Did central Australian megafaunal extinction coincide with abrupt ecosystem collapse or gradual climate change?

Aim: In central Australia, thegiant flightless bird Genyornis newtoni disappeared about 45–50thousand years ago (ka). It has been reported that coincident with thisextinction the carbon isotopic composition of preserved eggshells of the extantemu (Dromaius novaehollandiae) shows an abrupt dietary shift from tropicalgrasses (C4 photosynthesis) to temperate grasses and/or woody browse (C3 photosynthesis).This abrupt shift has been interpreted as signalling ‘ecosystem collapse’ dueto landscape burning by humans. We evaluate an alternative interpretation, thatthe shift in diet was not abrupt, but gradual, and caused by the weakening ofthe Australian monsoon.

 

Location: Lake Eyre, centralAustralia.

 

Methods: We re-analysed a large,published dataset of emu diet δ13C (inferred from δ13C of preserved eggshells)spanning the last 140,000 years, using time-series analysis. Using Akaike'sinformation criterion, we compared two contrasting models: (1) there was anabrupt shift in δ13C coincident with the extinction of Genyornis, assumed47.5 ka; and (2) there was a gradual shift in δ13C, correlated withreconstructed water level in Lake Eyre, a proxy for monsoon intensity.

 

Results: There was little evidenceof an abrupt shift in emu diet δ13C about 45–50 ka, but δ13C appeared tosteadily decrease between about 80 and 30 ka. Indeed, the modelrepresenting a correlation between δ13C and lake level was more than seventimes more likely than the model representing an abrupt shift at 47.5 ka.

 

Main conclusions: The emu eggshellisotopic record from Lake Eyre does not support the hypothesis that landscapeburning by humans transformed a savanna−grassland mosaic into the modern desertscrub, contributing to the extinction of Genyornis. While our findingscast strong doubt on the foremost line of evidence that landscape burning byhumans caused the megafaunal extinctions, and suggest that central Australiawas becoming increasingly arid in the Late Pleistocene, the relative roles ofhunting by humans and climate change in the megafaunal extinctions remainunresolved.