The Holocene extinction includes the disappearance of large land animals known as megafauna, starting at the end of the last Ice Age. Megafauna outside of the African continent, 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 (additionally, many African species have also gone extinct in the Holocene). These extinctions, occurring near the Pleistocene–Holocene boundary, are sometimes referred to as the Quaternary extinction event.
The arrival of humans on different continents coincides with megafaunal extinction. The most popular theory is that human overhunting of species added to existing stress conditions. 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" 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, ocean acidification and the decline in amphibian populations being a few broader examples of an almost universal, cosmopolitan decline in biodiversity. Human overpopulation (and continued population growth) along with profligate consumption are considered to be the primary drivers of this rapid decline.
Definitions
The Holocene extinction is also known as the "sixth extinction", as it is possibly the sixth mass extinct 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. Mass extinctions are characterized by the loss of at least 75% of species within a geologically short period of time. 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 200,000 and 100,000 years ago; this is supported by rapid megafaunal extinction following recent human colonisation in Australia, New Zealand and Madagascar, 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. Only during the most recent parts of the extinction have plants also suffered large losses.
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. But some contend that this biotic destruction has yet to reach the level of the previous five mass extinctions. Stuart Pimm, for example, asserts that the sixth mass extinction "is something that hasn't happened yet – we are on the edge of it." 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."
Anthropocene
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". "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.
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. 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 and also say that "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, a term which was considered for inclusion in the timeline of Earth's history by the International Commission on Stratigraphy in 2016. 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. 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.
Influences
Competition by humans
The Holocene extinction is mainly caused by human activity. 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 a correlation between megafaunal extinction and the arrival of humans, and human overpopulation and human population growth, along with overconsumption and consumption growth, most prominently in the past two centuries, are regarded as one of the underlying causes of extinction.
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. It has been suggested that the African megafauna survived because they evolved alongside humans. 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.
More recently, 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. 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".
Agriculture
Human civilization flourished in accordance to[clarification needed] the efficiency and intensity of prevailing subsistence systems. Local communities that acquire more subsistence strategies increased in number to combat competitive pressures of land utilization.[clarification needed][Gobbledegook. Please rewrite in English.] Therefore, the Holocene developed competition on the basis of agriculture. The growth of agriculture has then introduced newer means of climate change, pollution, and ecological development.
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), thus replacing the original 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 spread through livestock and crops.
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. 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. Scientists have questioned the correlation between population size and early territorial alterations. 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.
While a number of human-derived factors are recognized as potentially 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. 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. 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. 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.
Islands
Human arrival in the Caribbean around 6,000 years ago is correlated with the extinction of many species. 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.
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. 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.
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. 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.
Australia
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. 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.
Due to the older timeframe and the soil chemistry on the continent, very little subfossil preservation evidence exists relative to elsewhere. 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) and the fact that megafauna survived until a later date on the island of Tasmania following the establishment of a land bridge 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.
Madagascar
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. 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. 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.
The eight or more species of elephant birds, giant flightless ratites in the genera Aepyornis and Mullerornis, are extinct from over-hunting, 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.
New Zealand
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. The last moa, large flightless ratites, became extinct within 200 years of the arrival of human settlers. 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.
Americas
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 changes 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. 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.
Afroeurasia
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. 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). 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.
Climate change
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. 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. 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. 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.
Megafaunal extinction
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). 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. 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. The extinction of the mammoths allowed grasslands they had maintained through grazing habits to become birch forests. 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; some recent studies favor this theory.
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, contributing to the warmer climate of the time (up to 10 °C warmer than at present). 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.
Disease
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. 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.
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 Virus is unlikely to have caused extinction.
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.
Defaunation
Recent extinction
Recent extinctions are more directly attributable to human influences, whereas prehistoric extinctions can be attributed to other factors, such as global climate change. The International Union for Conservation of Nature (IUCN) characterises 'recent' extinction as those that have occurred past the cut-off point of 1500, and at least 875 species have gone extinct since that time and 2012. Some species, such as the Père David's deer and the Hawaiian crow, 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. Other populations are only locally extinct (extirpated), still existence elsewhere, but reduced in distribution, as with the extinction of gray whales in the Atlantic, and of the leatherback sea turtle in Malaysia.
Habitat destruction
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. Palm oil mainly serves as a cheap cooking oil, 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. Palm oil cultivation has also been criticized for other impacts to the environment, including deforestation, which has threatened critically endangered species such as the orangutan and the tree-kangaroo. 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.
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. 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.
Some researchers suggest that by 2050 there could be more plastic than fish in the oceans by weight, with about 8,800,000 metric tons (9,700,000 short tons) of plastic being discharged into the oceans annually. 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. 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.
Overexploitation
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. Populations located nearer to villages are significantly more at risk of depletion. 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".
The surge in the mass killings by poachers involved in the illegal ivory trade along with habitat loss is threatening African elephant populations. In 1979, their populations stood at 1.7 million; at present there are fewer than 400,000 remaining. Prior to European colonization, scientists believe Africa was home to roughly 20 million elephants. According to the Great Elephant Census, 30% of African elephants (or 144,000 individuals) disappeared over a seven-year period, 2007 to 2014. African elephants could become extinct by 2035 if poaching rates continue.
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. Humans are unique among predators in that they regularly predate on other adult apex predators, particularly in marine environments; bluefin tuna, blue whales, North Atlantic right whales and various sharks in particular are particularly vulnerable to predation pressure from human fishing. A 2016 study published in Science concludes that humans tend to hunt larger species, and this could disrupt ocean ecosystems for millions of years.
Disease
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, 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. 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.
Mitigation
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.
Source from Wikipedia
没有评论:
发表评论