Australia: The Land Where Time Began

A biography of the Australian continent 

Megafauna Extinction in Late Quaternary of Australia Not the result of Climate

Megafauna’ extinctions in the late Quaternary impoverished the diversity of mammals around the world. In Australia the cause of these extinctions are most controversial though essential to resolve, as this continent-wide event presaged similar losses that occurred thousands of years later on the other continents. In this study Saltré et al. applied rigorous metadata analysis and a new ensemble-hindcasting approach to 659 megafauna fossil ages. They showed that extinction of megafaunal extinctions were broadly similar among genera and independent of aridity of climate and variability of climate in Australia over the last 120,000 years. Climate change as the primary driver of megafauna extinctions in the most controversial context is rejected by their results, estimating instead that the megafauna disappeared Australia-wide ~13,500 years after the arrival of humans, and in some regions shorter periods of coexistence. This is the first comprehensive approach that incorporated uncertainty in fossil ages, timing of extinction and climatology, in order to quantify mechanisms of prehistoric extinctions.

The late Quaternary was a time of rapid and widespread extinction of ~65% of ‘megafauna’ genera (i,e,, large vertebrates with mature individuals >40 kg) (Koch & Barnosky, 2006). The role of climate in the collapse in the Quaternary, is still being hotly debated (Koch & Barnosky, 2006; Stuart, 2015; Sandom et al., 2014; Cooper et al., 2025), because of these extinctions coinciding with colonisation by humans (Martin & Steadman, 1999), though some of the largest mass extinctions earlier in the history of the Earth were probably caused by shifts in climate. The initial arrival of humans in Australia and New Guinea (together with Tasmania, collectively ‘Sahul’), that were connected by a land bridge at the time, and the megafauna occurred close to, or beyond, the limit of radiocarbon (Prideaux et al., 2007) dating (55-45 kyr), (kyr = 10³ years), which led to uncertainty as to whether humans had a specific and important role in driving continent-wide megafauna extinctions in Sahul (Koch & Barnosky, 2006). According to the ‘climate-driven’ extinction hypothesis changes in climate variability coupled with severe aridification from about 430 kyr was the cause of the extinctions (Roe et al., 2013), in spite of evidence that during the period of most disappearances (Brook et al., 2007; Rule et al., 2012), the climate was relatively stable. According to the alternative ‘human-driven’ hypothesis hunting was the major cause of extinctions (Brook et al., 2007; Miller et al., 2005) and/or the natural regimes were modified by humans sufficiently to alter vegetation communities and disrupt trophic dynamics (Miller et al., 2005). According to a 3^rd hypothesis that emphasises a possibly human-climate synergy, according to which populations that were already compromised by environmental change that was climate-driven, the coup-de-grace was delivered by the impact of humans.

The construction of an accurate chronology of species losses relative to major climatic or archaeological transitions has been made difficult by the limits of ¹⁴C dating. The primary data that describe the timing of the extinction of megafauna species are from estimated ages of fossilised remains (Robert et al., 2001; Miller et al., 1999; Prideaux et al., 2010; Prideaux et al., 2007; Johnson, 2006), though the time of final extinction of any population that has been long-disappeared inevitably diverges from the youngest records as a result of a bias that is introduced by incomplete sampling or taphonomy (the Signor-Lipps effect) (Signor & Lipps, 1982). There are many inferential methods that have been developed to estimate the time gap between the last dated fossil and the date of the final extinction, but their efficiency varies with the type of extinction and density of sampling over time (Saltré et al., 2015). According to Saltré the Australian fossil data set has traditionally been questionable (i.e., sparse and of variable quality) (Prideaux et al., 2007; Brook et al., 2013; Grūn et al., 2010), as well as these taphonomic and statistical issues, prior to the recent development of a general tool for assessing the reliability of age estimates for fossils (Rodríguez-Rey et al., 2015).

In this study Saltré et al. used the largest, most diverse data set in existence for Sahul, that was rigorously quality controlled, in order to investigate the role of climatic variation in the extinction of megafauna in Sahul as well as to investigate the window of coexistence of megafauna and humans, which indicates the likelihood of an alternative that was human-driven    (i.e., they hypothesise that the longer the overlap, the more time humans would have had to alter the environment and affect the megafauna). They achieved this by:

i)                   Developing a robust chronological framework that reduces timing uncertainties in megafauna extinctions and the first occurrence of humans, and

ii)                Testing the chronology of the extinctions of megafauna (i.e., the number of genera that went extinct over time) against 6 high-resolution climate records, and hindcasts.

They showed that:

i)                   The chronology of megafauna extinctions over the last 120 kyr is not correlated with variability of climate changes or events that increase aridity at a continental scale, and

ii)                The length of time of human-megafauna coexistence lasted ~13,5 kyr across the continent as a whole, though with shorter periods of overlap probably in an particular region.

It is implied by this data that human pressure, and not climate change, was most likely to be the main driver of these extinctions.  

Discussion

The timing of megafauna extinction and first human arrival in Sahul have both long been controversial given the lack of reliable data and bias that is introduced by Signor-Lipps effect.

Once these deficiencies were accounted for, the results of Saltré et al. support the general conclusion of previous studies (Roberts et al., 2001; Clarkson et al., 2015), though they are much better constrained by more diverse and expanded data sets that have been scrutinised rigorously by use of the best available analytical tools.

It is generally accepted that humans were present in Australia by 48 kyr (Roberts et al., 2001; Allen & O’Connell, 2014; Bird et al., 2013). He modelled age estimates of Saltré et al. of 55.6 kyr reflects the early colonisation by humans of northern Australia (Clarkson et al., 2015; Roberts et al., 1994), though a reliable interpretation of the pattern of human dispersal across Sahul is prevented by geographic gaps in high quality data. However, it is clear that humans were present in Tasmania by 39 kyr (Allen & O’Connell, 2014) and in the arid centra of Australia by 35 kyr (Smith, 2013).

According to Saltré et al. the 13.5 kyr window of coexistence that has been estimated is long enough for groups of hunter-gatherers to disperse and become established across Sahul (Johnson & Brook, 2011) and to extirpate many species of megafauna without a wave of rapid, continent-wide overkill. On more local/regional scales these results do not dismiss the possibility of rapid human overkill, nor that the longer the period of coexistence, the more time megafauna would have had to adapt to human presence and become more immune to their impacts. There is, however, a lack of direct evidence for human-megafauna interactions, possibly resulting from taphonomic loss and sampling bias (Surovell & Grund, 2012). It has been demonstrated, nevertheless, that even small groups of hunter gatherers that live across a vast continent and using technologies that are stone based could feasibly exterminate species that had low rates of population growth, such as large-bodied mammals (Brook & Johnson, 2006). Saltré et al. suggest that this conclusion could not be supported by results that are based only on a direct reading of the fossil record, as it would be biased by the uncorrected Signor-Lipps effect (Signor & Lipps, 1982). The duration of human-megafauna coexistence can be estimated with much higher statistical confidence, by including only reliably dated megafauna and archaeological records in the models of Saltré et al., and correcting explicitly for the Signor Lipps effect. Discrimination between various extinction mechanisms (Roberts et al., 2001) that have been proposed for the megafauna of the late Quaternary in Sahul has revealed that climate change was not a continental driver of extinction of these genera. The same approach could similarly provide insights into the cause of late Quaternary megafauna extinction events in other regions of the world.

Saltré, F., et al. (2016). "Climate change not to blame for late Quaternary megafauna extinctions in Australia." Nature Communications 7(1): 10511.