Quaternary extinction event

The Quaternary period saw the extinctions of numerous predominantly larger, especially megafaunal, species, many of which occurred during the transition from the Pleistocene to the Holocene epoch. However, the extinction wave did not stop at the end of the Pleistocene, but continued especially on isolated Islands in Holocene extinctions. Among the main causes hypothesized by paleontologists are natural climate change and overkill by humans, who appeared during the Middle Pleistocene and invaded many previously uninhabited regions of the world during the Late Pleistocene and Holocene. A variant of the latter possibility is the second-order predation hypothesis, which focuses more on the indirect damage caused by overcompetition with nonhuman predators. The spread of disease is also discussed as a possible reason.

Contents

The Pleistocene or Ice Age extinction event

The Late Pleistocene extinction event saw the extinction of many mammals weighing more than 40 kg.

The extinctions in the Americas entailed the elimination of all the larger (over 100 kg) mammalian species of South American origin, including those that had migrated north in the Great American Interchange. Only in North America, South America and Australia did the extinction occur at family taxonomic levels or higher.

There are two main hypotheses concerning the Pleistocene extinction:

There are some inconsistencies between the current available data and the prehistoric overkill hypothesis. For instance, there are ambiguities around the timing of sudden extinctions of Australian megafauna. Biologists note that comparable extinctions have not occurred in Africa and South or Southeast Asia, where the fauna evolved with hominids. Post-glacial megafaunal extinctions in Africa have been spaced over a longer interval.

Evidence supporting the prehistoric overkill hypothesis includes the persistence of certain island megafauna for several millennia past the disappearance of their continental cousins. Ground sloths survived on the Antilles long after North and South American ground sloths were extinct. The later disappearance of the island species correlates with the later colonization of these islands by humans. Similarly, woolly mammoths died out on remote Wrangel Island 7000 years after their mainland extinction. Steller's sea cows also persisted off the isolated and uninhabited Commander Islands for thousands of years after they vanished from continental shores of the north Pacific.[1]

Controversial alternative hypotheses to the theory of human responsibility include the Younger Dryas event and Tollmann's hypothetical bolide, which claim that the extinctions resulted from bolide impact(s). However, such hypotheses would predict an instantaneous, regional extinction(s), and thus cannot account for the planet-wide species losses that occurred over an interval of thousands of years.

Africa and Asia

The Old World tropics have been relatively spared by Pleistocene extinctions. Africa and Asia are the only regions that have megamammals weighing over 1000 kg today. However, during the early, middle and late Pleistocene some large animal forms disappeared from these regions without being replaced by comparable successor species. Climate change has been cited as most likely causing the extinctions in Southeast Asia.[2]

Large animals,which disappeared in Africa or Asia during the Early and Middle and Late Pleistocene:

Large animals, which disappeared in parts of Africa and Asia during the Late Pleistocene:

Australia and New Guinea

See also: Australian megafauna

The sudden spate of extinctions occurred 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:

Some extinct megafauna, such as the bunyip-like Diprotodon, may be the sources of ancient cryptozoological legends.

Northern Eurasia

(80,000–10,000 years ago)

North America

During the last 50,000 years, including the end of the last glacial period, approximately 33 genera of large mammals have become extinct in North America. Of these, 15 genera extinctions can be reliably attributed to a brief interval of 11,500 to 10,000 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.[4] In contrast, 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. (Moreover, previous extinction pulses were not comparable to the Quaternary extinction event; they involved primarily species replacements within ecological niches, while the latter event resulted in many ecological niches being left unoccupied.) The megafaunal extinctions include twelve genera of edible herbivores (H), and five large, dangerous carnivores (C). North American extinctions included:

The survivors are in some ways as significant as the losses: bison, moose (a late Pleistocene immigrant through Beringia), elk, caribou, deer, pronghorn, muskox, bighorn sheep, and mountain goat. All save the pronghorns descended from Asian ancestors that had evolved with human predators.[5] Pronghorns are the second fastest land mammal (after the cheetah), which may have helped them elude hunters. More difficult to explain in the context of overkill is the survival of bison, since these animals first appeared in North America less than 240,000 years ago [6][7][8] and so were geographically removed from human predators for a sizeable period of time. Because ancient bison evolved into living bison,[9][10] there was no continent-wide extinction of bison at the end of the Pleistocene (although the genus was regionally extirpated in many areas). The survival of bison into the Holocene and recent times is therefore inconsistent with the overkill scenario. By the end of the Pleistocene, when humans first entered North America, these large animals had been geographically separated from human hunters for more than 200,000 years. Given this enormous span of geologic time, bison would almost certainly have been very nearly as naive as native North American large mammals.

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 criticism of the "prehistoric overkill hypothesis" has been that the human population at the time was too small and/or not sufficiently widespread geographically to have been capable of such ecologically significant impacts. This criticism does not mean that climate change scenarios explaining the extinction are automatically to be preferred by default, however, any more than weaknesses in climate change arguments can be taken as supporting overkill. Some form of a combination of both factors could be plausible, although overkill would have the lion's share for the blame. It is easier to believe overkill would be a lot easier to achieve large-scale extinction with an already dying population due to climate change.

Lack of tameable megafauna was perhaps one of the reasons why Amerindian civilizations evolved differently than Old World ones.[11] Critics have disputed this by arguing that llamas, alpacas, and bison were domesticated.[12]

South America

At the Pleistocene-Holocene transition South America, which had remained largely unglaciated except for increased mountain glaciation in the Andes, saw an extinction wave, which carried off many large species. Unfortunately of those were the last of the Notoungulata (Toxodon, Macrauchenia) a group which have ruled South America for 57 million years and may have been common throughout the world before the rise of the modern ungulates. South America being an island continent it still had the primitive forms that have been replaced elsewhere, though most of them went extinct during the Great American Interchange. Of the megafauna survivors of the interchange, none survived the Quaternary one so only their smaller relatives remain (Anteaters, tree sloths, armadillos; New World Marsupials: Opposums, Shrew opossums, and Monito del Monte (actually more related to Australian marsupials, yet Australian marsupials evolved from a South American ancestor that may have been similar to Monito del Monte)). Of those that survived the interchange but not the Quaternary were the Ground sloths, Glyptodonts, Pampatheres, and the previously mentioned Notoungulatas; and of these all but the Notoungulatas migrated to North America, only to go extinct there too. Today the largest land mammals remaining in South America are the wild Lamini camels: Guanacos and Vicuñas and their domestic counterparts Llamas and Alpacas. Other notable surviving megafauna or large fauna are tapirs, rheas, Jaguars, Boa Constrictors, Anacondas, Caymans, and giant rodents like Capybaras.

Later extinctions

Mediterranean Islands

New Zealand

c. AD 1500, 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 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 arrival 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:

Caribbean

Hunting hypothesis

The hunting hypothesis suggests that humans hunted megaherbivores to extinction. As a result, carnivores and scavengers that depended upon those animals became extinct from lack of prey.[13][14][15] Therefore this hypothesis holds Pleistocene humans responsible for the megafaunal extinction. One variant, often referred to as blitzkrieg, portrays humans as hunting the megafauna to extinction within a relatively short period of time. Some of the direct evidence for this includes: fossils of some megafauna found in conjunction with human remains, embedded arrows and tool cut marks found in megafaunal bones, and European cave paintings that depict such hunting. Biogeographical evidence is also suggestive; the areas of the world where humans evolved currently have more of their Pleistocene megafaunal diversity (the elephants and rhinos of Asia and Africa) compared to other areas such as Australia, the Americas, Madagascar and New Zealand, areas where early humans were non-existent. In addition, where animals have not been hunted for several years they become naive. Based on this evidence, a picture arises of the megafauna of Asia and Africa evolving with humans, learning to be wary of them, and in other parts of the world the wildlife appearing ecologically naive and easier to hunt. This is particularly true of island fauna, which display a dangerous lack of fear of humans. Of course, it is impossible to demonstrate that continental mammals were possessed of a similar naïveté.

Circumstantially, the close correlation in time between the appearance of humans in an area and extinction there provides weight for this scenario. This is perhaps the strongest evidence, as it is almost impossible that it could be coincidental when science has so many data points. For example, the woolly mammoth survived on islands despite changing climatic conditions for thousands of years after the end of the last glaciation, but they died out when humans arrived around 1700 BC. The megafaunal extinctions covered a vast period of time and highly variable climatic situations. The earliest extinctions in Australia were complete approximately 50,000 BP, well before the last glacial maximum and before rises in temperature. The most recent extinction in New Zealand was complete no earlier than 500 BP and during a period of cooling. In between these extremes megafaunal extinctions have occurred progressively in such places as North America, South America and Madagascar with no climatic commonality. The only common factor that can be ascertained is the arrival of humans.[16][17] Even within regions, this phenomenon appears to be true. The mammal extinction wave in Australia about 50,000 years ago coincides not with known climatic changes, but with the arrival of humans. In addition, large mammal species like the giant kangaroo Protemnodon appear to have survived in Tasmania longer than on the Australian mainland. Tasmania was colonised by humans a few thousand years after mainland Australia, which argues for the hunting hypothesis.[18][19]

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 left). This suggests that in Africa, where humans evolved, prey animals and human hunting ability evolved together, so the animals evolved avoidance techniques. As humans migrated throughout the world and became more and more proficient at hunting, they encountered animals that had evolved without the presence of humans. Lacking the fear of humans that African animals had developed, animals outside of Africa were easy prey for human hunting techniques. It also suggests that this is independent of climate change (see figure at left).

Extinction through human hunting 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[20][21][22] and by computer models by Mosimann and Martin,[23] and Whittington and Dyke,[24] and most recently by Alroy.[25]

Overkill hypothesis

The overkill hypothesis, a variant of the hunting hypothesis, was proposed 40 years ago by Paul S. Martin, Professor of Geosciences Emeritus at the Desert Laboratory of the University of Arizona. It sparked debate which continues today. It explains why the megafaunal extinctions occurred within a relatively short period of time. The most convincing evidence of his theory is that 80% of the North American large mammal species disappeared within 1000 years of the arrival of humans in the Americas.

Arguments regarding the hunting hypothesis

The major objections to the theory are as follows:

Climate change hypothesis

At the end of the 19th and beginning of the 20th centuries, 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.

Critics object that since there were multiple glacial advances and withdrawals in the evolutionary history of many of the megafauna, it is rather implausible that only after the last glacial would there be such extinctions. However, this criticism is rejected by a recent study indicating that terminal Pleistocene megafaunal community composition may have differed markedly from faunas present during earlier interglacials, particularly with respect to the great abundance and geographic extent of Pleistocene Bison at the end of the epoch.[32] This suggests that the survival of megafaunal populations during earlier interglacials is essentially irrelevant to the terminal Pleistocene extinction event, because bison were not present in similar abundance during any of the earlier interglacials.

Some evidence weighs against climate change as a valid hypothesis as applied to Australia. It has been shown that the prevailing climate at the time of extinction (40,000–50,000 BP) was similar to that of today, and that the extinct animals were strongly adapted to an arid climate. The evidence indicates that all of the extinctions took place in the same short time period, which was the time when humans entered the landscape. The main mechanism for extinction was likely fire (started by humans) in a then much less fire-adapted landscape. Isotopic evidence shows sudden changes in the diet of surviving species, which could correspond to the stress they experienced before extinction.[33][34][35]

Some evidence obtained from analysis of the tusks of mastodons from the American Great Lakes region appears inconsistent with the climate change hypothesis. Over a span of several thousand years prior to their extinction in the area, the mastodons show a trend of declining age at maturation. This is the opposite of what one would expect if they were experiencing stresses from deteriorating environmental conditions, but is consistent with a reduction in intraspecific competition that would result from a population being reduced by human hunting.[36]

Increased temperature

The most obvious change associated with the termination of an ice age is the increase in temperature. Between 15,000 BP and 10,000 BP, a 6°C 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-volume ratio, would have fared worse than small mammals.

Arguments against the temperature hypothesis

More recent research has demonstrated that the annual mean temperature of the current interglacial that we have seen for the last 10,000 years is no higher than that of previous interglacials, yet some of the same large mammals survived similar temperature increases. Therefore warmer temperatures alone may not be a sufficient explanation.[37][38][39][40][41][42] However, a recent study has demonstrated that Pleistocene bison were more abundant and more widely distributed in North America at the end of the Pleistocene than at any previous time during the epoch, and in many continental regions was among the most common large herbivores.[32] The documented increase in abundance of bison in midcontinent North America preceded the extinction event, and so cannot have been a result of the extinction.[32] Although it is premature to suggest that the increasing abundance of bison may have driven the extinction, these data nevertheless demonstrate that terminal Pleistocene megafaunal community composition was significantly different during the end-Pleistocene warming phase than that from any earlier warming periods.

In addition, numerous species such as mammoths on Wrangel Island[43] and St. Paul Island survived in human-free refugia despite changes in climate. This would not be expected if climate change were responsible. Under normal ecological assumptions island populations should be more vulnerable to extinction due to climate change because of small populations and an inability to migrate to more favorable climes.

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 outlined below.

Vegetation changes: geographic

It has been shown that vegetation changed from mixed woodland-parkland to separate prairie and woodland.[39][40][42] This may have affected the kinds of food available. If so, herbivores might not have found the plants with which they had evolved and thus would have fallen prey to the anti-herbivory toxins in the plants that remained available. Shorter growing seasons may have caused the extinction of large herbivores and the dwarfing of many others. In this case, as observed, bison and other large ruminants would have fared better than horses, elephants and other monogastrics, because ruminants are able to extract more nutrition from limited quantities of high-fiber food and better able to deal with anti-herbivory toxins.[44][45][46] So, in general, when vegetation becomes more specialized, herbivores with less diet flexibility may be less able to find the mix of vegetation they need to sustain life and reproduce, within a given area.

Rainfall changes: time

Increased continentality resulted in reduced and less predictable rainfall limiting the availability of plants necessary for energy and nutrition.[47][48][49] Axelrod[50] and Slaughter[51] have suggested that this change in rainfall restricted the amount of time favorable for reproduction. This could disproportionately harm large animals, since they have longer, more inflexible mating periods, and so may have produced young at unfavorable seasons (i.e., when sufficient food, water, or shelter was unavailable because of shifts in the growing season). In contrast, small mammals, with their shorter life cycles, shorter reproductive cycles, and shorter gestation periods, could have adjusted to the increased unpredictability of the climate, both as individuals and as species which allowed them to synchronize their reproductive efforts with conditions favorable for offspring survival. If so, smaller mammals would have lost fewer offspring and would have been better able to repeat the reproductive effort when circumstances once more favored offspring survival.[52]

Arguments against the continentality hypotheses

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

The extinction of the megafauna could have caused the disappearance of the mammoth steppe. Alaska now has low nutrient soil unable to support bison, mammoths, and horses. R. Dale Guthrie has claimed this as a cause of the extinction of the megafauna there; however, he may be interpreting it backwards. The loss of large herbivores to break up the permafrost allows the cold soils that are unable to support large herbivores today. Today, in the arctic, where trucks have broken the permafrost grasses and diverse flora and fauna can be supported.[63][64] In addition, Chapin (Chapin 1980) showed that simply adding fertilizer to the soil in Alaska could make grasses grow again like they did in the era of the mammoth steppe. Possibly, the extinction of the megafauna and the corresponding loss of dung is what led to low nutrient levels in modern day soil and therefore is why the landscape can no longer support megafauna.

Arguments against both climate change and overkill

Detractors have asserted that neither the overkill nor climate change hypotheses can explain several observations. Among these are the observation that browsers, mixed feeders and non-ruminant grazer species suffered most, while ruminant grazers generally survive However a broader variation of the overkill hypothesis may predict this, because changes in vegetation wrought by anthropogenic fire preferentially selects against browse species. Because of perceived shortcomings of the overkill or climate change hypotheses alone, some scientists support a combination of climate change and overkill.

Hyperdisease hypothesis

Theory

The Hyperdisease Hypothesis attributes the extinction of large mammals during the late Pleistocene to indirect effects of the newly arrived aboriginal humans.[65][66][67] The Hyperdisease Hypothesis proposes that humans or animals traveling with them (e.g., chickens or 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.[65]

Diseases imported by people have been responsible for extinctions in the recent past; for example, bringing avian malaria to Hawaii has had a major impact on the isolated birds of the island.

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.

One suggestion is that pathogens were transmitted by the expanding humans via the domesticated dogs they brought with them.[68] Unfortunately for such a theory it can not account for several major extinction events, notably Australia and North America. Dogs did not arrive in Australia until approximately 35,000 years after the first humans arrived and approximately 30,000 years after the megafaunal extinction was complete and as such can not be implicated. In contrast numerous species including wolves, mammoths, camelids and horses had emigrated continually between Asia and North America over the past 100,000 years. For the disease hypothesis to be applicable in the case of the Americas it would require that the population remain immunologically naive despite this constant transmission of genetic and pathogenic material.

Arguments against the hyperdisease hypothesis

Second-order predation

Scenario

The Second-Order Predation Hypothesis says that as humans entered the New World they continued their policy of killing predators, which had been successful in the Old World but because they were more efficient and because the fauna, both herbivores and carnivores, were more naive they killed off enough carnivores to upset the ecological balance of the continent, causing overpopulation, environmental exhaustion, and environmental collapse. The hypothesis accounts for changes in animal, plant, and human populations.

The scenario is as follows:

Support

This has been supported by a computer model, the Pleistocene Extinction Model (PEM), which, using the same assumptions and values for all variables (herbivore population, herbivore recruitment rates, food needed per human, herbivore hunting rates, etc.) other than those for hunting of predators. It compares the Overkill hypothesis (predator hunting = 0) with Second-Order Predation (predator hunting varied between 0.01 and 0.05 for different runs). The findings are that Second Order-Predation is more consistent with extinction than is Overkill[70][71] (results graph at left).

The PEM is the only test of multiple hypotheses and is the only model to specifically 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 effect of Climate Change.[72] (results graph at right).

The second-order predation hypothesis is supported by the observation above that there was a massive increase in bison populations.(Scott)[73]

Second-order predation and other theories

Arguments against the second-order predation hypothesis

Arguments against the second-order predation plus climate hypothesis

Comet hypothesis

First publicly presented at the Spring 2007 joint assembly of the American Geophysical Union in Acapulco, Mexico, the comet hypothesis suggests that the mass extinction was caused by a swarm of comets 12,900 years ago. Using photomicrograph analysis, research published in January 2009 has found evidence of nanodiamonds in the soil from six sites across North America including Arizona, Minnesota, Oklahoma, South Carolina and two Canadian sites. Similar research found nanodiamonds in the Greenland ice sheet.[75][76][77]

Arguments against the comet hypothesis

Debate around this hypothesis has included, among other things, the lack of an impact crater, relatively small increased level of iridium in the soil, and the highly improbable nature of such an event.

See also

References

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External links

Hyperdisease hypothesis

Second-order predation

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