June 7, 2012
Ecosystem Changes Drove Extinction In Pleistocene Australia
Brett Smith for redOrbit.com
Scientists may have finally established the explanation for the disappearance of the giant koala and other Australian megafauna.Between 50,000 and 45,000 years ago, around 60 species of mammals, predominantly foraging herbivores called browsers, went extinct. These animals included 19 species that weighed over 100 kilograms, like the rhinoceros-sized giant wombat and half-ton marsupial Palorchestes azael. Slightly smaller animals like the flightless bird Genyornis newtoni were also mysteriously wiped out, according to a story published online in EARTH magazine.
While evidence of colonization has suggested humans were the cause of the animals´ disappearance, University of Colorado at Boulder geologist Gifford Miller and his fellow researchers think they have uncovered potential grounds for the megafauna´s disappearance: a change in the ecosystem that made their dietary staples no longer available.
The team´s research has been criticized for some of its finer technical points, but Miller told redOrbit they have the most comprehensive set of data available concerning the disappearance of these creatures.
For nearly two decades, Miller has collected bird eggshell fossils and marsupial teeth across the Australian Outback as part of the study. By analyzing these finds, the researchers have pieced together the ancient diets of three animals: Genyornis, along with the extant emu (another giant flightless bird) and the modern day wombat. They used these data to reconstruct the complex relationships among humans, vegetation, climate and animals on the continent over the last 140,000 years.
This historical model supports the theory of an abrupt ecological shift and a dramatic reduction in available food sources that occurred about the time of human colonization. Using carbon dating techniques, Miller and his team showed that 50,000 years ago the emu was an opportunistic feeder living in an environment where moisture varied considerably. During some years the climate was wet enough for grasslands to grow abundantly. Other, drier years were dominated by the prevalence shrubs and trees. From 50,000 to 45,000 years ago, the mean carbon isotope ratios in both emu shells and wombat teeth decreased, indicating an increased reliance by these animals on shrubs and trees, the report said.
This shift in the base of the food chain was likely coupled with clearing of landscape vegetation by human settlers and the combined effect permanently converted the previous ecosystem from life-sustaining savanna to a more highly selective desert scrub environment.
The earliest human remains found in Australia have been dated to around 40,000 years ago. These early remains found at Lake Mungo include evidence of cremation.
The researchers also decided to see if these human activities permanently altered the ecosystem through feedback loops that reduced the delivery of rain to the interior of the island continent. Using a general atmospheric circulation model, they assessed the sensitivity of monsoon precipitation to the continent´s vegetation and soil characteristics, especially rainfall ℠recycling´ through evaporation and plant transpiration.
Results from the model suggest that vegetation and soil characteristics play a significant role in determining the penetration of monsoon moisture into Australia´s interior, with the model showing that a vegetation shift may cut interior monsoon rain by as much as half under conditions normally favoring strong monsoon flow. This lack of monsoon rain recycling translates to moisture levels being higher along coastal regions, but lower on the Australian interior.
“If the vegetation is there, you´ll get that feedback of recycling, and so the argument is that the vegetation changed, and that´s what the isotopes in the emus tell us,” Miller said. “What was lost were those elements that allowed the recycling of the water.”
The article published in EARTH magazine included a criticism from one of Miller´s colleagues, Gilbert Price, a vertebrate paleoecologist at the University of Queensland in Australia who was not involved in the research.
“How reliable is the existing age model? A well-dated record is critical for looking at the speed of extinction and allows us to determine whether the extinctions or diet shifts were rapid or sudden,” Price said.
He adds, more than 60 percent of the apparent ages of the Genyornis eggshells are “butting right up close to the radiocarbon dating window.”
“The Miller work is fantastic, but I do think that their age model needs much further scrutiny before we accept the interpretation regarding the speed of extinction and diet shifts as gospel.”
Miller responded to the “fair” criticism by noting that his team used four independent dating methods in their study to accumulate their data: carbon dating, U-Th dating, optically stimulated luminance (OSL) dating, and Amino Acid Racemization (ARR).
“There are always uncertainties, but ours is probably the most dense set of dates available. I'm not interested in gospel acceptance, but I would like to see others develop similarly densely dated time series for comparison,” he told redOrbit.