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Lonesome George is
a large, mud-loving Pinta tortoise (Geochelone elephantopus
abingdoni), living out his long life in the Galapagos Islands. In
1971, George was found alone on Pinta Island and taken to the Charles Darwin
Research Station, where scientists theorized that he was the last of his
subspecies on the planet. When he dies, his genetic lineage will disappear
forever. Unfortunately, his companionless circumstances are not unique. In
fact, we are in the midst of the Sixth Great Extinction, an event
characterized by the loss of between 17,000 and 100,000 species each
year.
"Recent extinction rates are unprecedented in human history and
highly unusual in Earth's history," according to a study published
Friday (June 19, 2015) in the journal Science Advances. "Our global society has started to destroy
species of other organisms at an accelerating rate, initiating a mass
extinction episode unparalleled for 65 million years."
Researchers used "extremely conservative assumptions" to
determine extinction rates that prevailed in the past five annihilation
events. Still, they found the average rate of vertebrate species lost over
the past century was up to 114 times higher than normal.
MASS EXTINCTION DEFINITION(American Heritage
Science Dictionary):
The extinction of a large number of species within a relatively short period of geological time, thought to be due to factors such as a catastrophic global event or widespread environmental change that occurs too rapidly for most species to adapt. At least five mass extinctions have been identified in the fossil record, coming at or toward the end of the Ordovician, Devonian, Permian, Triassic, and Cretaceous Periods. The Permian extinction, which took place 245 million years ago, is the largest known mass extinction in the Earth's history, resulting in the extinction of an estimated 90 percent of marine species. In the Cretaceous extinction, 65 million years ago, an estimated 75 percent of species, including the dinosaurs, became extinct, possibly as the result of an asteroid colliding with the Earth. |
CURRENT
EXTINCTION OR SIXTH MASS EXTINCTION
The Holocene
extinction, sometimes called the Sixth
Extinction, is a name proposed to describe the currently ongoing extinction
event of species during the
present Holocene epoch (since
around 10,000 BCE) mainly due to human activity. The large number of
extinctions span numerous families of plants and animals including mammals, birds, amphibians, reptiles and arthropods. Although
875 extinctions occurring between 1500 and 2009 have been documented by the International
Union for Conservation of Nature and Natural Resources,[1] the vast
majority are undocumented. According to the species-area theory and based
on upper-bound estimating, the present rate of extinction may be up to 140,000
species per year.
The Holocene extinction includes the
disappearance of large mammals known as megafauna, starting between 9,000 and 13,000 years ago, the end of the last Ice Age. This may
have been due to the extinction of the mammoths whose
habits had maintained grasslands which became birch forests without them. The new
forest and the resulting forest fires may have induced climate
change. Such disappearances might be the result of the proliferation of modern
humans. These extinctions, occurring near the Pleistocene–Holocene
boundary, are sometimes referred to as the Quaternary extinction event. The
Holocene extinction continues into the 21st century.
There is no general agreement on whether to
consider this as merely part of the Quaternary extinction event, or just a
result of human-caused changes.[3][4] Only during
these most recent parts of the extinction have plants also
suffered large losses. Overall, the Holocene extinction can be characterized by
the human impact on the environment.
HUMAN ACTIVITIES
Extinction of animals, plants, and other organisms caused by human actions
may go as far back as the late Pleistocene, over 12,000 years ago. There is evidence that abrupt
climate change has especially played an enormous role in the extinction of
larger mammals.[22] However, while previous mass extinctions were due to
natural environmental causes, research shows that wherever on Earth humans have
migrated, other species have gone extinct, and human population growth, most prominently in the past two centuries, is regarded as one of the
underlying causes of this Holocene extinction event.[23] In terms of how humans have contributed to this mass
extinction, three major factors include: the increased global
concentration of greenhouse gases, affecting the global climate; oceanic
devastation, such as through overfishing and contamination; and the modification and destruction
of vast tracts of land and river systems around the world to meet solely
human-centered ends (with 10 to 15 percent of Earth's land surface now used as
urban-industrial or row-crop agricultural sites and 6 to 8 percent used as
pastures), thus ruining the local ecosystems.[24][25] Other, related human causes of the extinction event
include deforestation, hunting, pollution,[26] the introduction in various regions of non-native species, and the widespread transmission of infectious
diseases. At present, the
rate of extinction of species is estimated at 100 to 1,000 times higher than
the "base" or historically typical rate of extinction (in terms of
the natural evolution of the planet)[27] 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. On the other
hand, this extinction concerns a large number of plants, different from
previous extinctions.[citation needed]
The abundance of species extinctions considered anthropogenic, or due to human activity, have sometimes (especially when referring to
hypothesized future events) been collectively called the "Anthropocene
extinction".[28][29] The Anthropocene is a term introduced in 2000. Most biologists believe
that we are at the beginning of an anthropogenic mass extinction that is
accelerating at a large rate. In The Future of Life (2002), E.O. Wilson of Harvard calculated that, if the current rate of human
disruption of the biosphere continues, one-half of Earth's higher lifeforms will be extinct by 2100. A 1998 poll conducted by the American Museum of Natural History found that seventy percent of biologists believe that we
are in the midst of an anthropogenic extinction. Numerous scientific
studies—such as a 2004 report published in Nature, and papers authored by the 10,000 scientists who
contribute to the IUCN's annual Red List of threatened species—have since reinforced this conviction.
The evidence of all previous extinctions is geological in nature, and shorter
geological time scale is of the order of several hundred thousand to several
million years. Even extinctions caused by instantaneous events such as the
impact of the asteroid in Chicxulub, which is currently the best example,
extend the equivalent of many human lives, due to complex ecological
interactions that are triggered by the event.
Recent extinctions described are well-documented, but the nomenclature used varies. The term Anthropocene is a term that is used by few scientists, and some
commentators may refer to the current and projected future extinctions as part
of a longer Holocene extinction. The Holocene–Anthropocene boundary is
contested, with some commentators asserting significant human influence on
climate for much of what is normally regarded as the Holocene Epoch. Other commentators place the
Holocene–Anthropocene boundary at the industrial revolution while also saying that "Formal adoption of this term in the near
future will largely depend on its utility, particularly to earth scientists
working on late Holocene successions."
THE OTHER FIVE
During the 1980s,
using rigorous statistical analyses of the fossil record, John Sepkoski
(University of Chicago Paleontologist) and his Chicago colleague David Raup put
forth the controversial theory that catastrophic extinctions of marine animals
may have occurred approximately every 26 million years during the past 250
million years of Earth's history. These periodic events also included the
extinction of the dinosaurs 65 million years ago. The extinctions previously
were thought to have been random events.
The theory helped
open the possibility that mass extinctions both on land and in the oceans were
caused by some force external to Earth, such as catastrophic comet and asteroid
impacts of the type that inspired the films "Deep Impact" and "Armageddon"
and inspired the popular Shriekback song "Nemesis." These findings
prompted a major interdisciplinary research effort on extinction events in the
fossil record.
The exact cause of
periodic extinctions remains a mystery. "The theory has been under attack,
but I don't think anyone's been able to disprove it," said Sepkoski's
wife, Christine M. Janis, a Brown University paleontologist.
The first extinction,
named the Ordovician-Silurian extinction, occurred around 440
million years ago (m.y.a.). Scientists hypothesize that both a southerly
continental drift that led to a drastic decrease in temperatures and radiation
caused by the collapse of a massive star known as a hyper nova may have caused
this massive loss of diversity on Earth.
The second extinction
was the Late Devonian. Approximately 370 m.y.a., there was a sharp
decrease in marine reef biodiversity. Many factors may have played a part in
the Late Devonian extinction, but the causes remain mostly unknown.
Around 245 million
years ago, during the Permian-Triassic extinction event,
marine species died off to such an extent that oceanic reefs did not exist
anywhere on the planet for ten million years. A combination of factors,
including volcanic eruptions, climate change and a possible meteorite impact, made
this the largest historical extinction event.
In the Triassic-Jurassic extinction,
circa 210 m.y.a., 48 percent of genera vanished from the earth, including 80
percent of quadrupeds and half of all marine invertebrates. Although the causes
of this event are unknown, scientists believe that volcanic activity
contributed to extinctions.
The Cretaceous-Paleogene extinction
event, circa 65 m.y.a. (formerly known as the Cretaceous-Tertiary or K-T), is
best known for the extinction of the dinosaurs and nearly all large animal
species. During this event, temperatures increased by as much as 57 degrees
Fahrenheit (32 degrees Celsius) and sea levels rose as much as three hundred
meters.
During each extinction
event, between 50 and 95 percent of the planet’s life was lost, resulting in
dramatically changed biotic characteristics. Generally, ten million years pass
before biodiversity reaches pre-event levels.
WHY YOU SHOULD CARE
Since most people
probably cannot name a single recently extinct species, does it really matter
to the human race whether we save biodiversity or let much of it disappear into
the history books? The answer is a very strong and profound, Yes. By
failing to recognize the importance of biodiversity, we may be assuring the
demise of our own human species, as well as the destruction of most other
species on Earth. We need biodiversity.
Biodiversity provides
climate stability, nutritiously varied and abundant foods, medicines, clean
water, pollination of crops, disease-control, cultural diversity, environmental
knowledge, food-chain stability, and oxygen.
The world’s leading
scientists suggest that conservation measures, sustainable development,
stabilization of the human population and the support of environmentally
responsible economic development will be essential in halting the extinction
crisis.
http://voices.nationalgeographic.com/2012/03/28/the-sixth-great-extinction-a-silent-extermination/
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