Lynelle Munns
06056490
Over the planet earths lifetime it has seen a great many species come and go. Mass extinctions have occurred a number of times over this vast history and the megafauna extinctions of the late Pleistocene is one of them (Palmer, 2003). As with all of the mass extinctions there is a hotly contested debate over what caused these species to disappear forever. For more than a hundred years thought on the subject has generally fallen into the two categories of climate induced extinction, which focuses on the shift from a glacial to an interglacial and the associated global warming, and human-induced extinction, in which the evolution of humans and their activities either directly or indirectly caused the mass extinction. In recent years however a mixed model has emerged which appears to provide a much more comprehensive explanation of the extinction event by taking ideas from both schools of thought. The quaternary is the name givento the period of time beginning around 2 million years ago until the present day. This period is usually broken up into two epochs the Pleistocene (2 million years ago-10,000 years ago)and the Holocene (10,000 years ago-present day). The Pleistocene is characterized by a repeated cycle of glacial and interglacial periods. In the last of these glacial cycles a large number of megafauna, such as the wooly mammoth, the saber-toothed cat and the giant sloth, became extinct. These extinctions were not seen in the previous glacial cycles, so why did they suddenly occur in this one? (Anderson, Goudie & Parker 2007). Climatic change has been proposed as a major cause for the mega faunal extinctions. This theory focuses on the changes in the climate and habitat of the fauna. From around 15,000-13,000 years ago the earth came out of a glacial
Lynelle Munns
06056490
period and into the current interglacial. This change from a cold global climate to a warm global climate caused changes in habitats for many of the now extinct species. The flora of many regions would have been the first to change, as temperatures increased some forests gave way to grass lands, tundra s thawed and became forests, savannas became deserts. These changes would have heavily effected herbivores in the areas as the plants they had adapted to feeding on shifted or disappeared. The herbivores began to react to the change, some by adapting like the ruminant bison and sheep, while others who were unable to adapt, including the giant sloths and the mammoths, became extinct. The alterations to the dispersal and numbers of herbivores then in turn effected the carnivores causing further extinctions, in species such as the saber toothed cat and the short faced bear, as their prey animals moved to new territory or gradually disappeared(Huggett, 2004). This theory explains many of the extinction events that occurred but it is far from perfect opponents have pointed out locations where extinctions did not occur during the warming of the earth but did occur after humans settled. A prime example of this is New Zealand, the megafauna, such as the moa, survived the climatic shift of the late Pleistocene only to become extinct when humans arrived around 1000 years ago (Palmer, 2003). A similar story is emerging in the WestIndies where a species of sloth became extinct after human colonization. It has also been pointed out that there were many glacial-interglacial cycles throughout the Pleistocene why didn t any of the previous changes in climate cause the same or similar extinction rates when the changes in the earths average temperatures was similar (Koch & Barnosky, 2006).
Lynelle Munns
06056490
So if there was no difference in the average global temperatures, what made this interglacial so different to the others, wasit the evolution of man. The biggest difference between every other Pleistocene interglacial thaw and the last thaw is the presence of humans. Homo sapiens began to migrate around the world, slowly colonizing every continent. Although an exact time scale is still contested it appears that humans were present in all of the major continents (the Americas, Europe, Asia, Africa, and Australia) by around 12,000 years ago (Lewin, 2005). This colonization coincides with the timing of the mass extinctions, leading many to cite humans as a direct cause of the disappearances. By the time modern humans left Africa their ancestors had been using tools for close to four million years, they had advanced language skills and complex social structure, they were what humans are today and so they were amazing hunters (Duiker & Spielvogel 2011). A portion of the human -induced theories focuses on the idea that humans simply hinted all of these species to extinction. Some say that the megafauna where not aware of humans as a risk and so didn t flee from hunters, thus making them easy kills others say humans were just too good at killing and just over killed. The numbers of species that went extinct supports these theories. Africa, where the megafauna had co-evolved with humans, had a far lower rate of extinction than other continents such as South America, which lost 80% of its megafauna species (Huggett, 2004). Another group within the human-induced school of thought looks at humans as an indirect cause for the extinctions. A mass disease outbreak, spread by early humans, similar to the rats spreading the black plague in Europe, has been suggested as well as the introduction of pest species such as rats to previously pristine environments.
Lynelle Munns
06056490
The indirect theories have many holes in them; if a disease is to blame how did it kill so many different species over such a short period of time? The disease would have to evolve rapidly in order to infect that many species and it is difficult to explain how the disease also got down to geographically isolated locations such as Australia where species also became extinct. The human overkill works in many situations, including those that the climate change model doesn t explain such as the New Zealand moa, but there is limited evidence for such a high kill rate (Price et al., 2011). Many sites the lack of kill zones , areas near human settlement or campsites that have large numbers of bones which show evidence of butchering, but supporters of the hunted to extinction models propose that the killing took place over such a s hort amount of time that fossilization did not have time to occur. The lack of kill sites does not prove that humans did not hunt the animals (Elewa, 2008). If both human induced and climate based models do not explain the megafaunal extinctions, what does? The most widely accepted model is one of a mixed mode or a synthesis model. In these combined theories it looks at the effects the rapidly changing climate had on the species as well as human effects as a far more likely cause for the mega faunal extinctions. It is clear that both climatic and human induced factors fit the correct chronological scale but neither is adequate enough to fully explain the extinctions. By taking points from each school of thought to major theories have come about, the keystone species theory and a true mixed model. The keystone species concept follows the theory that a keystone species was wiped out by humans. A keystone species, like its namesake, keeps the ecosystem stable. When
Lynelle Munns
06056490
a keystone species is eliminated from an environment it disrupts the whole food web. A simple example ecosystem would be one with only one herbivore that also acts as a pollinator, if that herbivore was to go extinct the plant species would no longer be pollinated and the carnivores further up the food chain would have no prey species spelling the end for both the carnivores and the plant and therefore the ecosystem. This could also be caused by a climatic shift wiping out main food crops for the herbivores. The main problem with this model is lack of evidence. There is no evidence to show that there was one or two plant or animal species that became extinct first, causing the resultant chain of extinctions, but there is also no evidence against it. The second mix model hypothesis follows a combination of climatic change, which altered the distribution and caused fragmentation of habitat that on its own would strain populations but not cause them to go entirely extinct, and the human activity which was new to the continents. This combination of events may have been enough to push so many species over the edge. What s nice about this model is that it has the least disputed parts of each of the two hypotheses and combines them to make a hypothesis that is hard to find issues with. This model explains why so many species went extinct in so many different places, it takes into account the species that survived in Africa, because they where the predation by humans was not novel and to did not cause any extra pressure on already strained species, in the isolated Australia, which was subject to the same environmental pressures, and the lack of huge kill sites, as the numbers hunted weren t as huge as would be needed for human hunting alone (Koch & Barnosky, 2006).
Lynelle Munns
06056490
In conclusion explanations for the large-scale extinction of the megafauna in the late Pleistocene have come from two main schools in the past, the human induced and the climate induced. These theories both had various flaws ranging from anomalous locations, like the New Zealand moa to lack of evidence. However when the two schools of thought are combined to create a mixed model hypothesis where the new predation by humans placed an extra strain on species that were already struggling with a change in climate, there is very little evidence to disprove what I believe to be the most comprehensive theory proposed. References: Anderson, D. E., Goudi e, A., Parker, A. (2007) Global Environments Through The Quaternary: Exploring Environmental Change . New York: Oxford University Press pg. 265 Duiker, W. J., Spielvogel, J. J . (2011) The essential World History.Boston: Wadsworth, Cengage Learning. pg3 Elewa, A.M.T. (2008) Mass Extinction. Berlin: Springer. pgs 160-166 Huggett, R.J.(2004)Historical Biogeography. Fundamentals of Biogeography.Oxfordshire: Routledge. pgs.322-328 Koch, P. L., Barnosky, A. D.(2006) Late Quaternary Extinctions: State of the Debate. Annual Review of Ecology, Evolution, and Systematics. 37, pgs. 215-250. Lewin, R. (2005) Part 9: New Worlds. Human Evolution: an Illustrated Introduction.Victoria: Blackwell. pgs.237-241
Lynelle Munns
06056490
Palmer, D.(2003)Prehistoric Past Revealed: the Four Billion Year History of Life on Earth. Los Angeles: University of California Press. pgs. 41 Price, G. J., Webb, G. E., Zhao, J., Feng, Y., Murray, A. S., Cooke, B. N., Hocknull, S. A., Sobbe, I. H.,(2011)Dating Megafaunal extinction on the Pleistocene Darling Downs, eastern Australia: the promise and pitfalls of dating as a test of extinction hypotheses. Quaternary Science Reviews. 30, pgs 899-914
