Australasian Science: Australia's authority on science since 1938

The Devil Is in the DNA

Credit: Menna Jones

Credit: Menna Jones

By Anna Brüniche-Olsen & Jeremy J. Austin

DNA analysis reveals that Tasmanian devils survived a major population decline thousands of years ago, leaving them with low genetic diversity to withstand devil facial tumour disease.

When Europeans first arrived in Tasmania they encountered a strange creature. It was black with white markings, the size of a dog, and had an impressive set of teeth. This loud and rowdy scavenger, mainly active at night fighting over carcasses, puzzled them. They named it the “Tasmanian devil”. Two hundred years later we know a lot more about devil biology, but they stand at the brink of extinction and have become a symbol for the conservation of biodiversity worldwide.

Along with the Tasmanian tiger (or thylacine), devils were once widespread on mainland Australia. The first people in Australia depicted Tasmanian devils in their rock art, while fossils of devils have been found in caves across the mainland, from the south-west of Western Australia and across the Nullarbor to Victoria and New South Wales.

But around 3000 years ago the devils disappeared from mainland Australia. Recent analyses suggest that increases in Aboriginal populations during the past 10,000 years – along with a changing climate and the introduction of the dingo – have contributed to their mainland extinction.

After Europeans settled in Tasmania in 1803, devils suffered at the hands of the early farmers. They noticed that their chickens and sheep disappeared – and blamed the Tasmanian devils. To cope with the threat to their livestock, the Van Diemen’s Land Co. enforced an extensive bounty-hunting scheme to eradicate the devils. The program was a success and the population drastically declined, and became at risk of being hunted to extinction. In order to save the species, devils were declared rare and protected by law in 1941.

For decades the Tasmanian devils thrived due to their new protected status. Devils were more often encountered in the wild and tourists travelled to Australia’s island state to see this iconic animal.

All was blooming for the species until 1996, when sightings of devils with large facial disfigurations were reported in the Mount William National Park in north-eastern Tasmania. Devil facial tumour disease (DFTD) had emerged.

DFTD is a contagious cancer that spreads from devil to devil via biting. It is one of only two known transmissible cancers, and is 100% fatal. In less than two decades DFTD spread to the majority of the species’ geographic range, causing the population to decline by more than 80%. Once again the Tasmanian devils found themselves at risk of extinction.

Severe reductions in population size – such as those imposed by the bounty-hunting scheme or the spread of DFTD – are a concern. Each time a population decreases in size it is likely to lose genetic diversity. It is this genetic diversity that makes a species able to adapt to changes in the environment or diseases, so losing genetic diversity is a major concern.

The Tasmanian devil has low genetic diversity and its evolutionary potential is thereby limited. Combined with the spread of DFTD, this has raised concerns for the species’ long–term survival.

So when did the Tasmanian devil lose its genetic diversity? Was it a result of the bounty hunting? Was it due to the spread of DFTD? Or has the Tasmanian devils always had low genetic diversity? We decided to investigate this in a study that has now been published in Biology Letters.

The devil’s own DNA can provide clues to its past. When a species’ population size changes, up or down, it leaves behind traces of both the timing and size of the change in the DNA.

We sampled ear biopsies from Tasmanian devils from all parts of Tasmania. Using highly variable DNA sequences called microsatellites from more than 300 Tasmanian devils, we generated a genetic fingerprint for each individual. From this genetic data we could not only estimate the species’ current genetic diversity but also investigate changes in population size over the past 50,000 years.

What we found was a surprise. Contrary to expectations, neither the European bounty hunting nor the spread of DFTD had caused the current low genetic diversity. Rather, the devils lost the majority of their genetic diversity millennia before when the Earth’s climate went through massive changes.

Over the past two million years, the Earth’s climate has gone through natural fluctuations at semi-regular intervals, from warm and wet to cold and dry periods. Each period has had major impacts on the abundance and distribution of animals and plants.

The most recent “cold” period occurred 22,0000 to 18,000 years ago, and is known as the Last Glacial Maximum. During this time, large, cold-adapted megafauna such as the mammoth and the woolly rhinoceros were found across much of the Northern Hemisphere.

The Last Glacial Maximum also changed the climate in the Southern Hemisphere. Australia became more arid and much colder. The vegetation changed from eucalyptus woodland and rainforest to be dominated by grassland and scrub.

Around 10,000 years ago the climate warmed and was stable for thousands of years until around 5000 years ago when it got colder again. This was during a 2000-year period where an increase in El Niño–Southern Oscillation (ENSO) activity led to a climate mimicking the cold conditions of the ice age. During these cold periods the Tasmanian devil population size declined.

So how does climate change impact the population size of the largest marsupial carnivore? We believe that climate impacted devil populations through a sequence of events. A change in vegetation may have led to a loss of den sites, thus reducing the ability of devils to breed successfully. Vegetation change may also have negatively affected populations of prey species such as wallaby, kangaroo and smaller mammals. With less prey, devil populations may also have declined.

Our results suggest that 4000–2000 years ago during a period of increased ENSO activity the devil populations declined by more than 80%. Our results also suggest that an earlier decline took place approximately 30,000 years ago, prior to the Last Glacial Maximum.

The devil populations thus went through severe bottlenecks, and have existed for thousands of years with low genetic diversity. So the loss of genetic diversity took place long before Europeans settled in Tasmania and long before DFTD emerged and became a major extinction threat.

It is important that we conserve Tasmanian devils as they are an essential part of the Tasmanian ecosystem. Not only are they important for removing carcasses from the landscape, and indirectly help prevent the spread of diseases, they may also help suppress introduced pests such as foxes and feral cats. These introduced predators have had devastating impacts on small mammal and bird populations on mainland Australia. Research suggests that if a top–predator such as the Tasmanian devil is removed from the ecosystem, the abundance of smaller mammals are likely to decrease as a result. It is therefore important that we help to protect and conserve the devils to avoid a cascade of species declines.

Climate predictions for Australia point towards the climate becoming hotter and drier in the future. This raises concerns for the Tasmanian devil as well as other species. They will all have to adapt to the changing conditions. Given our findings that the Tasmanian devil population has drastically declined and lost genetic diversity during periods of climate change, we need to assure that we conserve as much as possible of the species’ genetic diversity to enable them to adapt so they can continue to fulfill their important role in the Tasmanian ecosystem and fascinate generations to come.

Anna Brüniche-Olsen is a PhD candidate at The University of Tasmania. Jeremy J. Austin is Deputy Director of the Australian Centre for Ancient DNA at The University of Adelaide.