See also[edit]

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Further reading[edit]

External links[edit]

External video
video icon Are We Living In the Sixth Extinction? on YouTube
video icon Extinction: The Facts in 6 minutes on BBC One