New World Pleistocene extinctions

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In the New World as well as other geographic areas, the Pleistocene epoch saw the extinctions of numerous species, many of which occurred during the transition to the Holocene epoch in what is termed the Holocene extinction event; see also Pleistocene extinctions. Among the main causes hypothesized by paleontologists are the spread of disease, natural climate change, and overkill by humans, which appeared during this epoch. A variant of this last possibility is the second order predation hypothesis, which focuses more on the indirect damage caused by overcompetition with nonhuman predators.

[edit] Overkill Hypothesis

The timing of extinctions follows the "March of Man"

The Overkill hypothesis suggests that humans hunted New World megaherbivores to extinction. As a result, carnivores and scavengers that depended upon those animals became extinct from lack of prey.^[1][2][3] The hypothesis was proposed by Paul S. Martin, now professor of geosciences emeritus at the Desert Laboratory of the University of Arizona 40 years ago. It sparked debate which continues today. The most convincing evidence of his theory is that that 80% of the North American animal population disappeared within 1000 years of the arrival of humans on the Western Hemisphere continents.

World wide extinctions seem to follow the migration of humans and to be most severe where humans arrived most recently and least severe where humans were originally – Africa (see figure at right).

Extinctions occur in relation to human artifacts regardless of climate change

This suggests that where humans had evolved – Africa – the prey animals and human hunting ability evolved together so the animals evolved avoidance techniques – they became sophisticated – as humans migrated throughout the world and became more and more proficient they encountered naive animals making it easier and easier to find and kill them. And that this is independent of climate change (see figure at left).

Overkill has been supported by archaeological finds of mammoths with projectile points embedded in their skeletons, by observations of modern naïve animals allowing hunters to approach easily ^[4][5][6] and by computer models by Mosimann and Martin,^[7]and Whittington and Dyke, ^[8] and most recently by Alroy.^[9]

[edit] Shortcomings of the Overkill Hypothesis:

The major objections to the theory are:

1. in predator prey models it is unlikely that predators could over-hunt their prey since predators need their prey as food to sustain life and reproduce. ^[10]
2. many animals (giant ground sloths, camelids, giant armadillos) that went extinct during the time period were probably not hunted by humans,
3. animals that were hunted (bison) did not go extinct.
4. the dwarfing of animals is not explained by overkill

[edit] Links on Overkill

Paul Martin pmartin@geo.arizona.edu

CV for Paul Martin – http://wwwpaztcn.wr.usgs.gov/paul_cv.html

John Alroy: http://www.nceas.ucsb.edu/~alroy/

[edit] Climate Change Hypotheses

At the end of the 19^thand beginning of the 20^thcentury when scientists first realized that there had been glacial and interglacial ages, and that they were somehow associated with the prevalence or disappearance of certain animals, they surmised that the termination of the Pleistocene ice age might be an explanation for the extinctions.

[edit] Increased Temperature

The most obvious change associated with the termination of an Ice Age is the increase in temperature. Between 15kya and 10kya, a 6 degree Celsius increase in global mean annual temperatures occurred. This was generally thought to be the cause of the extinctions.

According to this hypothesis, a temperature increase sufficient to melt the Wisconsin ice sheet could have placed enough thermal stress on cold-adapted mammals to cause them to die. Their heavy fur, which helps conserve body heat in the glacial cold, might have prevented the dumping of excess heat, causing the mammals to die of heat exhaustion. Large mammals, with their reduced surface-area-to-body ratio, would have fared worse than small mammals.

[edit] Shortcomings of the Temperature Hypothesis

More recent research has demonstrated that the annual mean temperature of the current interglacial that we have been for the last 10K years is no higher than that of previous interglacials so the same large mammals survived similar temperature increases.. Therefore warmer temperature alone is not a sufficient explanation ^{[11][12][13][14][15][16]}

[edit] Increased Continentality affects vegetation in time or space

Other scientists have proposed that increasingly extreme weather — hotter summers and colder winters — referred to as continentality or related changes in rainfall caused the extinctions. The various hypotheses are:

[edit] Shortcomings of the Continentality Hypotheses

Critics have identified a number of problems with the continentality hypotheses.

1. Megaherbivores have prospered at other times of continental climate. For example, megaherbivores thrived in Pleistocene Siberia, which had and has a more continental climate than Pleistocene or modern (post-Pleistocene, interglacial) North America.^[26][27][28]
2. The animals that went extinct actually should have prospered during the shift from mixed woodland-parkland to prairie, because their primary food source, grass, was increasing rather than decreasing. ^[29][30][28] Although the vegetation did become more spatially specialized, the amount of prairie and grass available increased, which would have been good for horses and for mammoths, and yet they went extinct.
3. Although horses went extinct in the New World, they were successfully reintroduced by the Spanish in the sixteenth century — into a modern post-Pleistocene, interglacial climate. Today there are feral horses still living in those same environments. They find a sufficient mix of food to avoid toxins, they extract enough nutrition from forage to reproduce effectively and the timing of their gestation is not an issue. Similarly, mammoths survived the Pleistocene Holocene transition on isolated, uninhabited islands in the Mediterranean^[31] and on Wrangel Island in the Siberian Arctic ^[32] until 4,000 to 7,000 years ago.
4. Large mammals should have been able to migrate, permanently or seasonally, if they found the temperature too extreme, the breeding season too short, or the rainfall too sparse or unpredictable ^[33]. Seasons vary geographically. By migrating away from the equator, herbivores could have found areas with growing seasons more favorable for finding food and breeding successfully. Modern-day African elephants migrate during periods of drought to places where there is apt to be water. ^[34]
5. Large animals store more fat in their bodies than do medium-sized animals^[35] and this should have allowed them to compensate for extreme seasonal fluctuations in food availability.

[edit] Shortcomings of Both Climate Change and Overkill

Neither Overkill nor the other major hypothesis – Climate Change explains why:

1. browsers, mixed feeders and non-ruminant grazer species suffered most, while ruminant grazers generally survived,
2. many surviving mammal species were sharply diminished in size; and
3. vegetative environments shifted from plaid to striped.^[36]

Because of the unsatisfactory nature of the Overkill or Climate Change hypotheses alone many scientists support some combination of Climate Change and Overkill.

[edit] Hyperdisease Hypothesis

[edit] Theory

The disease hypothesis attributes the extinction of large mammals during the late Pleistocene to indirect effects of the newly arrived aboriginal humans ^[37][38]

The disease hypothesis proposes that humans or animals traveling with them (e.g. domestic dogs) introduced one or more highly virulent diseases into vulnerable populations of native mammals, eventually causing extinctions. The extinction was biased toward larger-sized species because smaller species have greater resilience because of their life history traits (e.g. shorter gestation time, greater population sizes, etc). Humans are thought to be the cause because other earlier immigrations of mammals into North America from Eurasia did not cause extinctions ^[37]

If a disease was indeed responsible for the end- Pleistocene extinctions, then there are several criteria it must satisfy (see Table 7.3 in MacPhee & Marx 1997). First, the pathogen must have a stable carrier state in a reservoir species. That is, it must be able to sustain itself in the environment when there are no susceptible hosts available to infect. Second, the pathogen must have a high infection rate, such that it is able to infect virtually all individuals of all ages and sexes encountered. Third, it must be extremely lethal, with a mortality rate of c. 50–75%. Finally, it must have the ability to infect multiple host species without posing a serious threat to humans. Humans may be infected, but the disease must not be highly lethal or able to cause an epidemic.

[edit] Shortcomings of the Hyperdisease Hypothesis

1. No evidence of disease has been found
2. Generally speaking disease has to be very virulent to kill off all the individuals in a genera or species. Even such a virulent disease as West Nile Virus is unlikely to have caused extinction. ^[39]

[edit] Links on the Hyperdisease Hypothesis

http://www.amnh.biz/science/biodiversity/extinction/Day1/bytes/MacPheePres.html

http://biology.unm.edu/JHBrown/Published/WasAHyperdiseaseResponsible.x.pdf

[edit] Second Order Predation

Combination Hypotheses - Climate Change, Overkill + Climate Change, 2Order Predation + Climate Change

Overkill Hypothesis and Seconed Order Predation

[edit] Theory - Scenario

The Second Order Predation hypothesis says that as humans entered the New World they continued their policy of killing predators which upset the ecological balance of the continent causing over population, environmental exhaustion and environmental collapse. The hypothesis accounts for changes in animal, plant, and human populations The scenario is as follows:

• After the arrival of H. sapiens in the New World, existing predators must share the prey populations with this new predator. Because of this competition, populations of original, or first-order, predators cannot find enough food they are in direct competition with humans.
• Second-order predation begins as humans begin to kill predators.
• Prey populations are no longer well controlled by predation. Killing of nonhuman predators by H. sapiens reduces their numbers to a point where these predators no longer regulate the size of the prey populations.
• Lack of regulation by first-order predators triggers boom-and-bust cycles in prey populations. Prey populations expand and consequently overgraze and over-browse the land. Soon the environment is no longer able to support them. As a result, many herbivores starve. Species that rely on the slowest recruiting food become extinct, followed by species that cannot extract the maximum benefit from every bit of their food.
• Boom-bust cycles in herbivore populations change the nature of the vegetative environment, with consequent climatic impacts on relative humidity and continentality. Through overgrazing and overbrowsing, mixed parkland becomes grassland, and climatic continentality increases.

[edit] Support

This has been supported by a computer model - Pleistocene Extinction Model (PEM), using the same assumptions and values, compares hypotheses with Second Order Predation. The findings are that Second Order Predation is more consistent with extinction than is Overkill ^[40](Results graph at left). The PEM was run to test combination hypotheses by artificially introducing sufficient climate change to cause extinction. When Overkill and Climate Change are combined they balance each other out. Climate Change reduces the number of plants overkill removes animals therefore fewer plants are eaten Second Order Predation combined with Climate Change exacerbates the extinction ^[41] (results graph at right).

[edit] Second Order Predation (2OP) and other theories

[edit] 2 OP and Climate Change observations

• Second Order Predation accounts for the changes in vegetation, which in turn may account for the increase in continentality.
• Since the extinction is due to destruction of habitat it accounts for the loss of animals not hunted by humans.
• Second Order Predation accounts for the dwarfing of animals as well as extinctions since animals that could survive and reproduce on less food would be selectively favored.

[edit] 2 OP and Hyperdisease

The reduction of carnivores could have been from distemper or other carnivore disease carried by domestic dogs.

[edit] 2 OP and Overkill

The observation that extinctions follow the introduction of humans is supported by the Second Order Predation hypothesis.

[edit] Shortcomings of the Second Order Predation Hypothesis

No evidence of humans hunting predators has been found in the New World though it has been found in Siberia ^[42]

[edit] Links on Second Order Predation

Elin Whitney-Smith – http://quaternary.net

[edit] References

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