Did central Australian megafaunal extinction coincide with abrupt ecosystem collapse or gradual climate change?
Murphy, BP and Williamson, GJ and Bowman, DMJS, Did central Australian megafaunal extinction coincide with abrupt ecosystem collapse or gradual climate change?, Global Ecology and Biogeography, 21, (2) pp. 142-151. ISSN 1466-822X (2012) [Refereed Article]
Aim In central Australia, the giant flightless bird Genyornis newtoni disappeared
about 45–50 thousand years ago (ka). It has been reported that coincident with this
extinction the carbon isotopic composition of preserved eggshells of the extant
emu (Dromaius novaehollandiae) shows an abrupt dietary shift from tropical
grasses (C4 photosynthesis) to temperate grasses and/or woody browse (C3 photosynthesis).
This abrupt shift has been interpreted as signalling ‘ecosystem collapse’
due to landscape burning by humans. We evaluate an alternative interpretation,
that the shift in diet was not abrupt, but gradual, and caused by the weakening of
the Australian monsoon.
Location Lake Eyre, central Australia.
Methods We re-analysed a large, published dataset of emu diet d13C (inferred
from d13C of preserved eggshells) spanning the last 140,000 years, using time-series
analysis. Using Akaike’s information criterion, we compared two contrasting
models: (1) there was an abrupt shift in d13C coincident with the extinction of
Genyornis, assumed 47.5 ka; and (2) there was a gradual shift in d13C, correlated
with reconstructed water level in Lake Eyre, a proxy for monsoon intensity.
Results There was little evidence of an abrupt shift in emu diet d13C about
45–50 ka, but d13C appeared to steadily decrease between about 80 and 30 ka.
Indeed, the model representing a correlation between d13C and lake level was more
than seven times more likely than the model representing an abrupt shift at 47.5 ka.
Main conclusions The emu eggshell isotopic record from Lake Eyre does not
support the hypothesis that landscape burning by humans transformed a
savanna-grassland mosaic into the modern desert scrub, contributing to the
extinction of Genyornis.While our findings cast strong doubt on the foremost line
of evidence that landscape burning by humans caused the megafaunal extinctions,
and suggest that central Australia was becoming increasingly arid in the Late
Pleistocene, the relative roles of hunting by humans and climate change in the
megafaunal extinctions remain unresolved.