Our research focuses on the population dynamics of plants and how they are influenced by impacts of natural disturbances and global environmental change. We are particularly interested in the interactive effects of fire, grazing and drought in grasslands and woodlands in southern Australia, and how climate change, fragmentation and shrub encroachment affect ecosystems.

Sunday, 6 July 2014

A new population curve for prehistoric Australia

The arrival of Aboriginal people in Australia marked an important change in the continent's ecology. The loss of the mega-fauna and a rise in the use of fire are common themes that resonate when discussing the role of Aboriginal people in shaping the nature of Australia.

Up until now, most of the discussion has been about when aboriginal people arrived. The numbers fluctuate between 40,000 and 100,000 yrs ago, with a convergence of opinion in recent years (because of better carbon dating techniques) that it was probably about 52,000 yrs BP.

An interesting paper by Williams take this insight one step further.

Using radiocarbon dating techniques on >1700 sites from all over Australia, Williams has tried to reconstruct/assess population growth rates in Australia up until the time of European contact.

Using some modelling assumptions about the initial founder effect (that is, how many people first colonised), he interprets three key things:

1) Australia was settled by thousands, not just a handful, of humans, suggesting deliberate rather than accidental colonisation of the continent. The research suggests that it probably would have taken 1000 to 3000 people to reach the numbers of Aboriginal people observed at time of European contact.
2) the population size grew very slowly - constrained by glacial periods. It wasn't until the Holocene (from about 10,000 yrs BP) that the population grew substantially. The study shows during the glacial maximum 18,000 to 21,000 years ago, the population fell dramatically. The data suggests 60% of the population was lost during this time, a period of extreme dry and cold. It took 9000 years for the population to recover to the same levels.
3) the maximum population size before European settlement was approx. 1.2 M people (assuming a founder population of 1000-2000). This occurred only 500 years ago.

Distribution of archaeological sites contributing radiocarbon data to the study by Williams.
You can see there is a pretty good coverage across Australia.

You can see this population reconstruction clearly here in this figure from the paper. The effects of 'founder size' (how many people might have first colonised Australia) has a big effect on population size at the time of European contact, as does climate. The two graphs show the outcomes based on different model assumptions - they give pretty much the same results. They suggest that population growth really took off at about 12,000 yrs BP; this corresponds with increasing climate stability

This paper (along with others - see here) hints that the big changes in vegetation and mega-fauna extinctions observed in the last 50,000 yrs were probably driven by climate. Landscape burning by Aboriginal people has been linked to significant changes in the geographical range and demographic structure of many vegetation types but there is now an emerging lack of congruence between human activity and fire records during the period 20-40 kya. During a period of consistently low human populations, as posited by Williams, it is difficult to reconcile how Aboriginal people could have had the profound impacts that have been speculated by many.

I can't vouch for the veracity of the work reported by Williams, but it provides interesting food for thought when interpreting the recent history of Australia.

Williams (2013) A new population curve for prehistoric Australia. Proceedings of the Royal Society B 280, 20130486.

Sakaguchi et al. (2013) Climate, not Aboriginal landscape burning, controlled the historical demography and distribution of fire-sensitive conifer populations across Australia. Proceedings of the Royal Society B 280: 20132182

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