Sunday, March 25, 2012

Megafauna Extinction Caused by Overhunting

In last Friday's issue of Science, there's an article about the demise of the megafauna in Australia. Here's the reference:

Rule, Susan, Barry W. Brook, Simon G. Haberle, Chris S. M. Turney, A. Peter Kershaw, and Christopher N. Johnson (2012). The Aftermath of Megafaunal Extinction: Ecosystem Transformation in Pleistocene Australia. Science 335:1483-1486.

In case you didn't know, I should mention that, like the Americas, Australia had megafauna during the Pleistocene, but instead of mammoths, mastadons, camels, horses, dire wolves, and sabre-toothed tigers, down under they had giant marsupials, such as things resembling huge kangaroos and giant wombats. As in the Americas, the Australian megafauna disappeared near the end of the Pleistocene, and in both places there is a scientific debate about the cause of these massive extinctions. The usual suspects are either climate change and over-hunting by the first human populations, although in North America there has also been a recent flurry of interest surrounding the unlikely hypothesis that a meteor impact caused the die-off. What makes the debate tricky in North America in particular is that humans seem to have arrived (granted that the archaeological dates of the event leave a lot to be desired) at a time of rapid climate change. Therefore, distinguishing between the two causes is difficult because of the chronological overlap. In Australia, the situation is different because human arrived much earlier, well over 40,000 years ago, although (again, like in the Americas) there has also been plenty of dissension about the precise date.

The new article in Science contributes significantly to the Australian debate. The authors studied two cores, analyzing charcoal, pollen, and spores of the Sporormiella fungus. The charcoal told them about the changing regime of fires, the pollen provided information about the vegetation, and the spores, which grew primarily in the dung of the megafauna, allowed them to pinpoint their extinction. The spores disappeared about 41,000 years ago, about the same time that the megafauna are believed to have died out based on a variety a paleontological evidence.

(The use of spores from a fungus growing on the dung may seem like a crazy way to study the megafauna, but it's not novel; it's done in North America too. I once talked to a paleoethnobotanist who was studying plant remains from mammoth dung recovered in the Mid-Atlantic states. I asked her about her sample sizes, and she said something like, "It's not a problem. Their coprolites are huge. We have a warehouse full of poop." I guess when a mammoth took a dump, it created quite a pile. Now imagine herds of them slowly pooping their way across the continent. The dung becomes an ecosystem.)

The importance of the recent article in Science is that the researchers can show, through a high resolution chronological analysis, that the vegetation changes and increased burning came after the extinction, not before, and therefore they were evidently responses to the extinction rather than being related to its cause. Moreover, at the time of the extinction in Australia, they found no evidence of significant climate change. It occurred instead during a period relative climatic stasis. And of course the same megafauna had previously survived much more severe episodes of climate change.

The study adds to the growing evidence that human over-hunting caused the extinction of megafauna in several parts of the world. (New Zealand is another example.) This was probably the case in the Americas as well.

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