Holocene extinction event

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The Holocene extinction event is a name customarily given to the widespread, ongoing extinction of species occurring in the modern Holocene epoch. The extinctions vary from mammoths to Dodos, to countless species in the rainforest dying every year. Because some believe the rate of this extinction event is comparable to the "Big Five" mass extinctions, it is also known as the Sixth Extinction, though the actual numbers of extinct species are not yet similar to the major mass extinctions of the geologic past.

In broad usage, the Holocene extinction event includes the remarkable disappearance of large mammals, known as megafauna, near the end of the last ice age between 9,000 and 13,000 years ago. Such disappearances have been considered as either a response to climate change, a result of the spread and 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.

The observed rate of extinction has risen dramatically in the last 50 years. 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.

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The prehistoric extinction events

The current ongoing extinction event seems more outstanding if we follow tradition and separate the recent extinction (approx. since the industrial revolution) from the 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, 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. 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 both events. 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 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-level or higher.

There are two main hypotheses concerning this extinction:

  • The animals died off because of climate change: the retreat of the 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, 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)
  • 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
  • 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. Critics have disputed this by argueing that 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.

Younger prehistoric extinctions

New Zealand

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

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:

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

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-Triassic extinction event that extinguished some 90% of the Paleozoic biota, or the Cretaceous-Tertiary extinction event that eliminated all non-avian dinosaurs at the end of the Cretaceous, 65 million years ago.

The current Holocene extinction differs from all previous extinction events in that it appears to be caused by a single species—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.

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:


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:

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 --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.

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 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. Current socio-political trends, others argue, indicate that this idea is overly optimistic. Most hopes are set on sustainable development and moderate forms of primitivism.

External links

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
  • 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
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