Megamarsupial Extinction: The Carrying Capacity Argument
Webb, R. Esmee, Antiquity
An ancient and contentious debate in prehistory (Owen 1846; Lyell 1863) asks if the megafauna of newly colonized worlds was exterminated by human hunting, or whether other factors, such as changing climate, were decisive. Debate on this issue remains lively in Australia, because its harsh environments surely posed problems for very large mammals. A starting-point for this fresh look at megafaunal extinction was Flannery's (1994) adoption of the 'blitzkrieg' hypothesis, in which humanly caused kill-off was so rapid it left no decisive archaeological traces.
Prior to human arrival in Australia, Flannery (1994) argued that the endemic herbivores had developed a pattern of mosaic grazing that maintained patches of vegetation at different stages of growth. Thus, the herbivores avoided over-exploiting their fragile environment, while nutrients were rapidly recycled into the ecosystem through their dung. Patch exploitation permitted these animals to survive, despite the unpredictable Australian climate and poor food resources available, until people arrived and upset the delicate ecological balance maintained by the megaherbivores, who then disappeared. Flannery borrowed Martin's (1984) blitzkrieg argument to explain the absence of evidence for megafaunal kill sites in the Australian archaeological record: extermination was so rapid it left no identifiable traces. With the disappearance of the megaherbivores, the vegetational mosaics they had maintained were replaced by more homogenous plant communities, so people burnt the country to maintain its patchiness. This strategy recycled nutrients less efficiently than had the endemic fauna, because they were initially locked up in leaf litter and then lost in combustion. Flannery's argument implies that burning effectively reduced the carrying capacity of the Australian ecosystem as a whole; whereas archaeologists, particularly Hallam (1975) and Horton (1982), have generally assumed that firing increased carrying capacity by preventing major fires, maintaining patchiness and aiding the growth of new feed for herbivores - which people were then able to trap or hunt!
Megamarsupial extinction: the evidence
Any hypothesis explaining megamarsupial extinction is difficult to test; the evidence lacks precise dates. It is undoubtedly true that the bones of many species of now-extinct large marsupials have been found in Australian contexts thought to be of Quaternary age (Murray 1991); dating these fossils is another matter. Not only have few localities been dated radiometrically, but the ages obtained do not always relate to the bones recovered. It has long been recognized that the type of sample assayed and the quality of the association between it and the event being dated affect the ability of radiometric techniques, particularly radiocarbon, to date the past reliably (Waterbolk 1971). Meltzer & Mead (1985) critically evaluated the reliability of 14C assays relevant to the extinction of the North American megafauna. Baynes (1995) subsequently applied Mead & Meltzer's criteria to the 14C dates from Australian megafaunal sites. He concluded that there is no evidence that the megafauna survived long after human arrival (despite claims to the contrary, e.g. Gorecki et al. 1984; Wright 1986), because there is no association between the fossils and the samples dated at the 'young' localities. Moreover, when these unreliable dates are excluded, those remaining suggest that extinction occurred beyond the range of 14C dating. They date to [Greater than]35,000 b.p., when the 14C decay curve becomes asymptotic (Chappell et al. 1996), yielding dates that do not accurately reflect the true ages of the samples assayed. It is premature, Baynes (1995) concluded, to discuss why or how the Australian megafauna became extinct because the time of that extinction is unknown. While true, until the current programme to date Australian megafaunal sites yields precise results, that limitation need not inhibit speculation. …