The Uncertain Blitzkrieg Down Under

By the late Quartenary, Australia had lost 23 out of 24 genera of its megafauna. The 2 main explanations for this are human mediation – overkill and habitat destruction – and climate change. In this blog post I will look at each in turn, and conclude that humans are not the primary cause of megafauna extinction in Australia. While human ‘blitzkrieg’ was previously the consensus, new research has shown increasing evidence that it is likely climate change played a larger role in a majority of extinctions. 

Human Overkill and Habitat Destruction

The main argument for proponents of the blitzkrieg hypothesis is that most megafauna were present when humans arrived in Australia, and they were subsequently wiped out by interaction with humans. They point to the evidence that extinction of megafauna occurred around 45,000 calendar years ago, coinciding with human colonization of Australia and predating climate change (Roberts et al 2001). Miller et al (2005) also show in their research that records have shown a decline in food sources for the Australian emu and marsupial wombat, attributed to human modification of the drought- adapted environment of shrubs and grasses into the fire-adapted scrublands of today.

However, Trueman et al (2005) argue that fossil evidence from Cuddie Springs (Southeastern Australia) and other sites refute the consensus that megafauna became extinct about 45,000 calendar years ago. Their research has found that humans and megafauna co-existed for about 15,000 calendar years after the arrival of humans. Besides, Wroe et al (2004) points to the complete absence of any direct evidence implicating human mediation, e.g. kill sites, similar to those that have been found in North America.

Climate Change

Wroe and Field (2006) point to evidence from a variety of climate proxies (pollen, charcoal, hydrology, etc.) that there was a broad trend towards increasing aridification of the Australian climate in the Late Quarternary, which overlaid glacial cycles. Thus, the Holocene interglacial was drier than previous interglacials, disputing the idea that the Pleistocene-Holocene transition was similar to previous transitions. The chart below shows that lake levels and river flow in Northern and Southeastern Australia started declining from around 50,000 calendar years ago, while dust levels increased from around 200,000 calendar years ago. Other evidence from pollen records show that around 200,000 years ago, grasses became more prevalent relative to eucalyptus, indicating increased aridity. 

Fig. 1: Wroe and Field (2006)

Thus, non human-mediated climate change can be shown to have caused the extinction, as these natural processes were in motion long before humans arrived. However, it is certainly possible that humans played a role in further stressing megafauna already stressed by these climatic changes, although they cannot be seen to be a primary cause of the extinctions. 

An Alternative Model: Staggered Extinctions of Australian Pleistocene Megafauna

Wroe and Field (2006) suggest an alternative model of staggered extinctions. Their more recent research shows that at least 65% of the megafauna cannot be shown to have existed beyond 130,000 calendar years ago. Only 13% of megafauna species during the Pleistocene co-existed with humans, and at least half of these species survived 15,000 calendar years after humans arrived. Thus, they argue for an alternative model of repeated range contractions and limitation of refugia for megafauna as Australia’s climate became increasingly arid, resulting in extinctions which predated human contact.

Their model is supported by Cosgrove and Allen’s (2001) study of Tasmanian rockshelters. Since early humans could only have reached Tasmania by 37,000 calendar years ago through the development of a land bridge, if humans caused extinction then megafauna should be shown to survive there until the arrival of humans. However, no megafauna fossil remains younger than 46,000 calendar years ago could be found. 

References

Cosgrove, R. and Allen, J. (2001) ‘Prey choice and hunting strategies in the Late Pleistocene: evidence from Southwest Tasmania’ in Lilley, A. and  O’Conner, S. (Eds.), Histories of Old Ages: Essays in Honour of Rhys Jones. Canberra: Research School of Pacific and Asian Studies, Australian National University, Canberra, pp. 397–430.

Miller et al (2005) ‘Ecosystem collapse in Pleistocene Australia and a human role in megafaunal extinction’, Science, 309, 5732, pp. 287-290

Roberts, R.G. et al (2001) New ages for the last Australian megafauna continent-wide extinction about 46,000 years ago. Science, 292, 1888–1892.

Trueman et al (2005) ‘Prolonged coexistence of humans and megafauna in Pleistocene Australia’, Proceedings of the National Academy of Sciences of the United States of America, 102, 23, pp. 8381-8385

Wroe, S. et al (2004) ‘Megafaunal extinction in the Late Quaternary and the global overkill hypothesis’, Alcheringa, 28, pp. 291–331.

Wroe, S. and Field, J. (2006) ‘A review of the evidence for a human role in the extinction of Australian megafauna and an alternative interpretation’, Quaternary Science  Reviews, 25,21-22, pp. 2692-2703.