A changing Australian landscape
There was a time – yesterday, geologically speaking – when kangaroos 3 metres tall hopped about Australia, elephants roamed North America and 2-metre high birds strutted around in New Zealand. Since the end of the nineteenth century, many different writers have noticed that the world doesn’t have an abundant number of creatures over 40 kilograms. Collectively, these large beasts are known as ‘megafauna’. Alfred Wallace, who wrote the first paper on evolution by natural selection with Charles Darwin, noted that ‘we live in a zoologically impoverished world, from which all the hugest, and fiercest, and strangest forms have recently disappeared’. We now know that the extinction of many of these creatures was global and that they died out quite recently. Their bones, when explorers and researchers found them, were not fossilized, suggesting their death was a matter of thousands of years ago. But the extinction seems to have happened at different times in different places. Some regions even kept their megafauna. In Australia, 94 per cent became extinct, while at the other extreme, only 2 per cent were lost south of the Sahara. What happened?
As with any good mystery, there are two main suspects: in this case, climate and humans. The idea that our animal-skin clad ancestors may have hunted the huge beasts to extinction was first suggested as long ago as the mid-nineteenth century. Several major criticisms continue to be levelled at this theory. One is that many large animals are still present in Africa, despite it having the longest record of occupation by people. To its detractors, the fact that there were not huge numbers of our ancestors at the time of the main extinctions suggests they could not have caused large amounts of environmental damage. And it has also been controversially argued that most animals are shy of humans and are unlikely to hang around long enough to feel the hard end of a club.
|The strong tail kangaroo: Sthenurus|
The alternative is that a rapidly changing climate caused the habitat of the megafauna to shrink or disappear. This seems pretty attractive as an idea. During the last ice age, there were massive swings in climate. Animals that were adapted to icy conditions, the argument goes, were unlikely to be able to cope with a rapid transition to a warmer climate. A major criticism here is that there have been other major climatic changes in the past, some of which have been equally extreme and rapid. What could have been so different at some climatic boundaries to have caused widespread extinction when earlier shifts had had no discernible effect?
An excellent test of some of these ideas is what happened in Australia. As well as the giant kangaroos, a large number of different species are now sadly extinct. One of the best known is the giant herbivorous marsupial, the diprotodon. This creature was furry and wombat-like. Up to 2 metres high and around 3.5 metres long, it would have looked more at home in a Star Wars movie. When you throw in the now extinct marsupial lions, sheep-sized echidnas (spiny anteaters) and large goanna-like carnivores (monitor lizards) over 5.5 metres long, you have to wonder what happened.
Just this week I was fortunate to be involved in an exciting new research paper published in the prestigious journal Science that addressed just this issue. Called ‘The Aftermath of Megafaunal Extinction: Ecosystem Transformation in Pleistocene Australia’ led by Susan Rule of the of the Australian National University and Christopher Johnson at the University of Tasmania. Here we looked at ancient peat and lake sediments preserved within an extinct volcano known as Lynch’s Crater in tropical Queensland. Locked away within the sediments are a large range of minuscule fossils that record changes in the environment over some 250,000 years: pollen grains preserve a measure of vegetation shifts while charcoal fragments record changing fire activity over time. One of the other fossil types found in the Lynch’s Crater sediments is the spore of a fungus known as Sporormiella. In spite of it’s name, Sporormiella is something of a rising star in the science of reconstructing past environmental change. To complete it’s life cycle, Sporormiella has to be ingested where it produces spores, providing a rather nifty measure of megafaunal activity in the landscape. Effectively when you have a lot of herbivores, you have a lot of dung and the spores they contain can become preserved in basins of accumulating sediments.
Importantly, in Lynch’s Crater, the amount of Sporormiella shows a long-term decline shortly before 40,000 years ago. This seems to take place independent of climate change but appears to be at the same time as the arrival of humans in the area. It all points to humans playing a significant role in the cause of Australian megafaunal extinction. But perhaps equally importantly, this demise in the megafauna looks like it had a massive impact on the landscape. With the loss of these enormous beasts, the vegetation in the surrounding area significantly altered; bushfires increased significantly a century later, followed by a later expansion of grasslands and then dry-loving forest known as sclerophyll. The implication is that with the loss of large herbivores, there was a build-up of uneaten vegetation, creating fuel for fires, which when they came knocked out the vulnerable rainforest, creating room for the sclerophyll to thrive. Landscape change all bought about by extinction.
Whether these changes took place in other parts of the world remains to be seen but it’s a fascinating insight into the Australian landscape tens of thousands of years ago.