The extinction of Australia's megafauna led to major ecological changes and helped create the vegetation we recognise today, new research has found.
Australia's megafauna included giant marsupials, birds, reptiles and monotremes, which became extinct around 40,000 years ago, not long after humans are thought to have colonised Australia. The cause of their disappearance has been hotly contested for more than 100 years.
The study, published today in Science, was led by Dr Susan Rule from Australian National University and Professor Christopher Johnson from the University of Tasmania. The team of Australian researchers analysed fungal spores from two sediment cores taken from Lynch's Crater, a 'fossilised swamp' in Northeast Queensland.
They found that Sporormiella spores, which grow predominantly in the dung of large herbivores, virtually disappeared around 41,000 years ago.
"When there was lots of fungus, there was lots of dung and lots of big animals making it," says Johnson. "When they disappeared, their dung fungus went too."
The core samples revealed two climate change events in the previous 80,000 years both of which resulted in cooling and drying of the landscape. Neither of these had any impact on the presence of Sporormiella. However at the time of the megafauna's extinction (indicated by the fungus' disappearence) the samples indicate no climate transformation taking place.
The results were also compared with previous pollen and charcoal studies. These showed a transition from mixed rainforest species to grasses and leathery leaved, dry tolerant sclerophyll vegetation, and an increase in fire activity beginning at least 100 years after the animal extinctions - indicating they were the result of the extinction and not its cause.
The researchers argue that newly arrived humans hunted the animals to extinction, with the reduced grazing pressure causing an increase in the fuel load and fire intensity.
A rapid increase in grass was followed by a more gradual increase in the proportion of schlerophyllous plants in the forest until they came to dominate the forest structure.
"The study helps resolve the debate about the cause of the megafauna extinction," says Johnson. "By looking at the impact of these herbivores, and the consequences of their removal, it also answers an important question about Australia's environmental history."
Not enough dung
Archaeologist Dr Judith Field from the University of New South Wales says that on face value this is a really exciting find. However, she says she has a few reservations about the basis on which the conclusions are drawn.
"The researchers are assuming that the fungal spore in question is an accurate proxy for mega herbivores," she says. "This was first found in the US (and has since also been found in New Zealand and Madagascar), but there is not yet sufficient research to show this is accurate for Australian herbivores."
Johnson concedes this is a fair point, but says that the fungus appears to be ubiquitous in the dung of megaherbivores, and that it reappeared in volume in Australia only with the introduction of livestock like cattle.
"Until we can get our hands on a diprotodon dung bolus, we won't be able to validate Sporormiella as a proxy for the presence of mega-herbivores.
"But it does occur in living marsupials so it is a fair assumption that it was also present in their extinct relatives"
Field also questions the discounting of climatic flux as a potential cause of the disappearance of mega-herbivores and increase in schlerophyllus vegetation, with ice cores from Antarctica and Greenland showing that in fact this was a period of huge climatic fluctuations.
"Finally, there is no evidence for large-scale human hunting of megafauna, and only two sites - Cuddie Springs and Nombe Rockshelter in the New Guinea Highlands (which was joined to Australia at the time) - showing any evidence of temporal overlap," she says.s
"Correlating the continent-wide extinction of the megafauna with the disappearence of an unproven proxy might be drawing a very long bow."