Term Paper on "Earth History"

Term Paper 5 pages (1656 words) Sources: 0 Style: Harvard

[EXCERPT] . . . .

Large Mammal Extinction

Ice Age Large Mammal Extinction

The end of the last ice age was marked by the extinction of hundreds of species of large mammals. Whether this extinction occurred in the late Pleistocene or the early Holocene era is a subject of debate, but it seems clear that these mass extinctions happened at approximately the same time as the end of the last ice age. In North America alone, at least 35 different mammals disappeared. These mammals included animals that are now extinct worldwide, including: mammoths, mastodons, and giant ground sloths; relatives of animals that still live in the United States, such as bears; and animals that continued to survive and thrive in other places in the world, such as camels and horses. Scientists have developed several theories as to why so many large mammals went extinct. These reasons include: climate change, changing weather and vegetation patterns, overkill, disease, a meteor, or for about the last 30 years, the most accepted theory of mass extinction was overkill as the result of human hunting. There is, of course, evidence demonstrating that humans hunted some of the now-extinct large mammals. Furthermore, in modern times humans have caused the extinction or near-extinction of animal species, especially in island-settings. However, the fact that humans did hunt large mammals does not mean that human hunting caused their extinction; one must determine whether there are other factors suggesting that humans played a role in the extinction of the large mammals. One such factor is that mass extinctions followed shortly after humans migrated into the effected area. In fact, Africa was the continent least-impacted by the mass extinction 10,000-12,000 years ago, which makes sense in the overkill theory, because humans were leaving Africa and migrating to other areas, and the prey evolved at the same time as humans in Africa, so they would have evolved avoidance techniques, as well. If humans hunted large herbivores to extinction, it would have had the secondary result of killing large predators, because they would be deprived of their food sources. In North America, the theory seems to have merit; the vast majority of North American large mammal extinctions occurred within 1,000 years of man's arrival in North America. Some opponents of the overkill theory suggest that it is impossible for a predator (including humans) to hunt prey to extinction, but that does not take into account the fact that humans had several prey species and could switch prey after exhausting one food source. Others suggest that there is only evidence that a few species of mega fauna were ever hunted; however, new discoveries demonstrate that humans hunted horses in North America in addition to animals that were typically considered prey. Finally, some point to the fact that not all mega fauna became extinct to suggest that human overkill could not have been responsible for the mass extinctions, but those surviving animals may have been better at evasion or been capable of more rapid reproduction than animals who became extinct. Even the dwarfing of animals is explained by the overkill theory, because humans may have systemically harvested the largest of the prey species. However, one of the criticisms of the overkill theory has tremendous merit; large animal extinction happened at roughly the same time in Eurasia as North America, despite the fact that humans had been present in Eurasia much longer than humans. Finally, there is only clear evidence to establish that humans caused the extinction of one species of large mammal: the giant lemur in Madagascar.

The second-order predation theory was closely linked to the overkill theory. In this theory, human migration into the New World and human hunting are linked to extinction. Humans who entered the new world did not only kill prey animals, but also predators, which resulted in overpopulation of the prey species. In addition, because of competition with humans, pre-existing predators no longer have a sufficient food supply, resulting in a dwindling natural predator population. The resulting prey species overpopulation led to the exhaustion of environmental resources, like the food supply. Eventually, the environment can no longer support the prey species, resulting in death by starvation. In addition, the overgrazing by uncontrolled prey populations results in changes in the environment and climate, which further impacts the survival of animal species, leading to the eventual extinction of large herbivores. Most importantly, the second-order predation hypothesis would contribute to the damage caused by other hypotheses. Despite the strength of the second-order predation hypothesis, scientists have yet to discover evidence of humans hunting predators in America. Furthermore, the vegetation changes proposed by the second-order predation hypothesis should have caused substantial extinctions among small vertebrates, but these extinctions did not occur. Furthermore, the second-order predation hypothesis would suggest higher rates of extinction in grasslands, but large-mammal extinctions occurred in all types of vegetation. In addition, several large carnivore species survived, and it is likely that they would have been able to control animal populations sufficiently to prevent the overpopulation aspect of the hypothesis.

Some scientists believe that climate change was the cause of the large animal extinctions. As the last ice age ended, the world experienced a fairly dramatic increase in temperature. The theory is that large mammals that had evolved to live in extreme cold weather were unable to adapt to warming temperatures, and that large mammals were impacted more than small mammals. However, the change in temperature, alone, probably would not have been sufficient to cause extinction. First, the large mammals had survived other interglacial periods, which had similar average temperatures to the current interglacial period. Furthermore, many of the mega fauna species survived on islands, though not on mainlands, which is the opposite of what one would anticipate if the change in temperatures was directly responsible for the extinction.

However, the change in temperature could have led to extinction in other ways. For example, the change in climate could have resulted in changes in vegetation. Vegetation did undergo a change at around the same time as the mass extinctions, transforming from mixed woodland-parkland to separate prairies and woodland. The change may have impacted what food was available, depriving animals of their traditional food sources. This theory is supported by the fact that many of the large herbivores survived in dwarfed forms. It is also supported by the fact that ruminants faced less extinction than monogastrics, because they had more dietary flexibility. In addition, climate changes could have led to changes in rainfall patterns, which would have affected the availability of food, water, and shelter. This would have negatively impacted large mammals more than small mammals, because the longer gestation cycles and more inflexible mating periods, may have resulted in young being born at unfavorable times, with no opportunity to replace the young who died at those times. However, prior periods of continental climate have not been marked with the same type of mass extinctions that marked the end of the Pleistocene. In addition, the animals that went extinct were largely grass-eaters like horses and mammoths, and grass became more widely available. Finally, when faced with adverse weather conditions, like drought, modern large herbivores migrate to more favorable conditions.

Another theory is that the large mammals were killed by some type of disease. Like the overkill theory, the disease theory is linked to human infiltration of new areas. It supposes that humans or the animals traveling with them carried some type of disease with them, which was capable of causing sufficient deaths to bring about extinction. Humans are suggested as the vector for such disease, because prior animal immigrations did not result in extinctions. Furthermore, large mammals would be disproportionately impacted because of their longer life span, longer gestation period, and smaller populations. This theory seems meritorious because of what is known about the impact of diseases… READ MORE

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