The koala was a victim of prehistoric climate change

Image by John Llewelyn (Global Ecology @Flinders)

By Farzin Shabani, Frédérik Saltré, Corey Bradshaw, Katharina Peters and Antoine Champreux

Along with the kangaroo and the emu, the tree-living marsupial koala is the most recognised symbols of Australian wildlife. However, the koala might also soon become an icon of Australia’s past as these loveable marsupials struggle to survive throughout much of their remaining range. Distributed as far north as Cairns in Queensland, to as far east as Kangaroo Island in South Australia, the koala’s biggest threats today are undeniably deforestation, road kill, dog attacks, disease, and climate change. With increasing drought, heatwaves, and fire intensity and frequency arising from the climate emergency, it is likely that koala populations and habitats will continue to decline throughout most of their current range.

But fossil records tell us that in the past, the distribution of koalas was not always a zoological feature of only the eastern regions of Australia. While there are unfortunately not many dated koala fossils relative to many other Australian wildlife species (living or extinct), a few have been unearthed. These few fossils suggest that around 150 000 years ago, the distribution of the koala expanded into the Nullarbor Plain and over to south-western Western Australia.

Green indicates present koala distribution and red crosses show where koala fossils have been found.

This remarkably different distribution to today’s in what is — in geological terms anyway — the recent past, suggests that the eucalypt forests on which koalas rely for food and shelter must have had rather different distributions than they do today. This led us to question whether the eucalypt forests characteristic of the relatively wet east and southeast of Australia covered a much broader range in the past than they do today. If so, did they extend across the now-treeless Nullarbor and into Western Australia during at least some periods over the last few hundred thousand years as the fossil record implies?

In our recently published Ecography paper, we address these very questions by using mathematical models (correlative species distribution models) to predict the past distributions of 60 different trees (mostly Eucalyptus species) eaten by koalas today.

We applied these models based on the present distributions of the koala-browse species to construct a set of environmental ‘rules’ that dictate where each species can grow. Using climate data such as temperature and rainfall, as well as soil data, we then applied these environmental rules to simulated past climates from general circulation models. Projecting backward (hindcasting) to three different periods in Australia’s past — (1) during the Last Interglacial period from 128 000 to 116 000 years ago, (2) the Last Glacial Maximum from 23 000 to 19 000 years ago, and (3) the Mid-Holocene between 7000 and 5000 years ago — we reconstructed the most probable distributions of koala-suitable forests across the continent.

Our models predict a marked increase in the amount of suitable habitat for koalas between the Last Interglacial and the Last Glacial Maximum, followed by a rapid loss of forest extents up to the mid-Holocene and to the present day. These changes in the distribution of koala-suitable forests over these intervals therefore support our hypothesis that coastal forests likely extended from the east through the Nullarbor, and into Western Australia. This would have been enough to allow koala populations to cover this entire region. However, as the forests retracted eastward, they left only small populations of koalas in habitat refugia in the southwest, or disappeared entirely in the case of the Nullarbor.

Confident in our hindcast projections, we used the same approach to project the distribution of koala habitats into the future. And it is not good news — our projections for 2070 confirm that this trend of retracting habitat is likely to continue. Faced with all of the other threats koalas — and most other native Australian species — endure today, this might mean that we could lose the koala forever. But there is hope if do whatever it takes to protect existing koala habitats as well as replant those already destroyed.

Full reference: Shabani, F, M Ahmadi, KJ Peters, S Haberle, A Champreux, F Saltré, CJA Bradshaw. 2019. Climate-driven shifts in the distribution of koala browse species from the Last Interglacial to the near future. Ecography 42. doi:10.1111/ecog.04530

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