Australia is important to the debate of megafaunal extinction because it is very different (“a separate creation” - Charles Darwin) and humans arrived much earlier 40-50 kyr BP, well before the end of the ice age, so there is less confusion with climate change. It is a valuable independent test case.
It has been said there are not enough data; Chris disagrees. He presents the radiocarbon extinction chronology of megafauna in Australia. Shows clearly they all go around 40-5 kyr. There is archaeological evidence that humans were widespread by 45 kyr. Sporomielia at Lynch’s crater also decline at the same time. Pretty convincing there was a megafaunal mass extinction.
Could climate play a role? He shows a climate-niche model for megafaunal species for the last 120 kyr using Hadley Centre GCM. They matched location records to climate at that time and place. The model shows climate was improving and range sizes would have peaked at around 30 kyr. There is no signal at all of a climate shift that could have caused megafaunal range reduction at the time they went extinct.
The extinctions were severe: nothing above 40 kg survived - more severe than even the Americas. Plots risk of extinction as body mass. He replots this as extinction risk vs fecundity and the American and Australian curves collapse into one curve. If you were an animal with less than one offspring per year you were highlight likely to go extinct. Exactly the same story in Australia and the Americas. The best predictor of megafaunal extinction is reproduction rate. “Mega” is a red herring - size did not matter - what mattered is fecundity and rate of population growth. The sustainable harvest rate for kangaroos is 40%, for diprotodons it would have been around 5%.
Pretty convincing evidence that the Australian megafaunal collapse was driven by human arrival. What about the ecological impacts of the extinction?
Evidence from Lynch’s crater in Queensland. There were rainforest angiosperms and gymnosperms until around 40 kyr, then shift to fire-prone sclerophyll forest (eucalyptus). That shift has always been blamed on human arrival and fire use. How do animals like Diprotodon fit into that time sequence? They look at dung spores as an indicator of megafauna. The Sporomiella drops around 40 kyr, at the same time when charcoal appears in the record, with a charcoal spike around 500 years after the arrival, then increase in grasslands. He shows the velocity of change in each of these proxies (100 years sampling resolution). High inverse correlation with abundance of megafauna and total tree influx.
Summary: started with mixed sparse rainforest and sclerophyll forest, little fire (no modern analogue to this ecosystem). Changed to uniform dense sclerophyll forest; this area changed to wet rainforest in the Holocene.
Where would we expect the most change in vegetation? At the broadest scale vegetation is controlled by climate, but there is a sliver of climate space where you cannot predicted vegetation type. William Bond argues in those climates vegetate type is controlled by the removal of biomass by herbivory and fire. This climate space is a huge part of the Earth’s land surface.
Lynch’s Crater is within this uncertain zone. Their second site (Caledonia Fen) is outside this uncertain zone, and there is little evidence of vegetation change after megafaunal loss.
To summarise:
Human impact, not climate, caused megafaunal extinction
Extinction was closely followed by vegetation change and increased fire in some places but not others, depending on the climate space.
Sandra Diaz: analogy with kangaroo survival in Australia and guanaco/vicuna survival in South America. Both have reasonably high fecundity.
It has been said there are not enough data; Chris disagrees. He presents the radiocarbon extinction chronology of megafauna in Australia. Shows clearly they all go around 40-5 kyr. There is archaeological evidence that humans were widespread by 45 kyr. Sporomielia at Lynch’s crater also decline at the same time. Pretty convincing there was a megafaunal mass extinction.
Could climate play a role? He shows a climate-niche model for megafaunal species for the last 120 kyr using Hadley Centre GCM. They matched location records to climate at that time and place. The model shows climate was improving and range sizes would have peaked at around 30 kyr. There is no signal at all of a climate shift that could have caused megafaunal range reduction at the time they went extinct.
The extinctions were severe: nothing above 40 kg survived - more severe than even the Americas. Plots risk of extinction as body mass. He replots this as extinction risk vs fecundity and the American and Australian curves collapse into one curve. If you were an animal with less than one offspring per year you were highlight likely to go extinct. Exactly the same story in Australia and the Americas. The best predictor of megafaunal extinction is reproduction rate. “Mega” is a red herring - size did not matter - what mattered is fecundity and rate of population growth. The sustainable harvest rate for kangaroos is 40%, for diprotodons it would have been around 5%.
Pretty convincing evidence that the Australian megafaunal collapse was driven by human arrival. What about the ecological impacts of the extinction?
Evidence from Lynch’s crater in Queensland. There were rainforest angiosperms and gymnosperms until around 40 kyr, then shift to fire-prone sclerophyll forest (eucalyptus). That shift has always been blamed on human arrival and fire use. How do animals like Diprotodon fit into that time sequence? They look at dung spores as an indicator of megafauna. The Sporomiella drops around 40 kyr, at the same time when charcoal appears in the record, with a charcoal spike around 500 years after the arrival, then increase in grasslands. He shows the velocity of change in each of these proxies (100 years sampling resolution). High inverse correlation with abundance of megafauna and total tree influx.
Summary: started with mixed sparse rainforest and sclerophyll forest, little fire (no modern analogue to this ecosystem). Changed to uniform dense sclerophyll forest; this area changed to wet rainforest in the Holocene.
Where would we expect the most change in vegetation? At the broadest scale vegetation is controlled by climate, but there is a sliver of climate space where you cannot predicted vegetation type. William Bond argues in those climates vegetate type is controlled by the removal of biomass by herbivory and fire. This climate space is a huge part of the Earth’s land surface.
Lynch’s Crater is within this uncertain zone. Their second site (Caledonia Fen) is outside this uncertain zone, and there is little evidence of vegetation change after megafaunal loss.
To summarise:
Human impact, not climate, caused megafaunal extinction
Extinction was closely followed by vegetation change and increased fire in some places but not others, depending on the climate space.
Sandra Diaz: analogy with kangaroo survival in Australia and guanaco/vicuna survival in South America. Both have reasonably high fecundity.