by bystander » Sat Jun 19, 2010 6:19 pm
Here's some more theories to think about.
Research Suggests Large Mammals Influenced Global Climate
University of New Mexico | 2010 May 24
More than 13,000 years ago, millions of large mammals such as mammoths, mastodon, shrub-ox, bison, ground sloths and camels roamed the Americas and may have had profound influences on the environment according to research in a paper titled, “Methane Emissions from Extinct Megafauna” released in the publication Nature Geosciences Sunday.
The extinction of these large herbivores, which also include horses, llamas and stag moose in addition to the giant wooly mammoth, probably led to an abrupt decrease in methane emissions and atmospheric concentrations of the gas with potential implications for climate change says Dr. Felisa Smith, Associate Professor of Biology at the University of New Mexico.
...
Approximately 13,400 years ago, the Americas supported a mammal fauna that was richer than that of Africa today explained Smith. “Around 11,500 years ago and within 1,000 years of the arrival of humans in the New World, 80 percent of these large-bodied mammals were extinct,” said Smith in the paper.
“This is arguably the first detectable influence of humans on the environment going back 13,400 years to when humans first got to the continent,” said Smith. “I think that it’s intriguing because there are a lot of ramifications. Potentially, if the decrease in methane, which is synchronous with this ice spell, was actually the cause, then humans contributed to the Younger Dryas cold episode.”
Methane emissions from extinct megafauna
Mass Animal Extinctions, Not Climate Change, Killed Plants
Live Science | Environment | Behind the Scenes | 14 May 2010
Jack Williams is a plant ecologist at heart. He likes to figure out how — and why — plant communities change over time.
...
Recently, these interests took on a new dimension for him. In November, Williams and his colleagues, including graduate student Jacquelyn Gill, released research on how the extinction of ancient large plant-eating animals, such as mammoths and mastodons, affected ecosystems when the
enormous mammals began their decline in North America about 15,000 years ago.
However, in doing so, the researchers also raised intriguing new questions about how those animals might have died in the first place.
...
They found that the death of these creatures apparently prompted a proliferation of broadleaved trees, and ultimately, the accumulation of woody debris that contributed to a dramatic increase in wildfires. They also determined that the animals' decline probably was gradual, meaning they didn't die from some sudden event.
Taken together, the evidence appeared to eliminate some popular theories about what caused their
mass extinction, including the
impact of a meteor or comet, a "blitzkrieg" of human hunting, or a
loss of habitat due to climate changes.
Pleistocene Megafaunal Collapse, Novel Plant Communities, and Enhanced Fire Regimes in North America
Where Did The Mammoths Really Go?
University of Washington |
Northwest Science & Technology | Spring 2010
Fifteen thousand years ago, North America was home to roughly 35 different types of large mammals which no longer exist today, such as saber tooth cats, giant ground sloths, and mammoths. Many species weighed over two thousand pounds; a full-grown adult male mammoth could weigh up to 20,000 pounds, the weight of a city bus. Why did so many large animals die off by the end of the last ice age? It's rare to find a mystery as enigmatic or hotly debated as the cause for the disappearance of North America's largest known mammals. Scientists have worked to elucidate the cause of this extinction for over fifty years. Investigation of fungus that lived in megafaunal dung has led to a new timeline for the extinction, and the results were published in the November 2009 issue of Science.
Several major events occurred in North America during megafaunal population decline which have fueled speculation about the causes of extinction. Climate changes, over hunting by humans, and a purported cosmic impact are all posited as causing megafaunal extinction. While there is some support to link these events to extinction, there is not overwhelming evidence for any one hypothesis. Furthermore, parsing correlation from causation for events ten thousand years in the past is difficult, to say the least.
Traditionally, tracking the decline of these giant animal populations has relied on radiocarbon dating of fossils. Based on such evidence, most extinctions are dated somewhere between 13.3 to 12.9 thousand years ago. ... Gill's data indicate that megafauna populations began to decline earlier than previously expected: beginning at 14.8 thousand years ago. ...
Although Gill's paper refutes the Clovis blitzkrieg hypothesis, it leaves a window open for a gradual human over-hunting hypothesis, perhaps by pre-Clovis people. Haynes points out that as soon as modern humans arrive in locations all over the world, there is very quick collapse of megafauna populations. ...
A small cadre of scientists have recently proposed a cosmic impact 12.9 thousand years ago as the cause for megafaunal extinction. The impact hypothesis is inconsistent with Gill's research establishing an earlier date for population collapse beginning at 14.8 thousand years ago that declined over a longer period of time. ...
Most scientists studying the mystery of megafauna decline do not accept the newer cosmic impact hypothesis. Critics point to the absence of an appropriately aged impact crater. In addition, Donald Grayson, archeologist at the University of Washington, points out that not all of the big mammals died off at the same time. ...
Climate change is another potential culprit for megafaunal demise. At the end of the last ice age, in the Pleistocene, there was a series of warming and cooling cycles. The Bølling-Alerød warming period occurs at about the same time the megafaunal population began to decline according to Gill's work. The Younger Dryas, a colder era, lasted from 12.8 to 11.5 thousand years ago, close to the date of final megafaunal extinction. ...
Hybrid models that combine climate change and human hunting also have support in the scientific community. Megafauna may have been stressed by the various consequences of climate change, but human hunting, even mild hunting, may have pushed the massive mammals over the edge ...
Here's some more theories to think about.
[size=120][url=http://www.unm.edu/~market/cgi-bin/archives/005200.html]Research Suggests Large Mammals Influenced Global Climate[/url][/size]
University of New Mexico | 2010 May 24
[quote]More than 13,000 years ago, millions of large mammals such as mammoths, mastodon, shrub-ox, bison, ground sloths and camels roamed the Americas and may have had profound influences on the environment according to research in a paper titled, “Methane Emissions from Extinct Megafauna” released in the publication [i]Nature Geosciences[/i] Sunday.
The extinction of these large herbivores, which also include horses, llamas and stag moose in addition to the giant wooly mammoth, probably led to an abrupt decrease in methane emissions and atmospheric concentrations of the gas with potential implications for climate change says Dr. Felisa Smith, Associate Professor of Biology at the University of New Mexico.
...
Approximately 13,400 years ago, the Americas supported a mammal fauna that was richer than that of Africa today explained Smith. “Around 11,500 years ago and within 1,000 years of the arrival of humans in the New World, 80 percent of these large-bodied mammals were extinct,” said Smith in the paper.
“This is arguably the first detectable influence of humans on the environment going back 13,400 years to when humans first got to the continent,” said Smith. “I think that it’s intriguing because there are a lot of ramifications. Potentially, if the decrease in methane, which is synchronous with this ice spell, was actually the cause, then humans contributed to the Younger Dryas cold episode.” [/quote]
[b][i]Methane emissions from extinct megafauna[/i][/b]
[list][i]Nature Geosciences[/i] [b]3[/b], 23 May 2010, doi: [url=http://dx.doi.org/10.1038/ngeo877]10.1038/ngeo877[/url][/list]
[size=120][url=http://www.livescience.com/environment/plant-ecology-animal-extinctions-bts-100514.html]Mass Animal Extinctions, Not Climate Change, Killed Plants[/url][/size]
Live Science | Environment | Behind the Scenes | 14 May 2010
[quote]Jack Williams is a plant ecologist at heart. He likes to figure out how — and why — plant communities change over time.
...
Recently, these interests took on a new dimension for him. In November, Williams and his colleagues, including graduate student Jacquelyn Gill, released research on how the extinction of ancient large plant-eating animals, such as mammoths and mastodons, affected ecosystems when the [url=http://www.livescience.com/animals/091119-mammoth-megafauna-extinction.html]enormous mammals began their decline[/url] in North America about 15,000 years ago.
However, in doing so, the researchers also raised intriguing new questions about how those animals might have died in the first place.
...
They found that the death of these creatures apparently prompted a proliferation of broadleaved trees, and ultimately, the accumulation of woody debris that contributed to a dramatic increase in wildfires. They also determined that the animals' decline probably was gradual, meaning they didn't die from some sudden event.
Taken together, the evidence appeared to eliminate some popular theories about what caused their [url=http://www.livescience.com/strangenews/070808_GM_mass_extinctions.html]mass extinction[/url], including the [url=http://www.livescience.com/animals/061128_dinosaur_extinct.html]impact of a meteor or comet[/url], a "blitzkrieg" of human hunting, or a [url=http://www.livescience.com/environment/090824-assisted-migration.html]loss of habitat due to climate changes[/url]. [/quote]
[b][i]Pleistocene Megafaunal Collapse, Novel Plant Communities, and Enhanced Fire Regimes in North America[/i][/b]
[list][i]Science[/i], Vol 326, No 5956, 20 Nov 2009, DOI: [url=http://dx.doi.org/10.1126/science.1179504]10.1126/science.1179504[/url][/list]
[size=120][url=https://depts.washington.edu/nwst/issues/index.php?issueID=spring_2010&storyID=1753]Where Did The Mammoths Really Go?[/url][/size]
University of Washington | [i]Northwest Science & Technology[/i] | Spring 2010
[quote]Fifteen thousand years ago, North America was home to roughly 35 different types of large mammals which no longer exist today, such as saber tooth cats, giant ground sloths, and mammoths. Many species weighed over two thousand pounds; a full-grown adult male mammoth could weigh up to 20,000 pounds, the weight of a city bus. Why did so many large animals die off by the end of the last ice age? It's rare to find a mystery as enigmatic or hotly debated as the cause for the disappearance of North America's largest known mammals. Scientists have worked to elucidate the cause of this extinction for over fifty years. Investigation of fungus that lived in megafaunal dung has led to a new timeline for the extinction, and the results were published in the November 2009 issue of [i]Science[/i].
Several major events occurred in North America during megafaunal population decline which have fueled speculation about the causes of extinction. Climate changes, over hunting by humans, and a purported cosmic impact are all posited as causing megafaunal extinction. While there is some support to link these events to extinction, there is not overwhelming evidence for any one hypothesis. Furthermore, parsing correlation from causation for events ten thousand years in the past is difficult, to say the least.
Traditionally, tracking the decline of these giant animal populations has relied on radiocarbon dating of fossils. Based on such evidence, most extinctions are dated somewhere between 13.3 to 12.9 thousand years ago. ... Gill's data indicate that megafauna populations began to decline earlier than previously expected: beginning at 14.8 thousand years ago. ...
Although Gill's paper refutes the Clovis blitzkrieg hypothesis, it leaves a window open for a gradual human over-hunting hypothesis, perhaps by pre-Clovis people. Haynes points out that as soon as modern humans arrive in locations all over the world, there is very quick collapse of megafauna populations. ...
A small cadre of scientists have recently proposed a cosmic impact 12.9 thousand years ago as the cause for megafaunal extinction. The impact hypothesis is inconsistent with Gill's research establishing an earlier date for population collapse beginning at 14.8 thousand years ago that declined over a longer period of time. ...
[b]Most scientists studying the mystery of megafauna decline do not accept the newer cosmic impact hypothesis. Critics point to the absence of an appropriately aged impact crater. In addition, Donald Grayson, archeologist at the University of Washington, points out that not all of the big mammals died off at the same time. [/b] ...
Climate change is another potential culprit for megafaunal demise. At the end of the last ice age, in the Pleistocene, there was a series of warming and cooling cycles. The Bølling-Alerød warming period occurs at about the same time the megafaunal population began to decline according to Gill's work. The Younger Dryas, a colder era, lasted from 12.8 to 11.5 thousand years ago, close to the date of final megafaunal extinction. ...
Hybrid models that combine climate change and human hunting also have support in the scientific community. Megafauna may have been stressed by the various consequences of climate change, but human hunting, even mild hunting, may have pushed the massive mammals over the edge ...[/quote]