Fires set by humans may have led to animal extinction

Genyornis

The earliest humans in Australia may have inadvertently disrupted the continent’s food chain by burning vast areas of native vegetation, resulting in the extinction of most large animal species about 50,000 years ago.

The evidence, reported in this week’s Science, runs counter to a century of debate pointing to either climate change or human predation as the reason why 85 percent of the Australia’s animals weighing more than 100 pounds went extinct.

Gone were animals including a wombat the size of a hippopotamus, a 23-foot-long monitor lizard and a snake 25 feet long and more than a foot in diameter, according to Beverly Johnson, a UW post-doctoral fellow. Johnson is a co-author on the Science paper for work she did while at the University of Colorado at Boulder, work she’s continuing to pursue at the UW’s School of Oceanography.

With funding from the National Science Foundation and the Australian National University, the researchers broke new ground in dating this “megafauna” extinction by documenting the demise of Genyornis newtoni, an enormous flightless bird that looked something like a large stocky emu. Johnson helped collect and analyze fossilized eggshells that allowed researchers to determine what Genyornis ate, to date the timing of their extinction and to rule out climate change as the reason.

Johnson looked at the stable isotope geochemistry of the eggshells and determined that Genyornis was a fairly selective eater, choosing primarily browse (foliage and twigs from trees and shrubs) and some grasses. Her geochemical results on nearly 100 eggshells support the evidence from scant skeletal remains that Genyornis had a strong, shearing beak that made it able to eat browse. In contrast, emus living alongside Genyornis had a much more varied diet and could subsist on either all grasses or all browse.

The research team speculates that the use of fire by the earliest human immigrants, beginning about 55,000 years ago, changed the ecosystem by reducing the amount of trees and shrubs in the Australian interior. Because Genyornis and other browsers lost their food source, only animals with more opportunistic feeding patterns could survive, Johnson says. Emus, for example, survive today.

Like any other group of people, the early Australians were just trying to keep their families fed, according to Gifford Miller, a professor of geological sciences at the University of Colorado at Boulder and lead author of the Science paper. “We suspect the systematic burning by the earliest colonizers—used to secure food, promote new vegetation growth, signal other groups of people and perhaps for other purposes—differed enough from the natural fire cycle that key ecosystems were pushed past a threshold from which they could not recover.”

Although fire was common before the first humans arrived, the natural fire season occurred in November and December due to lightning strikes during the build-up to the wet season when sufficient fuel loads had accumulated, Miller says. “The possibility of human burning at other times of the year and at a greater frequency may have inhibited the regeneration of the natural tree and shrub vegetation in the interior.”

With more than 700 dated Genyornis eggshells, the researchers are confident they have documented the bird’s presence from more than 100,000 years ago to their disappearance 50,000 years ago. Furthermore, they argue that the other megafauna extinctions likely coincided with that of Genyornis in part because the birds’ bones are found in association with those of the last megafauna species known to have roamed the continent.

Climate change was ruled out as the cause for the disappearance of Genyornis after eggshells from three different climate regions showed that the birds had survived much more severe conditions during their existence than they confronted at the time of their disappearance.

It’s possible that Genyornis was less agile than flightless birds that survived and thus were hunted by humans. However, the evidence of direct predation on Genyonris is limited to a single site and kill sites for other megafauna species are equally rare.

Today much of what once was the range of Australia’s megafauna consists of desert scrub and grassland. “Australia’s soils have always been poor,” Johnson says. “An increase in fires, even small ones, could have changed the balance of what was already a fragile ecosystem.”

Johnson and Miller’s co-authors are John Magee, Linda Ayliffe, Malcolm McCullock and Nigel Spooner of the Australian National University, and Marilyn Fogel of the Carnegie Institution of Washington, D.C. ¶

Sandra Hines, News and Information