More Info

Wednesday, November 5, 2008

Holocene extinction event

The Holocene extinction event is the widespread, ongoing mass extinction of species during the modern Holocene epoch. The large number of extinctions span numerous families of plants and animals including mammals, birds, amphibians, reptiles and arthropods; a sizeable fraction of these extinctions are occurring in the rainforests. This extinction event is sometimes referred to as the sixth extinction following the previous five extinction events. Since 1500 CE/AD, 784 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. Up to 140,000 species per year (based on Species-area theory) may be the present rate of extinction based upon upper bound estimating.

In broad usage, the Holocene extinction event includes the notable disappearance of large mammals, known as megafauna, starting 100,000 years ago as humans developed and spread- (poul s. martin). 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 Quaternary extinction event or Ice Age extinction event. However the Holocene extinction event continues through the events of the past several millennia and includes the present time.

The observed rate of extinction has accelerated 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 human-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 glacial period. 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. 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 increase in human population, so there would be no need to separate the Pleistocene extinction from the recent one. Nevertheless, the Pleistocene extinction event is large enough and has not been resolved completely.

Younger extinctions

New Zealand

c. AD 1500, 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 that 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.


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:
o Giant aye-aye (Daubentonia robusta)
o sloth lemurs, including chimpanzee-sized Palaeopropithecus and gorilla-sized Archaeoindris
o Megaladapis, an orangutan-sized arboreal lemur
* Giant tortoise
* Malagasy 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.

Ongoing Holocene extinction

Significantly, the rate of species extinctions at present is estimated at 100 to 1000 times "background" or average extinction rates in the evolutionary time scale of planet Earth.

Megafaunal extinctions continue 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.), 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 include:

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

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
* the Carolina Parakeet of the American southeast

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 disruption 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.

Peter Raven, past President of the American Association for the Advancement of Science, states in the foreword to their publication AAAS Atlas of Population and Environment: "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." The reasons for the current mass extinction are all human related and include deforestation and other habitat destruction, hunting and poaching, the introduction of non-native species, pollution and climate change.
The Golden Toad of Costa Rica, extinct since around 1989. Its disappearance has been attributed to climate change.

Evidence for all previous extinction events is geological in nature, and the shortest scales of geological time 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 was a limited debate as to the extent to which the disappearance of megafauna at the end of the last glacial period 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.

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. Many hopes are set on sustainable development and conservation. 189 countries which are signatory to the Rio Accord have committed to preparing a Biodiversity Action Plan, a first step at identifying specific endangered species and habitats, country by country.

No comments: