WHAT HAPPENED DURING THE AUSTRALIAN MEGAFAUNAL EXTINCTION: TROPHIC LEVEL UPSET, ECOSYSTEM CHANGE, OR HUNTING PRESSURE?

Stable carbon and nitrogen isotopes in the organic fraction of dated, fossil eggshells from emu and an extinct bird, Genyornis, reflect the diet of these birds and record changes through time. Data from three geographically distinct sites provides evidence that large-scale changes in the ecosystem happened from the Lake Eyre basin to Southern Australia after megafauna went extinct at 45-50 kA. Carbon isotopic shifts record diminishing C-4 grasslands; nitrogen isotopic shifts at Lake Eyre and Darling Lakes, NSW, record increasing aridity. Isotopic compositions of Genyornis and fossil emu will be compared to the C and N isotopic compositions of >500 modern plants collected in the continental rainfall gradient from 100-1500mm/year. In South Australia, N isotopic compositions of plants growing in very dry conditions reach values of +10 per mil, whereas in high rainfall regions N isotopes are as low as –2 per mil. Shifts in rainfall that affect the N isotopic compositions of plants can explain most of the changes measured in the eggshells over time. Genyornis eggshells were typically collected along side lakeshores and other water courses, often in great abundances indicating that they might have been colonial nesting birds. Multiple eggshells from the same nest are compared with different nests to determine whether these birds were capital breeders, relying on a stable, year-round food source. Both factors may have contributed to their demise from human predation, relative to the emu, solitary nesters that rely on recently ingested food for egg-laying.