Brush-tailed rock-wallabies hop in from the cold

Innovative fine-scale climate maps help explain the distribution of an endangered species.

The endangered brush-tailed rock-wallaby (Petrogale penicillata) prefers to live on rocky hills, and until now, we assumed this was just because rocky habitats helped them avoid or escape predators. However, by looking at fine-scale variations in temperature, we have discovered that it may also have a lot to do with avoiding the cold. This finding may help predict how this endangered species might be affected by future changes in climate or land-use.

Within any one region, local temperatures can vary a lot. Due simply to the impact of hills, forests and proximity to the ocean, temperature may vary by as much as 20°C within a region such as the Hunter Valley (~60,000 km2). Even small hills can be over 9°C warmer than the valleys below as cold air drains downhill on frosty winter mornings.So it makes sense that the endangered brush-tailed rock-wallaby may be seeking hills to live not only because of the rocks, but also because of temperature.

We recently set out to investigate this, using novel climate maps we’ve produced for the Hunter Valley region of NSW. We’ve collected more than four years of hourly temperatures and humidities across a variety of habitats including coastal dunes, rainforest gullies, eucalypt woodlands, and rocky hills, and then used the data to map the warmest and coldest locations across the region.

There are several reasons why the brush-tailed rock-wallaby might be sensitive to cold. They are descended from tropical ancestors, they love basking in the sun even though this exposures them to predators, and they tend to stay in their rocky shelters rather than forage when it is wet and cold. We also had a hunch that they are usually found in the warmer locations in the landscape.

We therefore set out to determine whether brush-tailed rock-wallabies were often found on rocky hills because of the underlying topography, the warmer minimum temperatures, or higher vegetation and less disturbance in these rugged locations. Using occurrence records of brush-tailed rock-wallabies from the Atlas of Living Australia and a personal database from the NSW Office of Environment and Heritage we modelled their distribution using a variety of topographic, climatic, and habitat factors.

As predicted, we found that wallabies were found less often in locations exposed to extreme cold. In addition, minimum temperatures were able to predict their distribution better than either vegetation or topography (elevation, slope and aspect). We found that rock-wallabies occurred more frequently where minimum temperatures remained high, rainfall was low, and there was ample tree cover in the surrounding area.

Our study highlights how knowledge of fine-scale variations in temperature can help us understand the distribution of plants and animals and make predictions of how they might be affected by future changes in climate or land-use.

It is frequently reported that species must make large shifts in altitude and latitude to cope with a changing climate, but there are also opportunities for species to seek refuge by making use of the variations in temperature that occur within regions.

Dr Mick Ashcroft
Scientific Officer

More information:
Ashcroft M.B., Cavanagh M., Eldridge M.D.B., Gollan J.R. (2014) Testing the ability of topoclimatic grids of extreme temperatures to explain the distribution of the endangered brush-tailed rock-wallaby (Petrogale penicillata). Journal of Biogeography, in press.

This research was a collaboration between the Australian Museum Research Institute, University of Technology Sydney and NSW Office of Environmental and Heritage.