Size alone ‘did not cause ancient megafauna to go extinct’

Around 40,000 years ago mega-sized animals disappeared from the ancient continent of Sahul, which comprised mainland Australia, Tasmania, New Guinea and numerous smaller adjacent islands.

What happened? Was climate change to blame? Or was it people who hunted them into extinction? Or both at the same time?

Scientists have been seeking to find the answers to these questions, and a team of researchers led by Prof. Corey Bradshaw of Flinders University set out to do the same.

For a new study the Australian team devised complex mathematical models to assess how susceptible different species were to extinction based on several of their features, including 13 extinct species of ancient megafauna and eight species alive today.

The experts created simulations by taking into account such variables as body size, weight, lifespan, survival rate and fertility to predict how likely the survival of the various species was under different environmental circumstances such as prolonged droughts and increased hunting by people.

Their findings show that species that grew slowly and had lower fertility rates like the giant wombat called Diprotodon were generally at higher risk of going extinct than those that were more fertile the marsupial predator thylacine, which survived into the early part of the 20th century in Australia.

That is no surprise as biologists have long predicted that. Yet just because these larger animals were more susceptible to extinction did not mean they went extinct as predicted, based on the actual fossil record.

“We found no clear relationship between a species’ inherent vulnerability to extinction — such as being slower and heavier and/or slower to reproduce — and the timing of its extinction in the fossil record,” Bradshaw explains. “In fact, we found that 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,” he adds.

The conclusion the scientists have drawn is that it was probably because of a complex array of environmental stressors that caused various species to go extinct and not a singular cause.

“The relative speed of different species to escape hunters, as well as whether or not a species dug protective burrows, also likely contributed to the mismatch between extinction susceptibility and timing,” observes Prof. Vera Weisbecker of Flinders University who was a co-author of the study.

“For example, 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,” she adds. “Small wombats that dug burrows might also have been more difficult for people to hunt than the bigger, non-burrowing megafauna.”

What their findings mean for today’s species is that it may not always be a straightforward matter to predict which endangered species are at highest risk.

“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,” Bradshaw stresses.