Australian Megafaunal Extinctions Essay, Research Paper
The possible causes of the Australian ‘megafauna’ extinctions at the end of the Pleistocene fall into two basic categories, human impact and climatic change. Many models based on one or both of these are used to try to explain the extinction. The timing of the event is important in determining what is influential in the extinctions. This paper will consider the timing of relevant climate and human events and then evaluate five explanations for the Pleistocene extinctions. The explanations are: disease, direct overkill (blitzkrieg), human fire use, climatic catastrophe and the ecophysical model. The paper will show that none of them are totally adequate, that more data needs to be gathered, and that different circumstances apply in different regions.
Timing is a critical issue in the Pleistocene extinctions. The timings of human arrival in Australia, climatic events and the Pleistocene extinctions are all important in determining the causes of the extinctions (Furby, et. al. 1993:204). The date of human arrival in Australia is important in evaluating the influence of humans in the Pleistocene ‘megafauna’ extinctions. The prevailing date is around 40 000 BP, however Hiscock claims a date from artefacts at Malakunanja II in excess of 50 000 BP (Hiscock 1990:122). The arrival of humans in Australia is significant regarding the coexistence of humans and Pleistocene fauna. Coexistence of up to 10 000 years is demonstrated at many sites in South East Australia (O’Connell 1990:56) Human arrival is also relevant to the use and effect of fire on the environment. It is assumed that human fire use patterns are almost unchanged since humans occupied Australia until English colonisation (Clark 1983).
The Pleistocene was a period of fluctuating conditions, from cold and dry to moist and temperate. A glacial period is recognised at 18 000 BP (25 000-15 000 BP), producing low sea levels and extreme aridity in Australia (Hope 1983:52). Information about Australian Pleistocene conditions is restricted to about 40 000 BP because of the limitations of Carbon dating (Hope 1984:70). Other climatic events relevant to the Pleistocene are similar glacial phases occurring before 18 000 BP, producing cool, dry conditions. The frequent natural occurrence of fire during the Pleistocene seems to reduce the impact of human firing on the environment, which would have adapted to it (White, et. al. 1982:93).
Since Australia was occupied by 40 000 BP, well before the climatic extremes of the glaciation at 18 000 BP, the question can be resolved to simply dating the Pleistocene extinctions. If the extinctions occurred at or shortly after the arrival of humans but before the beginning of the glacial period (25 000 BP), then humans probably played some role. However if the extinct mammals coexisted for thousands of years with humans, then died out after 18 000 BP, climate related increased aridity (at the peak of the glacial) could be implicated as a cause (Hope 1984:75).
The timing of the extinctions is the most uncertain, and the dates are as divergent as 6000 BP to before 35 000 BP (Flannery 1990:45). Cuddie Springs has one of the oldest discovered assemblages of megafauna (in association with artefacts) that dates to a minimum of 29 570×280 BP (Furby, et. al. 1993:206). Lancefield Swamp has extinct Pleistocene faunal remains reliably dated to 25 000×800 BP (Flannery 1990:48). Some of the youngest sites so far are Beginners Luck Cave, with bone dated to 9 360×140 BP and Lime Springs with a date suggesting extinct megafauna surviving until 6000 BP (Flannery 1990:48). The dates vary from being close to the advent of human occupation to after the glacial event, and are spread across Australia. The inland and southwestern extinctions occurred well before the height of the glacial, while the southeastern extinctions may have occurred after 18 000 BP and finishing at about 15 000 BP (Hope 1984:79).
The term ‘megafauna’ extinctions is a misnomer. Although the extinction is characterised by the disappearance of large (100-2000 kg) mammals, not all of the mammals that disappeared were large. Some birds (flightless) and reptiles became extinct also (Flannery 1990:46). Extinct megafauna include Diprotodon optatum and Procoptodon rapha (1000-2000 kg). Mammals the size of the extant Macropus greyi (grey kangaroo) that became extinct are Macropus rama, Zaglossus hacketti, and Simostheurus gill. Smaller than the grey kangaroo include Warendja wakefieldi and Zaglossus ramsayi (Flannery 1990:47). Some mammals are not believed to be extinct but reduced in size after the Pleistocene, such as Macropus giganticus being the dwarf form of M. titan (Flannery 1990:46). The nature of the animals that became extinct is important in determining what caused it.
The possible causes of the extinctions can now be approached, having considered the conditions that surrounded it, and what became extinct. The least likely explanations are disease and direct overkill. Disease has not been seriously considered in Australia, and it has not been studied (White, et. al. 182:92). There is no data on the pathological state of the extinct mammals that would suggest widespread disease as a cause of the extinctions. Direct overkill has been considered in the form of Martin’s blitzkrieg hypothesis, after the American model (Hope 1984:75).
Overkill is where the Pleistocene fauna becomes extinct at the same time as humans populate Australia, as a result of direct hunting. This model says that the extinctions happened so rapidly that kill sites will be hard to find (Flannery 1990:50). The dwarfism is explained as a result of selective hunting resulting in a genetic shift in size (Flannery 1990:51). Evidence for the blitzkrieg is that no site yet has revealed extinct Pleistocene mammals with human occupation as early as 35 000 BP. However the amount of sites after 35 000 BP and up to 10 000 BP with human and megafauna remains weakens the model. There is also no published, undisputed account of humans actually hunting and eating megafauna (Rich 1984:997). It has been shown that humans and Pleistocene fauna coexisted for around 10 000 years (Horton 1980:94). The blitzkrieg model is not suitable as an explanation for the Pleistocene extinctions because of the existence of evidence against it.
The blitzkrieg model suggests humans are completely responsible for the extinctions. Another explanation that implicates humans involves the use of fire by the Aboriginals and the way the fire alters the environment. Fires were common during the Pleistocene before human occupation, and the human use of fire would have only altered the pattern (White, et. al. 1990:93). The change in plant communities resulting from Aboriginal firing ‘may have contributed to the extinction of the large Australian Pleistocene fauna’ (Flannery 1990:55). Flannery suggests the opposite; that human firing was a response to the extinctions, and an attempt to assist the survival of medium-sized fauna (Flannery 1990:55). Any effects of Aboriginal fire use would be confined to regional areas because of the small-scale firing they practised (Archer 1984:154). The Aboriginal firing seems to have been a substitute rather than an addition, and its effects somewhat overrated (Horton 1990:60). There is no doubt that humans were and still are directly and indirectly for many animal extinctions, but the Pleistocene extinctions are more complex and controversial (Hope 1984:75).
Climate is the other main explanation for the Pleistocene extinctions. There are two models that use climate to explain the extinctions; the catastrophe model and the ecophysiological model. The catastrophe model suggests that a freak climatic event such as a severe drought caused the animals to die of starvation and thirst (White, et. al. 1982:94). The event occurred during the glacial peak at 18 000 BP, when extreme aridity resulted in large scale drought. The main problem of the catastrophist model, similar to the blitzkrieg model, is that it is demonstrated by a lack of evidence to disprove it. In this case, the pollen record would not show up the droughts because they would be of such short duration (White, et. al. 1982:95). The Pleistocene had extreme glacials periodically, and the animals would be adapted to fluctuating conditions, rather than dying out (Hope 1984:70).
Another flaw in the catastrophist model is that many extinct faunal remains are found in areas when the climate was not dry, such as the south-west or the tropical north. At the Lake Tandou site in western New South Wales, extinctions are believed to have happened before 27 000 BP during the Mungo lacustral period (Hope 1983:52). The Mungo lacustral period is characterised by high lake levels and regionally high watertables. Extinctions did occur away from the critical climatic event- the last glacial- and in non-arid conditions. The catastrophe model is applicable in certain regions, such as the centre of Australia, but not only during the glacial event. The Cuddie Springs site suggests that extinctions occurred in an arid zone but before 30 000 BP (Furby 1993:209). So the catastrophe model has problems, and it is not appropriate for a general explanation, but it may be viable in certain areas at certain times.
The ecophysiological model as proposed by Main offers an explanation for the extinctions based on climatic events, as well as a description of how the mammals became large. Main also suggests how dwarfism occurs. Changes in the environment at the end of the Cainozoic resulted in a reduction in vegetable food quality (Galloway and Kemp 1984:83-93). The mammals responded by increasing in size to process more of the less nutritious food, a side effect of this was an increased need for ‘free’ water (Archer 1984:154). The last glacial event of the Pleistocene was at 18 000 BP and was a period of an all time low precipitation. The shortage of water would have stressed some mammals beyond their physiological capacity and they would have become extinct. Other mammals responded by reducing their size (dwarfism) and therefore their water requirement and generation time (Archer 1984:154).
The ecophysiological model seems the most convincing explanation, but it does not consider previous Pleistocene climatic events that would have stressed the animals. The animals may have adapted to periodic climatic extremes, especially as the glacial periods were much longer than the interglacials (Hope 1984:70). Methodological concerns are raised, as the studies sometimes used very small sample populations (Archer 1984:154). There are also other studies that show megafauna and the equivalent dwarf species both reducing in size, when they are supposed to be the same species in succession, with the dwarf species not changing size (Archer 1984:154). This explanation takes little notice of human contribution to the extinctions. It is possible that the Pleistocene fire regime that was perpetuated by the Aboriginals may have regionally altered plant species that led to ecological pressure on the mammals, similar to the end-Cainozoic conditions. However, during the glacial event the Aboriginals may have stuck closely to the well watered coastal regions, and were probably as stressed as the megafauna (Archer 1984:154).
The ecophysiological model relies on a rare and brief (almost catastrophic) convergence of poor conditions in the climate and environment. Like the other possible causes of the extinctions, the ecophysiological model is too restrictive with its chronological boundaries to be a definite solution. If the extinctions were greater at the glacial event, than this model would seem more plausible, but presently they do not appear to be concentrated at any time. More data is needed to fix the time of the extinctions (Flannery 1990:49), or a multivariate theory to accommodate the variety in the timing of the extinctions.
The ideal explanation for the extinctions would provide the causes of a series of local extinctions, that culminated in the total loss of the megafaunal species (Hope 1984:76). All of the models put forward so far require the extinctions to take place under the same particular climatic and environmental conditions, at about the same time. The archaeological evidence suggests that the extinctions occurred at different times at different conditions. The evidence for direct human predation is scant, as is proof for a climatic catastrophe. Ecological stresses have more credibility as resulting in the extinctions because they can local and general, and over varying periods of time. The timing of climatic and human events is very important in determining the influence of each on the extinctions. The timing of the extinctions is the most crucial question regarding its cause. The variable timing of the extinctions necessitates more data about the extinctions and climate to formulate a satisfactory explanation.