From Wikipedia, the free encyclopedia
The
Holocene extinction, otherwise referred to as the
sixth extinction or
Anthropocene extinction, is the ongoing
extinction event of
species during the present
Holocene epoch, mainly due to human activity. The large number of extinctions spans numerous families of
plants and
animals, including
mammals,
birds,
amphibians,
reptiles and
arthropods. With widespread degradation of
highly biodiverse habitats such as
coral reefs and
rainforest, as well as other areas, the vast majority of these extinctions is thought to be
undocumented. The current rate of extinction of species is estimated at 100 to 1,000 times higher than natural background rates.
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.
The ecology of humanity has been noted as being that of an
unprecedented "global superpredator" that regularly preys on the adults
of other
apex predators and has
worldwide effects on
food webs. Extinctions of species have occurred on every
land mass and
ocean, with many famous examples
within Africa,
Asia,
Europe,
Australia,
North and
South America, and on smaller islands. Overall, the Holocene extinction can be characterized by the
human impact on the environment. The Holocene extinction continues into the 21st century, with
meat consumption,
overfishing,
ocean acidification and the
amphibian crisis 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.
[1][2]
Definitions
The
Holocene extinction is also known as the "sixth extinction", due to its
possibly being 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.
[3][4][5][6][1] 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.
[7][8]
Some have suggested that anthropogenic extinctions may have begun as
early as when the first modern humans spread out of Africa between
100,000 and 200,000 years ago, which is supported by rapid megafaunal
extinction following recent human colonisation in
Australia,
New Zealand and
Madagascar,
[3] in a similar way that any
large, adaptable predator moving into a new ecosystem would. In many cases, it is suggested even minimal hunting pressure was enough to wipe out large fauna, particularly on
geographically isolated islands.
[9][10] Only during the most recent parts of the extinction have
plants also suffered large losses.
[11]
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 acknowledge the existence of the anthropogenic extinction.
[12] Numerous scientific studies — such as a 2004 report published in
Nature,
[13] and papers authored by the
IUCN's annual
Red List
of threatened species — have since reinforced this conviction. 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)
[14][15][16] and also the current rate of extinction is, therefore, 10 to 100 times higher than any of the previous
mass extinctions in the history of Earth. It is also the only known mass extinction of plants.
[citation needed] 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.
[17] Stuart Pimm stated "the current rate of species extinction is about 100 times the natural rate" for plants.
[18] Mass extinctions are characterized by the loss of at least 75% of species within a geologically short period of time.
[19][20][21]
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.
[22][23]
While there is widespread consensus in the scientific community that
human activity is accelerating the extinction of many animal species
through the destruction of wild lands, the consumption of animals as
resources or luxuries, and the persecution of species that humans view
as threats or competitors,
[24]
some contend that this biotic destruction has yet to rise to 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."
[25]
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".
[26][27][24] "
Anthropocene"
is a term introduced in 2000. It is now posited by some 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.
[3]
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.
[28][29]
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.
[30] Other commentators place the Holocene–Anthropocene boundary at the
industrial revolution
while also saying 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
following the mid-20th century different enough from the rest of the
Holocene to consider it a new
geological epoch, known as the
Anthropocene,
[31] which was considered for implementation into the timeline of Earth's history by the
International Commission on Stratigraphy in 2016.
[32][33] 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 at a global scale
and when these alterations caused changes to global climate. Employing
chemical proxies from Antarctic ice cores, researchers have estimated
the fluctuations of
carbon dioxide (CO
2) and
methane gases (CH
4) in the earth’s atmosphere for the
late Pleistocene and Holocene epochs.
[30]
Based on studies that estimated fluctuations of carbon dioxide and
methane in the atmosphere using chemical proxies from Antarctic ice
cores, general argumentation of when the peak of the Anthropocene
occurred pertains to the timeframe within the previous two centuries;
typically beginning with the
Industrial Revolution, when greenhouse gas levels were recorded by contemporary methods at its highest.
[34][35]
Influences
Competition by humans
The percent of
megafauna on different land masses over time, with the arrival of humans indicated.
The Holocene extinction is mainly caused by human activity.
[4][5][24][6][1] 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.
[24] 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.
[4][36][1][2][37]
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.
[9][10] It has been suggested that the African megafauna survived because they evolved alongside humans.
[3] 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.
[3]
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.
[38]
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.
[8][3][24]
Agriculture
Human civilization flourished in accordance to the efficiency and intensification of prevailing
subsistence systems.
[39]
Local communities that acquire more subsistence strategies increased in
number to combat competitive pressures of land utilization.
[30][39]
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.
[40]
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
[42]), thus replacing the original local ecosystems.
[43] Other, related human causes of the extinction event include
deforestation,
hunting,
pollution,
[44] the introduction in various regions of
non-native species, and the
widespread transmission of
infectious diseases spread through livestock and crops.
[15]
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.
[39] 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.
[45][46] Scientists have questioned the correlation between population size and early territorial alterations.
[46]
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.
[39]
While a number of human-derived factors are recognized as potentially contributing to rising atmospheric concentrations of CH
4 and CO
2,
deforestation and territorial clearance practices associated with
agricultural development may be contributing most to these
concentrations globally.
[34][47][39] 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.
[30][45][47] He argued that the patterns of the significant decline of CO
2
levels during the last ice age of the Pleistocene inversely correlates
to the Holocene where there has been dramatic increases of CO
2 around 8000 years ago and CH
4 levels 3000 years after that.
[47] The correlation between the decrease of CO
2 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.
[30][47]
Islands
Human arrival in the Caribbean around 6,000 years ago is correlated with the extinction of many species.
[48] 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.
[49]
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.
[50]
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.
[51]
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.
[52]
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.
[22]
Australia
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 on
Australia.
[53]
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.
[3]
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.
[54]
Due to the older timeframe and the soil chemistry on the continent, very little
subfossil preservation evidence exists relative to elsewhere.
[55]
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)
[56] and the fact that megafauna survived until a later date on the island of
Tasmania following the establishment of a land bridge
[57] 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.
[53]
Madagascar
Radiocarbon dating of multiple subfossil specimens shows that now extinct
giant lemurs were present in Madagascar until after human arrival.
Within 500 years of the arrival of
humans between 2,500–2,000 years ago, nearly all of Madagascar's distinct,
endemic and
geographically isolated megafauna became extinct.
[58]
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.
[10] 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 and
Mullerornis, are extinct from over-hunting,
[59] 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.
[60]
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.
[61] The last
moa, large flightless
ratites, became extinct within 200 years of the arrival of human settlers.
[9] 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 and NZ's national emblem, the
kiwi, is on the endangered bird list.
[61]
Americas
The
passenger pigeon was a species of
pigeon endemic to
North America.
It experienced a rapid decline in the late 1800s due to 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
[62] of prey populations. Discoveries at Monte Verde in South America and at
Meadowcroft Rock Shelter in Pennsylvania have caused a controversy
[63] 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.
[63] 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)
[64]
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.
[3]
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[65]
but it is not known if this hunting was the cause of the subsequent
massive ecological changes, widespread extinctions and climate changes.
[7][8]
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.
[3]
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.
[66] 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).
[67]
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.
[68]
Climate change
Top: Arid ice age climate
Middle: Atlantic Period, warm and wet
Bottom: Potential vegetation in climate now if not for human effects like agriculture.
[69]
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.
[28][70]
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.
[3][71][72] 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.
[73][74][75][76][77][78] 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.
[79][80][81]
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).
[82] 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.
[83][84] 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.
[84] The extinction of the
mammoths allowed
grasslands they had maintained through grazing habits to become birch forests.
[7] The new forest and the resulting forest fires may have induced
climate change.
[7] Such disappearances might be the result of the
proliferation of
modern humans.
[24]
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,
[85] contributing to the warmer climate of the time (up to 10 °C warmer than at present).
[85][86]
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.
[85]
Recent studies have indicated that the extinction of megafaunal herbivores may have caused a reduction in
atmospheric methane. This hypothesis is relatively new.
[87] 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.
[88] 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.
[87] 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.
[87]
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.
[89][90][91] 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.
[89]
There are many problems with this theory in the scientific community,
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.
[92]
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.
[93]
Defaunation
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.
[94]
There are roughly 880
mountain gorillas remaining in existence. 60% of
primate species face an anthropogenically driven extinction crisis and 75% have declining populations.
[95]
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.
[98][24] This is not to be confused with extinction, as it includes both the disappearance of species and declines in abundance.
[99]
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.
[100] Since then, the term has gained broader usage in conservation biology as a global phenomenon.
[101][100]
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.
[102] 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.
[103]
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.
[104]
“ |
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[102] |
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.
[105] Pollinators, which are necessary for 75% of food crops, are declining globally in both abundance and diversity.
[106] 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."
[107]
“ |
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[108] |
Various species are predicted to
become extinct in the near future,
[109] among them the
rhinoceros,
[110] primates,
[95] pangolins,
[111] and
giraffes.
[112][113] Hunting alone threatens bird and mammalian populations around the world.
[114][115][116] 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.
[6][117][118][119] 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.
[120] 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.
[121] According to the WWF's 2016
Living Planet Index,
global wildlife populations have declined 58% since 1970, primarily due
to habitat destruction, over-hunting and pollution. They project that
if current trends continue, 67% of wildlife could disappear by 2020.
[122][123] 189 countries, which are signatory to the
Convention on Biological Diversity (Rio Accord),
[124] have committed to preparing a
Biodiversity Action Plan, a first step at identifying specific
endangered species and habitats, country by country.
[125]
“ |
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[126] |
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.
[4][5] The
International Union for Conservation of Nature (IUCN) characterises 'recent' extinction as those that have occurred past the cut-off point of 1500,
[127] and at least 875 species have gone extinct since that time and 2012.
[128] Some species, such as the
Père David's deer[129] and the
Hawaiian crow,
[130] 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 that they essentially have no impact on the ecosystem.
[131]:318 Other populations are only
locally extinct (extirpated), still existence elsewhere, but reduced in distribution,
[131]:75–77 as with the extinction of
gray whales in the
Atlantic,
[132] and of the
leatherback sea turtle in Malaysia.
[133]
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.
Satellite image of rainforest converted to
oil palm plantations.
[135]
The removal of land to clear way for
palm oil plantations releases carbon emissions held in the
peatlands of Indonesia.
[136][137] Palm oil mainly serves as a cheap
cooking oil,
[138] 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.
[139] Palm oil cultivation has also been criticized for other impacts to the environment,
[140][141] including deforestation,
[142] which has threatened critically endangered species such as the
orangutan.
[143][144]
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.
[145] Tree-kangaroos are also threatened with extinction as the result of palm oil deforestation.
[146]
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.
Some researchers suggest that by 2050 there could be more
plastic than fish in the oceans by weight.
[37]
Overexploitation
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.
[147] The species could be extinct by autumn of 2017, according to WWF conservationists.
[148]
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.
[149] Populations located nearer to villages are significantly more at risk of depletion.
[150][151]
The surge in the mass killings by
poachers involved in the illegal ivory trade along with habitat loss is threatening
African elephant populations.
[152][153] In 1979, their populations stood at 1.7 million; at present there are fewer than 400,000 remaining.
[154] Prior to European colonization, scientists believe Africa was home to roughly 20 million elephants.
[155] According to the
Great Elephant Census, 30% of African elephants (or 144,000 individuals) disappeared over a seven-year period, 2007 to 2014.
[153][156] African elephants could become extinct by 2035 if poaching rates continue.
[113]
The collapse of
Atlantic cod off the coast of Newfoundland in 1992 as a result of
overfishing. The population never recovered, completely altering the ecosystem and rendering the species
locally extinct.
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.
[157] Humans are unique among predators in that they regularly predate on other adult
apex predators, particularly in marine environments;
[38] bluefin tuna,
blue 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.
[158]
“ |
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[159] |
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,
[3] 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.
[3]: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.
[citation needed]