Ice Age flora extinction reveals fresh plant biodiversity clues

The new study suggests extinction driven by climate instability may be just as important as evolution as a driver of plant biodiversity.

Ice Ages caused a mass extinction of plants in south-eastern Australia around a million years ago, according to a new study that presents a fresh take on how extinction shapes biodiversity.

Scientists previously believed that the rate at which new species evolve was the key to rich biodiversity.

However, the new study published in the journal US Proceedings of the National Academy of Sciences, provides a counterpoint to that view.

The researchers analysed fossils of sclerophyll, or tough-leaved plants found at a site near Daylesford in Central Victoria to estimate the plant biodiversity in the region 1.5 million years ago.

They found the vegetation that produced the fossils is comparable to samples found in south-western Australia, one of the richest locations for plant diversity on Earth.

That suggests a widespread extinction in south-eastern Australia’s past, they concluded.

Scientists had previously thought the common climate attributes shared by south-western Australia and the Cape region of South Africa – including a lack of summer rain and poor soil – meant that those conditions were the drivers of plant biodiversity in these regions.

But a co-author of the study, Dr Kale Sniderman of the University of Melbourne, said their study showed that was not necessarily the case.

“We know from the fossil locations that we did not have a climate anything like that of south-western Australia. Instead, we had a climate with a lot of summer rain, much higher levels than are now found in the region,” he said.

In fact, unstable climates and extinction are just as important to biodiversity as evolution in favourable conditions, Dr Sniderman said.

“Regions that are very high in biodiversity are regions that have had more climate stability over the last million years or so,” he said.

“The climates of south-eastern Australia have been less stable than those in south-western Australia and it was the onset of a new level of climate instability during the Ice Age that led to the extinction of all these tough-leaved plants in south-eastern Australia, but the greater stability in south-western Australia allowed more species to co-exist.”

Dr Sniderman said the study provided fresh clues on how climate change might affect plant biodiversity.

“In south-eastern Australia, existing vegetation has been through quite variable climate changes in the past and may be more tolerant to climate changes in the future,” he said.

“Conversely the vegetation in south-western Australia is only biodiverse for the very reason that it has not experienced climate changes in the past, therefore it may be especially sensitive to climate change in the future.”

Dr Susan Lawler, Head of La Trobe University’s Department of Environmental Management and Ecology said the study provided a new perspective on how biodiversity works.

“We always think of lots of species as being a healthy environment but this study has pointed out that having lots of species is not just about new species being born but about old species not going extinct,” she said.

“Some hyper diverse areas in Western Australia and South Africa are not diverse because their speciation was any different but because they managed to escape extinction during the last Ice Age.”

Dr Lawler said the study showed how drastically landscapes change even with subtle climate shifts.

“When there’s a big climate shift, it’s still really hard to predict what is going to happen in the future,” she said.

“We will see changes in species distribution and extinction due to climate change but we aren’t sure where that’s going to happen yet.”

Dr Lawler said the study also demonstrated the value of research collaboration across regions.

“These are botanists in different parts of the world, South Africa and Australia, and by comparing knowledge of their local species they were able to find out something interesting about the rest of the Earth.”