Possum magic survives Black Saturday

31 March 2014

This story originally appeared on Radio National. Listen to the audio here.

After the 2009 Black Saturday bushfires in the Central Highlands of Victoria, only one species of possum endured in the blackened areas: the mountain brushtail.

It's the biggest of Australian mainland possums, and is also a part of one of the longest running ecological surveys in the country.

The survey is led by Professor David Lindenmayer of the Fenner School of Environment and Society at the Australian National University, a passionate and outspoken conservationist and ecology expert.

The survey started 31 years ago when Lindenmayer was given the job as a technical assistant to Dr Andrew Smith, studying the habitat requirements of the Leadbeater's possum.

'The work started on Leadbeater's possum,' says Lindenmayer, 'but very quickly we realised that to understand where that animal occurs and why it occurs where it does, we needed to know not only about all of the other possums and gliders in this magnificent forest, but we also needed to know how the forest was disturbed by fire, how it was disturbed by logging, how it was changed by those things and how the animal responded to those disturbances.'

Even before the fire, the mountain brushtail was one of the more populous species in the survey area. It kept on turning up in the Leadbeater's survey sites, but it wasn't until after the fire that the true adaptability of the species became apparent.

Dr Sam Banks now leads the mountain brushtail section of the study, looking at various aspects of the species including genetics and behavioural research.

Banks is a tall, calm, salt-of-the-earth type, whose gentle manner with students and volunteers hides a keen scientific mind.

'As a matter of fact in the week leading up to the fires we were out here trapping and fitting radio collars to these animals to do a study of what sort of habitat and what sort of things they eat and things like social behaviour,' he explains.

'We fitted radio collars to, I think, 13 animals in the week leading up to the fire and promptly left the night before the fires for good reason. But as soon as we could get in here, about two months after the fires happened, we were surprised that all of them were perfectly fine, alive and well.'

Lindenmayer explains that many of the bird species that were on those sites also remained there, even though the forests had been fried.

'But it was not so for possums and gliders, even five years after the fire we know that there is only one species that seems to have weathered the storm: and that's the mountain brushtail possum. There are no possums and gliders of any other species on any other [fire affected] sites.'

The survival of this species could be in part due to its ability to adapt its foraging and also its flexibility within social behaviour.

'You'd call them a herbivore, but they're not far from being omnivorous,' says Banks.

'Around here we're trying out some new GPS radio collars, and we were finding last winter that they would cross about 500m of forest, and ten other possums' territories, to go and feed on pine cones from an old pine tree that was planted at the old Cambarville settlement.'

It some ways, it's not surprising to the lay person that the brushtail is such a tenaciously adaptable beastie-it's family trait.

Its cousin, the common brushtail (Trichosurus vulpecula), is the possum that has boisterous dance parties complete with the possum version of heavy metal mosh-pit in your roof at all hours of the morning.

Those common brushtail interactions you might hear-the hissing and galumphing around-are also something that the researchers have been looking into with their bush cousins.

Banks admits that the studies of social interaction might seem esoteric at first, but they have uncovered social adaptability that might be contributing the survival of the possum in the post-fire landscape.

'[Banks has] made remarkable discoveries about the social behaviour and the social cohesion of kinship groups in this marsupial. Nobody had even contemplated anything near as complex as Sam has been able to find,' says Lindenmayer.

'These animals are actually quite flexible in their social behaviour in response to things like how many hollow trees you get in the landscape, so the loss of hollow bearing trees is one of the most important threats to these animals.'

After the fires, some burnt areas were 'salvage felled'-a practice where loggers take any large trees still left standing-which further reduced living quarters for possums.

And that pressure is unlikely to reduce any time soon. Even if the area is then left to grow new forest, a mountain ash will take at least 70 years to grow to maturity and produce liveable possum condos, and probably more than 100 years to be a suitable habitat for multiple species. In such an atmosphere, the mountain brushtails' social flexibility has been the key to their success.

'Here at Cambarville we've noticed that they essentially start forming these tight, quite sociable, family groups in areas where you don't have many hollow trees yet.'

'When things are going down the tube, you after your own and become a bit more sociable,' says Dr Banks. 'Whereas when conditions are good, you couldn't really care about close relatives.

The data collected in the specific study on mountain brushtails adds to the 31 years of data collected from trapping, counting and surveying flora and fauna across the region, including significant data on the Leadebeater's possum. And though the brushtail isn't the most charismatic, fluffy or sexy of all of the animals, for Lindenmayer, the only conservation ecologist to be with the project since the beginning, the study of the species has brought him immense pride and led to the naming of a new species.

'I'll never forget the day I was in Canberra, and we were looking at some data on measurements on mountain brushtail possums we had made from here in the Central Highlands, but also in Gippsland and right up to Gladstone in Queensland.'

Lindenmayer told statistical science colleague Adjunct Professor Ross Cunningham that he was expecting the data to follow the course of Bergmann's rule: that in warmer climates the specimens would be smaller, and further south in cooler climes the specimens would be larger.

'Ross said "no, let's have a look of this, we don't ever think about jumping to conclusion without looking at the data,"-typical statistician,' says Lindenmayer.

'So he plotted the data and we looked at it and, lo and behold, there were two completely separated groups of animals, and that was the start of this process of having discovered the northern species, which we now call the short eared possum, and the mountain brushtail possum.'

'Then the genetics confirmed that these animals were as far apart as, for example, two or three of the species of gazelles are in Africa.'

'So we named the scientific name of the southern species, the mountain brushtail possum, Trichosurus cunninghami after Ross Cunningham, the statistician,' says Lindenmayer.

'We had to take his name out of the paper, because that's scientific etiquette, but it was quite a remarkable thing.'

'This is only 10 years ago, that in this amazing country, one and a half hours from the MCG, you can still make a discovery of a new species-and it's not a little beetle, not small flies-this is a four kilogram possum!'