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Australia: The Land Where Time Began |
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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 3rd
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. |
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |