RECONSTRUCTING ECOSYTEM CHANGE FROM D13C IN DROMAIUS EGGSHELLS

Modern humans colonized Australia 55 to 50 ka; all of Australia’s large mammalian vertebrates became extinct about 50 ka, shortly after human colonization. Between 60 and 40 ka climate was similar to present and not changing rapidly. Consequently, attention has turned toward plausible human mechanisms for the extinction, with proponents for over-hunting, ecosystem change, and introduced disease. To differentiate between these options we rely on isotopic tracers of diet preserved in avian eggshells to track changes in ecosystems before and after human colonization. δ¹³C preserved in eggshell calcite and in organic residues preserved within the calcite crystals, monitors a bird’s dietary intake in the weeks to months before egg laying. Late Quaternary eggshells of two large flightless birds are locally abundant across semi arid Australia: the Australian emu (Dromaius novaehollandiae), an opportunistic dominantly herbivorous feeder, and a heavier bird, Genyornis newtoni. We developed a continuous, 140 ka δ¹³C reconstruction for Dromaius diet, and for Genyornis diet from 140 ka until its extinction about 50 ka based on >900 dated eggshells from central Australia. Less continuous dietary records for both species were developed from two distant regions (>250 additional dated eggshells <130 ka old). Dromaius eggshell δ¹³C reveals an unprecedented reduction in food sources about 50 ka in both carbon reservoirs from all three regions. Genyornis diet everywhere differed from co-existing Dromaius, occupying a more restricted isotopic space that implies a more specialized feeding strategy. We suggest that the dietary shift observed in Dromaius eggshells ca. 50 ka is diagnostic of ecosystem collapse throughout the semi-arid zone shortly after humans colonized Australia, possibly a consequence of systematic burning by early humans. Those animals with feeding strategies that allowed them to adapt to a changed vegetation regime, such as Dromaius, survived; those with restricted dietary sources, such as Genyornis, became extinct.