Holocene extinction

The Holocene extinction event is a name customarily given to the widespread, ongoing mass extinction of species during the modern Holocene epoch. The extinctions vary from mammoths to dodos, to countless species in the rainforest dying every year. Because the rate of this extinction event appears to be much more rapid than the "Big Five" mass extinctions, it is also known as the Sixth Extinction. Since 1500 AD, 698 extinctions have been documented by the International Union for Conservation of Nature and Natural Resources. However, since most extinctions are likely to go undocumented, scientists estimate that during the last century, between 20,000 and two million species have become extinct, but the precise total cannot be determined more accurately within the limits of present knowledge. The high estimates rely upon an annual loss of up to 140,000 species per year (based on Species-area theory).

In broad usage, the Holocene extinction event includes the remarkable disappearance of large mammals, known as megafauna, by the end of the last ice age 9,000 to 13,000 years ago. Such disappearances have been considered as either a response to climate change, a result of the proliferation of modern humans, or both. These extinctions, occurring near the Pleistocene / Holocene boundary, are sometimes referred to as the Pleistocene extinction event or Ice Age extinction event. However the Holocene extinction event continues through the events of the past several millenia and includes the present time.

The observed rate of extinction has accelerated dramatically in the last 50 years, to a pace which appears to be greater than the rate seen during the Big Five. There is no general agreement on whether to consider more recent extinctions as a distinct event or merely part of a single escalating process. Only during these most recent parts of the extinction have plants also suffered large losses. Overall, the Holocene extinction event is most significantly characterised by the presence of man-made driving factors and its very short geological timescale (tens to thousands of years) compared to most other extinction events.

The prehistoric extinction events

The ongoing extinction event seems more outstanding if we follow tradition and separate the recent extinction (approximately since the industrial revolution) from the Pleistocene extinction near the end of the last ice age. In popular imagination the latter is exemplified by the extinction of the woolly mammoth and, incorrectly, the Neanderthal people.

However, modern climatology suggests the current Holocene epoch is no more than the latest in a series of interglacial intervals between glaciation events, perhaps one that will be artificially extended by global warming. Furthermore, there is a continuum of extinctions between 13,000 years ago and now. If only considering human impact, the vulnerability and extinction rate of species simply rises with the rise of technology, so there would be no need to separate the Pleistocene extinction from the recent one. Nevertheless, the Pleistocene extinction event is large enough and hasn't been resolved completely.

The Pleistocene or Ice Age extinction event

The Ice Age extinction event is characterised by the extinction of many large mammals weighing more than 40 kg. In North America around 33 of 45 genera of large mammals went extinct, in South America 46 of 58, in Australia 15 of 16, in Europe 7 of 23, and in Subsaharan Africa only 2 of 44. The South American extinction witnessed the aftermath of the Great American Interchange. Only in South America and Australia did the extinction occur at family-levels or higher.

There are two main hypotheses concerning this extinction:

• The animals died off due to climate change: the retreat of the polar ice cap.
• The animals were exterminated by humans: the "prehistoric overkill hypothesis" (Martin, 1967).

The prehistoric overkill hypothesis is not universally applicable and is imperfectly confirmed. For instance, there are ambiguities around the timing of sudden extinctions of marsupial Australian megafauna. Biologists note that comparable extinctions have not occurred in Africa, 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 which claims that the Holocene was initiated by an extinction event caused by bolide impacts.

Major megafaunal extinctions

Europe

(circa 15,000 years ago)

• Woolly Mammoth
• Woolly Rhinoceros
• Irish Elk
• Cave Lion
• Cave Bear
• Giant Hyaena

Mediterranean Islands

(by 9000 years ago)

• a pygmy hippopotamus (Phanourios minutus) of Cyprus
• the Balearic Islands Cave Goat (Myotragus balearicus) of Majorca and Minorca
• dwarf elephants: Palaeoloxodon cypriotes of Cyprus, P. falconeri of Sicily and Malta
• Giant Swan (Cygnus falconeri) of Malta
• Giant Rat of Majorca

North America

During the last 50 thousand years, including the end of the last ice age, approximately 33 genera of large mammals have gone extinct in North America. Of these, 15 genera extinctions can be be reliably attributed to a brief interval of 11.5 to 10 thousand radiocarbon years before present, shortly following the arrival of the Clovis people in North America. Most other extinctions are poorly constrained in time, though some definitely occurred outside of this narrow interval. Contrary to this, only about half a dozen small mammals disappeared during this time. 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)
• Saber-toothed cats (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
• The giant progenitor 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.), who were 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. Critics have disputed this by arguing that, with llama, vicuña, and bison, there was no such lack.

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

South America

In 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

The sudden spate of extinctions came earlier than in the Americas. Most evidence points to the period immediately after the first arrival of humans — thought to be a little under 50,000 years ago — but scientific argument continues as to the exact date range. 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.

Younger prehistoric extinctions

New Zealand

c. 1500 AD, several species became extinct after Polynesian settlers arrived, including:

• Ten species of Moa, giant flightless ratite birds.
• The giant Haast's Eagle, Harpagornis
• The flightless predatory Adzebills.

Pacific, including Hawaii

Recent research, based on archaeological and paleontological digs on 70 different islands, has shown the numerous species went extinct as people moved across the Pacific, starting 30,000 years ago in the Bismarck Archipelago and Solomon Islands (Steadman & Martin 2003). It is currently estimated that among the bird species of the Pacific some 2000 species have gone extinct since the arrivial of humans (Steadman 1995). Among the extinctions were:

• The Moa-nalos, giant grazing ducks from Hawaii.
• A giant megapode from New Caledonia.
• Mekosuchine crocodiles from New Caledonia Fiji and Samoa.

Madagascar

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

• the Aepyornis, or Elephant Bird, a giant flightless ratite bird.
• 17 of 50 species of lemur, including:
  • giant aye-aye (Daubentonia robusta); last known individual killed 1930
  • sloth lemurs, including chimpanzee-sized Palaeopropithecus and gorilla-sized Archaeoindris
  • Megaladapis, an orangutan-sized arboreal lemur
• giant tortoise
• pygmy hippopotamus

Indian Ocean Islands

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

• several species of giant tortoise on the Seychelles and Mascarene Islands
• 14 species of birds on the Mascarene Islands, including the Dodo, the Rodrigues Solitaire, and the unrelated Réunion Solitaire.

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.) and by unregarded extinctions among arthropods. Some notable examples of modern extinctions of "charismatic" mammal fauna:

• Aurochs, Europe
• Thylacine or Tasmanian Tiger, Thylacinus cynocephalus, Tasmania [extinction disputed]
• Quagga, a zebra relative, Southeast Africa
• Steller's Sea Cow

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:

• the Dodo, the giant flightless pigeon of Mauritius, Indian Ocean
• the Great Auk of islands in the north Atlantic
• the Passenger Pigeon of North America
• several species of Moa, giant flightless birds from New Zealand

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 The Future of Life (2002), 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—such as a 2004 report from Nature, and those 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.

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 another important factor, anthropogenic explanations have become predominant.

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 hundred, the overall effect may be acceptable. There is still hope, argue some, that humanity can eventually slow the rate of extinction through proper ecological management. Current socio-political and overpopulation trends, others argue, indicate that this idea is overly optimistic. Most hopes are set on sustainable development and moderate forms of primitivism.

See also

• Overpopulation
• Slash-and-burn

External links

• The Current Mass Extinction Event
• American Museum of Natural History official statement on current mass extinction
• The Extinction Website

References

• Leakey, Richard and Roger Lewin, 1996, The Sixth Extinction : Patterns of Life and the Future of Humankind, Anchor, ISBN 0385468091.
• Martin, P.S. & Wright, H.E. Jr., eds., 1967. Pleistocene Extinctions: The Search for a Cause. Yale University Press, New Haven, 440 pp., ISBN 0300007558
• Oakes, Ted, Kear, Amanda, Bates, Annie, Holmes, Kathryn, 2003, Monsters we met. Man's prehistoric battle for the planet, BBC Worldwide Ltd., Woodlands, ISBN 159258005X
• Pielou, E. C., 1991, After the Ice Age: the return of life to glaciated North America, University Of Chicago Press, ISBN 0226668118
• Steadman, D.W., 1995. Prehistoric extinctions of Pacific island birds: biodiversity meets zooarchaeology. Science 267, 1123–1131.
• Steadman, D.W., Martin, P.S., 2003. The late Quaternary extinction and future resurrection of birds on Pacific islands. Earth-Science Reviews 61, 133–147

Footnotes