The swift-running, flightless Australian emu is helping scientists piece together the puzzle on the respective roles that humans and natural climatic changes have had on the Australian continent over the millennia. Emus eat a variety of different plants, which carry different compositions of carbon isotopes. The isotopes stay intact once metabolized and are found in the shells of the eggs that the animals lay. This information is used by the scientists as a marker to reveal aspects of the past climate and vegetation of Australia.
Scientists Beverly J. Johnson and Marilyn L. Fogel of the Geophysical Laboratory of the Carnegie Institution of Washington, G. H. Miller of the University of Colorado and others* report their findings in the May 14, 1999, issue of Science. The researchers establish, using contemporary eggshell samples, that the carbon isotopes varied in response to the inclusion of certain grasses in the diet of the emu. The grasses of Australia fall into two distinct sets based on their photosynthetic pathways. One set converts carbon dioxide to a compound with 3 carbon atoms ( C3) during the process of photosynthesis, while the other changes the CO2 into a compound with four atoms of carbon (C4). The distribution of the C3 and C4 grasses over the continent depends on the season of rainfall-C3 grasses dominate southern Australia where it rains in the winter and has cooler temperatures, while C4 grasses are dominant in northern Australia with summer precipitation and warmer temperatures.
After dating and analyzing fossilized eggshell samples from the Lake Eyre Basin of Australia, the investigators conclude that between 65,000 and 45,000 years ago the C4 grasses were abundant and readily available for the emu to consume during its breeding season. The summer rainy season influenced by the monsoon from the north, they argue, was more extensive at this time, so that C4 grasses grown in summer were still abundant when emu were laying their eggs in winter.
Between 28,000 and 15,000 years ago, during the peak of the last glacial period, C4 grasses were almost totally absent because of the colder temperatures. Their range should have expanded again when the Earth warmed during the Holocene (10,000 years ago). However, data from the emu eggshell documents that the ecosystem did not revert to its state that prevailed prior to the arrival of humans. C4 grass abundances were below the levels established before the glacial period. Over the last 65,000 years, the researchers conclude, environmental factors other than climate have significantly influenced Australian ecology.
Vegetation changes in north- and south-eastern Australia, brought about by more frequent fires, is attributed to the arrival of the first human immigrants at about 60,000 years ago. The transfer of moisture from the plants and soils to the atmosphere is an important feedback mechanism that enhances the transfer of monsoon rainfall into the interior of the continent. Changes in the types of plants across northern Australia, brought about by the burning practices of the first human colonizers, may have reduced this mechanism and thus diminished the amount of rainfall reaching what is now the arid Australian interior. The influence of humans with their recurrent fires has been suggested as the cause of extinction for a least one Australian megafauna, the flightless bird Genyornis newtoni at 50,000 years ago (Science, vol. 283, no. 5399, p. 182).
Measurements since European colonization (about 150 years ago) indicate a further depletion of the C4 grasses. Instead of being related to climate, the shift is believed to result from overgrazing, the introduction of exotics and other activities associated with human expansion.
According to Marilyn Fogel, "Our isotopic data on the emu's eggshells are consistent with a human overprint on natural climate changes. The effect of precipitation from the summer monsoon in central Australia decreased significantly at approximately the same time as many of the large Australian animals went extinct. Data from the emu's eggshells has allowed us to piece together the story of climate changes since that time, and it is clear from the data that the ecosystem never fully recovered."
* J. W. Magee and M. K. Gagan of the Australian National University, and A.R. Chivas of the University of Wollongong, Australia.
This research was funded by the National Science Foundation, the Australian National University and the Carnegie Institution of Washington.