CONTINENTAL-WIDE ECOSYSTEM RESTRUCTURING COINCIDENT WITH HUMAN COLONIZATION AND MEGAFAUNAL EXTINCTION IN AUSTRALIA

The links between megafaunal extinction and human colonization across the Americas and Australia have long been debated, as well as ancillary evidence for widespread landscape modification following human arrival. Here we use d¹³C in the organic and inorganic components of eggshells of the Australian flightless bird, Dromaius (emu), as tracers of palatable vegetation ecosystems over the past 100 ka, from well before human colonization, continuously through to modern times. Our sites now include most of arid and semi-arid Australia, greatly expanding our geographic coverage from earlier report. The emu is a generalist feeder, typically foraging on whatever elements of the ecosystem provide the most nutritious foods. Palatable C4 summer grasses have d¹³C values of ca. -10‰, whereas C3 trees, shrubs and southern winter-rainfall grasses have d¹³C values around -26‰. Emus breed at the same time throughout the continent (hatching in winter, June-August), and the isotopic composition of their eggshells is set by food sources consumed in February, March, just before egg laying. Our time series consist of over 2,500 individual measurements of d¹³C, each from a dated emu eggshell. Samples were collected from six regional study areas scattered across 3000 km of arid and semi-arid Australia. We find that in all six areas a major reduction in the isotopic composition of emu diet occurred sometime between 50 and 45 ka ago, with the older time interval characterized by a wide isotopic dietary intake, ranging from pure C3 to pure C4 diets, and widely scattered between these two end members. By contrast, after 45 ka, most of the C4 dietary component had disappeared from their dietary intake, with rarely more than 50% of emu diet derived from C4 plants. These data are consistent with earlier assertions that the Australian landscape underwent a major reorganization shortly after human colonization ca. 50 ka and was associated with megafaunal extinction. Whether the change in ecosystems was due to human agency and contributed to megafaunal extinction, or was itself a result of reduced browse pressure because of megafaunal extinction remains debated, but the failure of ecosystems to recover to their state prior to 50 ka at any time in the past 45 ka suggests a continued human impact on ecosystem services since colonization.