FAUNAL EXTINCTION AND DRAMATIC ECOSYSTEM CHANGE IN THE LATE QUATERNARY OF SEMI-ARID AUSTRALIA

Extinction of the Australian megafauna is now dated with reasonable certainty at about 50±5 ka, shortly after the arrival of the first human colonizers about 55±5 ka. A link between the two has been suggested, including the possibility that landscape modification was influential, but rigorously evaluating the role of humans remains elusive. To evaluate changes at the ecosystem level we utilize the environmental information recorded by the ratios of light stable isotopes preserved in the carbonate and organic carbon within eggshells of two large flightless birds: Dromaius (the extant emu) and Genyornis (an extinct element of the megafauna). Emus are opportunistic omnivores, but are dominantly herbivorous. Their diet provides a realistic reflection of the range of suitable food sources available in the weeks before nesting (June). Without isotopic data, little was known about Genyornis feeding strategies.

A 130,000-year record of dietary input and potential water sources for both birds has been reconstructed from stable C and O isotopes in more than 1000 individuals from south-central Australia, each dated by ¹⁴C, luminescence and/or racemization. Prior to 50 ka Dromaius diet was highly variable, ranging from 100% C-3 to 100% C-4. However, immediately after 50 ka, Dromaius diet shifted dramatically. C-4 plant contribution to their diet never exceeded 50% (n=200) after 50 ka, whereas more than half the samples older than 50 ka contain >50% C4 dietary sources. The dietary C input of Genyornis changed very little through the extinction window, although dramatic changes in d¹⁸O at the time of extinction imply great environmental stress. At Lake Eyre, Genyornis’ diet included a significant component of C-4 grass throughout the fossil record. We attribute the observed changes in Dromaius diets to reflect a fundamental rearrangement of the plant ecosystems favoring C-3 plants in semi-arid central Australia. As a generalist, Dromaius was able to adjust its feeding strategy to the changed environmental conditions, but Genyornis and other large herbivores were not. The coincidence in time of megafauna extinction and ecosystem collapse shortly after the arrival of modern humans, and the lack of similar changes during dramatic climate change earlier in the Quaternary, suggests there may be a causal link.