Ice age extinction shaped Australian plant diversity

Researchers have shown that part of Australia’s rich plant diversity was wiped out by the ice ages, proving that extinction, instead of evolution, influences biodiversity.

The research led by the University of Melbourne and University of Tasmania has shown that plant diversity in south-east Australia was as rich as some of the most diverse places in the world, and that most of these species went extinct during the ice ages, probably about one million years ago. 

The team’s work was published in the prestigious US Proceedings of the National Academy of Sciences recently.

Dr Kale Sniderman of the University of Melbourne’s School of Earth Sciences says the findings show extinction is just as important to diversity of organisms as evolution.

Traditionally scientists believed some places have more flora species than others because they evolved more rapidly in these locations. 

“We have overthrown this theory, which emphasises evolution, by showing that extinction may be more important,” Dr Sniderman says.

“The general understanding of the history of Australian vegetation over the past 20 million years is one in which slow drying of the continent drove formerly widespread rainforests to extinction.

“It was thought the rainforests were replaced by sclerophyll vegetation, which now covers most of Australia (which we in south-east Australia know as the typical bush landscape). 

“The onset of the ice ages in the past million years or so was thought to have simply reinforced this by introducing even drier climates during glacial periods.”

However, Dr Sniderman says fossil pollen extracted from a remarkable sediment drillcore at Stony Creek Basin near Daylesford hinted there was much more to the story.

“The millions of pollen grains I found showed that, around one-and-a-half million years ago, the vegetation cycled back and forth between rainforest (now extinct in Central Victoria) and sclerophyll forest. We found many leaf fossils that were beautifully preserved, often in full anatomical detail, which was exciting because most of the fossil record is of rainforest species, even though most of Australia is now dominated by sclerophylls.”

Dr Sniderman’s colleague Dr Greg Jordan of the School of Plant Sciences at the University of Tasmania used electron microscopy to painstakingly study about 1200 individual sclerophyll leaf fossils, and found a surprisingly large number of species. 

“Even those fossils which looked quite similar under a low power light microscope were actually quite distinct,” Dr Sniderman says.

“We then asked how diverse this was, compared with different vegetation types in the modern world. We knew that the fossils could not have been transported very far before they ended up together in our lake mud, and therefore this fossil diversity must reflect the diversity of a fairly small area of ancient vegetation. 

“By comparing our fossil diversity with previous studies on the transport of modern leaves into lakes, we realised suddenly that we were looking at something very interesting. Our ‘Eureka!’ moment came when we realised eastern Australia once had a sclerophyll flora as diverse as south-west Australia does today.”

This realisation had important implications for understanding the history of Australian flora, but more generally had implications for understanding what allows the floras of some regions to be much more diverse than others.

“South-western Australia has a huge diversity of tough-leaved shrubs and trees such as Eucalypts, Banksia, Grevilleas and Acacias, making it one of the most biodiverse places on Earth,” Dr Sniderman says.

“The southern tip of South Africa is even richer, with astonishing numbers of similar kinds of plants like Proteas and Ericas.”

Dr Jordan says not only has the study overturned current thought on the role of extinction in plant diversity, it has implications for understanding how Australian plant diversity will deal with current and future climate change.

“The species that went extinct in south-east Australia during the ice ages were likely to be the ones most sensitive to rapid climate change, meaning the species that now grow in eastern Australia may be more capable of tolerating rapid changes than predicted by current science.

“However, the species in hotspots of diversity like Western Australia may be much more sensitive to future climate change, because they have been protected from past climate changes.”

The study was carried out in collaboration with the Nelson Mandela Metropolitan University in South Africa.

www.earthsci.unimelb.edu.au/