Australian extinctions walking on eggshells

A few weeks ago, I posted on recent work by Clive Trueman et al. (2005) that showed a prolonged coexistence of some extinct Australian megafauna with early humans.

Today, I saw this new paper by Gifford Miller and colleagues. Here's the abstract:

Most of Australia's largest mammals became extinct 50,000 to 45,000 years ago, shortly after humans colonized the continent. Without exceptional climate change at that time, a human cause is inferred, but a mechanism remains elusive. A 140,000-year record of dietary delta13C documents a permanent reduction in food sources available to the Australian emu, beginning about the time of human colonization; a change replicated at three widely separated sites and in the marsupial wombat. We speculate that human firing of landscapes rapidly converted a drought-adapted mosaic of trees, shrubs, and nutritious grasslands to the modern fire-adapted desert scrub. Animals that could adapt survived; those that could not, became extinct.

The paper has an accompanying commentary by Christopher Johnson that explains it well:

Miller et al. studied past diets of the emu (Dromaius novaehollandiae) and an even larger flightless herbivorous bird, the extinct Genyornis newtoni (see the first figure), in the arid and semi-arid regions of the south Australian interior. By analyzing carbon isotopes in individually dated eggshells, they were able to compare the contributions of plants that use the C4 photosynthetic pathway (mainly tropical and arid-adapted grasses) and those that use the C3 pathway (most shrubs, trees, and nongrass herbs) to the diet of the birds that laid the eggs. Their collection of eggshells covers the past 140,000 years, encompassing the whole of the last glacial cycle.
Miller et al. found a sudden change in emu diet between 50,000 and 45,000 years ago. Before 50,000 years ago, emus had variable diets, with a strong contribution from C4 plants; after 45,000 years ago, they ate mostly C3 plants. Genyornis eggshells were common before 50,000 years ago, but they abruptly disappeared at the same time as the diet of the emu changed. Before then, Genyornis also ate a mixture of C3 and C4 plants, but its diet was much less variable than that of the emu through the same period, which suggests that it was a more specialized feeder (Johnson 2005:255).

This paper argues that climate change could not be responsible because there was no climate change 45,0000 years ago. Trueman et al. (2005) argued that climate change was likely responsible because the animals survived alongside humans until 30,000 years ago, when climate changes did happen. Neither the paper nor the commentary note or cite Trueman and colleagues' work. This is not necessarily a surprise, since the papers were in press at the same time, but seems like an omission.

An article by Nature news does notice the contradiction, and has this to say:

Fossils found at Cuddie Springs, New South Wales, seem to indicate that ancient fauna lived side-by-side with humans for several thousand years before finally succumbing to encroaching desertification as little as 30,000 years ago (see "Did climate shift kill off giant Australian animals?").
But Miller argues that more accurately dated fossils are needed to support this theory. "The Cuddie Springs dating remains very contentious," he says. "Most agree that the extinction event occurred between 50,000 and 45,000 years ago."

Well it is always true that better dates would help. But I think we have a lot to learn about the dynamics of ancient mass extinctions, also. Burning could well have changed the Australian habitat in ways congenial to some animals (that is, after all, why people started burning to begin with) but ultimately bad for others. But could it have wiped out the entire ecological niche of a species? Or was a reduced population ultimately rendered more susceptible to human predation? Or climate change? Or were they simply outcompeted by other animals who could handle the loss of C4 [corrected on 8/1/05] resources? Lots of questions, few answers.

References:

Johnson CN. 2005. The remaking of Australia's ecology. Science 309:255-256. Full text online

Miller GH. et al. 2005. Ecosystem collapse in Pleistocene Australia and a human role in megafaunal extinction. Science 309:287-290. Full text online