Megafauna mystery – size isn’t everything

GREAT or small, the future of Australia’s iconic wildlife appears uncertain in the face of climate change and human impacts such as habitat loss and environmental degradation.
A new paper on Australia’s megafauna (extinct animals 45kg-plus), shows contrary to expectations, today’s smaller relatives face higher extinction risks than their super-sized ancestors, “so if their predecessors couldn’t survive, things look a bit grim for some of our current iconic species”.
Researchers at Flinders University and the Australian Research Council Centre of Excellence of Australian Biodiversity and Heritage revisited the fossil and archaeological records of the ancient continent of Sahul, comprising Australia, Tasmania, New Guinea and neighbouring islands.
Why giant herbivores such as diprotodons, ‘thunder’ birds two metres tall and lizards seven metres long went extinct remains a puzzle, but there is a growing consensus that multiple factors contributed to the extinction of megafauna in Australia including climate change, the impact of people on their environments and access to freshwater.
Flinders Professor Corey Bradshaw applies sophisticated mathematical modelling to assess how susceptible different species went extinction and what it means for the survival of species in Australia today.
Using various characteristics such as body size, weight, lifespan, survival rate, and fertility, his team created population models to predict the likelihood of these species surviving under different types of environmental disturbance.
 Simulations included increasing drought and hunting pressures to see which species of 13 extinct megafauna, as well as 8 comparative species still alive today, had the highest chances of survival.
Published in the journal ‘eLife’, the researchers compared results to what we know about the timing of extinction for different megafauna species derived from dated fossil records. They expected to confirm the most extinction-prone species were the first species to go. But it wasn’t the case.
They found slower-growing species with lower fertility, such as the rhino-sized wombat relative diprotodon, were generally more susceptible to extinction than more-fecund species such as the marsupial thylacine. “In fact, we found most of the living species used for comparison, such as short-beaked echidnas, emus, brush turkeys, and common wombats, were more susceptible on average than their now-extinct counterparts,” Professor Bradshaw said, adding the true ‘extinction cascade’ was a result of, “complex, localised scenarios, including impacts of regional climate variation, and different pressures from people across regions”.
Flinders Associate Professor Vera Weisbecker said: “Fast-hopping red kangaroos still alive today might have had an escape advantage over some of the slower-striding short-faced kangaroos that went extinct. Small wombats that dug burrows might also have been more difficult for people to hunt than the bigger, non-burrowing megafauna.”
Flinders co-author Frédérik Saltré added: “We determined the kangaroo species were the least-susceptible to extinction based on their biology, followed by the monotremes (echidnas), and the giant ‘wombat’ species. Interestingly, the large, flightless birds, like emu and the giant mihirung ‘thunderbird’ Genyornis, had the highest susceptibilities.”
Professor Bradshaw said: “Our results support the notion that extinction risk can be high across all body sizes depending on a species’ particular ecology, meaning that predicting future extinctions from climate change and human impacts aren’t always straightforward based on the first principles of biology.”
Readers can view some of the megafaunal fossils at the WA Museum which in 2002 unearthed the biggest fossil find in a century when scientists abseiled into Nullarbor caves near the WA-South Australian border (https://streetwisemedia.com.au/wp-content/uploads/2020/04/Freo-Streetwise-Easter-Edition-2020.pdf).