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

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.