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Holocene extinction event

From Academic Kids

The Holocene extinction event is a name customarily given to the widespread, ongoing extinction of species occurring in the modern Holocene epoch. In particular, it refers to the remarkable disappearance of the large animals known as megafauna across the world between 13,000 and 9,000 years ago, a disappearance that may well be connected with human beings.

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The prehistoric Holocene extinctions

The current ongoing extinction event seems important if we follow tradition and separate 'recent' (Holocene) time from the Pleistocene, or 'ice age' extinctions, exemplified in popular imagination by the extinction of the woolly mammoth and the Neandertal people. However, it is worth remembering that modern climatology suggests the 'Holocene' epoch we live in is no more than the latest in a series of interglacial intervals between glaciation events, one that will perhaps become artificially extended by global warming.

The rate of extinction today appears to be similar to, or perhaps greater than, the rate during the five 'classic' extinction events in deep geological time, such as the Permian extinction that extinguished some 90% of the Paleozoic biota, or the Cretaceous extinction event that eliminated all dinosaurs (except for the birds) at the end of the Cretaceous, 65 million years ago.

The Holocene extinction differs from all previous extinction events in that it appears to be caused by humans. The coincidence between the appearance of modern humans and the extermination of large mammalian Ice Age biota ('megafauna') is thought significant by many authorities, and has been given the name 'the prehistoric overkill' by Paul S. Martin, University of Arizona.

The 'prehistoric overkill' hypothesis is not universally applicable, and is imperfectly confirmed. For instance, the timing of sudden megafaunal extinctions of large Australian marsupials and a giant lizard, events that followed the arrival of human beings in Australia by many thousand years, need examining. Biologists note that comparably scaled extinctions have not occurred in Africa either, where the fauna evolved with hominids. Post-glacial megafaunal extinctions in Africa have been spaced over a longer interval.

An alternative to the theory of human responsibility is Alexander Tollmann's bolide theory, a more controversial hypothesis that claims that the Holocene was initiated by an extinction event caused by bolide impacts.

Major Megafaunal extinctions

Europe

(circa 15,000 years ago)

Mediterranean Islands

(by 9000 years ago)

North America

(circa 12,000-9000 years BP), 35 to 40 species of large mammals (and only about half a dozen small mammals, such as mice and rats) disappeared. Previous North American extinction pulses had occurred at the end of glaciations, but not with such an imbalance between large mammals and small ones. The megafaunal extinctions include twelve genera of edible grazers (G), and five large, dangerous carnivores (C). North American extinctions included

  • American Horses, five species (Asian horses survived) (G)
  • a few species of Western Camels (G)
  • North American llamas (G)
  • Deer, two genera (G)
  • Pronghorn, two genera (one survived) (G)
  • Stag-Moose, Shrub-Oxen, Woodland Muskoxen (an Arctic one survived) (G)
  • Giant Beaver
  • Shasta Ground Sloth and other Ground Sloths
  • Short-Faced Bears (larger than the present Grizzly Bear), cf Cave Bear (C)
  • Sabertooth Cat (C)
  • American Lion (larger than the current African Lion but probably a fairly recent immigrant through Beringia) (C)
  • American Cheetah (C)
  • Dire Wolf (C)
  • Mammoth, several species
  • American Mastodont, Mammut americanum
  • Bison, a giant sub-species of the surviving Bison
  • Giant Peccary

The survivors are as significant as the losses: Bison, Moose (recent immigrants through Beringia), Wapiti (Elk), Caribou, Deer, Pronghorn, Muskox, Bighorn Sheep, Mountain Goat. All save the Pronghorns descended from Asian ancestors that had accommodated with human predators. This connection has recently been expanded upon and supported in detail by R. D. E. MacPhee, Extinctions in Neartime, 1999, an outgrowth of an American Museum of Natural History conference on extinctions, 1997.

The culture that has been connected with the wave of extinctions in North America is the paleo-Indian culture associated with the Clovis people (q.v.), which was thought to use spear throwers to kill large animals. The chief opposition to the 'prehistoric overkill' hypothesis has been that population of humans such as the Clovis culture were too small to be ecologically significant. Other generalized evocations of climate change fail under detailed scrutiny.

According to Jared Diamond's Guns, Germs and Steel, the lack of tameable megafauna was one of the reasons why Amerindian civilizations developed at a slower rate than Old World ones. Which critics dispute by saying there was no lack of tameable megafauna; llama, vicuna, bison were all there.

Reference : E. C. Pielou, After the Ice Age: the return of life to glaciated North America, 1991

South America

South America, which had remained largely unglaciated, except for increased mountain glaciation in the Andes, there was a contemporaneous but smaller wave of extinctions.

Australia

(ca. 26000-15000 BP) the sudden spate of extinctions came earlier than in the Americas, but lagged well after the first arrival of humans. The Australian extinctions included:

  • diprotodons (giant relatives of the wombats)
  • Zygomaturus trilobus (a large marsupial herbivore)
  • Palorchestes azael (a marsupial "tapir")
  • Macropus titan (a giant kangaroo)
  • Procoptodon goliah (a hoof-toed giant short-faced kangaroo)
  • Wonambi naracoortensis (a five-to-six-metre-long Australian constrictor snake)
  • Thylacoleo carnifex (a leopard-sized marsupial lion)
  • Megalania prisca (a giant monitor lizard)

Some extinct megafauna, such as the bunyip-like diprotodon, may be the sources of ancient cryptozoological legends.

New Zealand

c. 1200 years ago, several species became extinct after Polynesian settlers arrived, including:

Madagascar

Starting with the arrival of humans c. 2000 years ago, nearly all of the island's megafauna became extinct, including:

Indian Ocean Islands

Starting c. 500 years ago, a number of species became extinct upon human settlement of the islands, including:

The Ongoing Holocene Extinction

Megafaunal extinctions have continued to the present day. Modern extinctions are more directly attributable to human influences. Extinction rates are minimized in the popular imagination by the survival of captive trophy populations of animals that are merely "extinct in the wild," (Père David's Deer, etc) and by marginal survivals of highly-publicized megafauna that is "ecologically extinct" (Giant Panda, Sumatran Rhinoceros, the North American Black-Footed Ferret, etc.). Some notable examples of modern extinctions of "charismatic" mammal and bird fauna:

Many birds have become extinct as a result of human activity, especially birds endemic to islands, including many flightless birds (see a more complete list under extinct birds). Notable extinct birds include:

There still is debate as to the extent to which the disappearance of megafauna at the end of the last ice age can also be attributed to human activities, directly by hunting or indirectly, by decimation of prey populations. While climate change is still cited as an important factor, such anthropogenic explanations have become predominant.

Most biologists believe that we are at this moment at the beginning of a tremendously accelerated anthropogenic mass extinction. E.O. Wilson of Harvard, in his book The Future of Life (ISBN 0679768114), estimates that at current rates of human destruction of the biosphere, one-half of all species of life will be extinct in 100 years. In 1998 the American Museum of Natural History conducted a poll of biologists that revealed that the vast majority of biologists believe that we are in the midst of an anthropogenic mass extinction. Numerous scientific studies since then-- led by the 10,000 scientists who contribute to the IUCN's annual Red List of threatened species-- have only strengthened this consensus.

Our evidence for all previous extinction events is geological evidence, and the shortest scales of geological time usually are in the order of several hundred thousand to several million years. Even those extinction events that were caused by instantaneous events -- the Chicxulub asteroid impact being currently the demonstrable example -- unfold through the equivalent of many human lifetimes, due to the complex ecological interactions that are unleashed by the event. Those who are skeptical about the impending mass extinction argue that even if the current rate of extinction is higher than the rate during a great mass extinction event, as long as the current rate does not last more than a few thousand years, the overall effect will be small. There is still hope, argue some, that humanity can eventually slow the rate of extinction through proper ecological management (see sustainable). Current socio-political trends, argue others, indicate that this idea is overly optimistic (cf Sustainable development and search 'Sustainable' for further topics).

See: Primitivism

External links

  1. The Current Mass Extinction Event: http://www.well.com/user/davidu/extinction.html
  2. American Museum of Natural History official statement on current mass extinction: http://www.amnh.org/museum/press/feature/biofact.html
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