Features

Brushtail and ringtail possums are familiar animals to most people, even to those living in the cities. Both seem to have adjusted well to suburban and town life, though still fall prey to domestic cats and dogs. However, there is an alarming trend evident in the western woodlands that towns may be one of their last refuges.
Factors in the decline of possums

The decline in possum numbers has been documented in historical evidence and recent data across Australia. The western ringtail is rare in the woodlands of WA; centralian populations have suffered major declines; and possums are becoming rarer in the woodlands of Queensland and Victoria.

What has caused these declines of once common species? There are probably a number of causes, but which are the most important? Dr Anne Kerle, who has studied possums for many years particularly in the Northern Territory, has suggested a sequence of critical events which could help an understanding of possum decline. This pattern also gives some insight into the decline of a variety of native mammal species1.

Brushtail possum hiding out
Photo: David Paull

The key elements are habitat loss; cycles of resource-poor seasons; and predation. The trend appears to be most evident in drier woodland environments, where the balance between resource availability and population size is most finely tuned. Times of long dry periods can bring resource decline; in response, wild populations adjust by slowing down reproduction. Predation also becomes a significant factor on wild possums during this time, as animals become more stressed and therefore need to travel further over the ground to reach food as well as being more inclined to seek water. Ringtails are particularly susceptible to predation on the ground, especially to foxes and cats.

Most important for possum food and shelter are mature hollow-bearing trees. In drier areas it is well documented that both brushtails and ringtails require hollows to live, and ringtails are not inclined to build dreys or outdoor nests. This may be because the very cold nights and temperature fluctuations in the more arid areas necessitate more reliable shelter. It is an important factor in the successful rearing of a litter of young.

When conditions improve, surviving animals are able to recover and re-populate. The habitat is still there for them to disperse into and utilise. The effect of predators even during scarce times is never great enough to entirely wipe out local populations because the bush remains.

What can tip this natural balance is the widespread removal of their trees. Dr Kerle found in the Northern Territory that there was once widespread removal of red gums for building and a whole range of other uses, including clearing for agriculture. The red gum woodlands in the riparian zones and on floodplains were important habitats for the centralian possums, but also had the better soils for stock use. Over-stocking further inhibits the natural regeneration of native vegetation, creating ongoing problems for tree recruitment. Widespread habitat loss, when combined with poor seasons, puts additional stress on populations.

Photo: Andrew Cox

David Paull (R) escorts John Ryan (L), State Liberal MLC, on a tour of Goonoo State Forest, another site of important western woodlands. John described Goonoo as "Dubbo's Kakadu" and thinks increased visitation as a national park would be a boon for Mendooran and its famous murals (background) by local Karin Guce.

For possums facing the problem of the loss of habitat, finding secure areas from predators becomes even harder. They need to spend increasing amounts of time on the ground. Possums require about four nest trees per hectare in order to support their social life, and this does not take into account the other fauna which also need shelter in a woodland habitat – owls, gliders, bats, parrots and cockatoos, goannas and pythons. Foxes now take their toll and local populations of possums are not able to recover.

Population crashes have occurred all through the arid regions of the continent and the possums are becoming extinct in the NT. Declines in WA, Victoria and Queensland have been attributed to the loss and fragmentation of their habitat, that is, box and ironbark woodlands. In NSW, these woodlands have been reduced by three-quarters of the pre-European coverage as a result of clearing.

There is evidence in some records of disease being a factor, showing huge declines in apparently good habitat. Disease is also more likely to have a greater effect during times of stress.

More recently, Dr Kerle has found evidence of historic declines of possums in NSW and there are more signs of a recent decline in State forests of the Pilliga2. Possums were recorded in their "millions" in the central west of NSW in the 1800s, and recent records show that possums still survive in three areas – wooded riparian zones, naturally wooded Crown lands and in towns. Recent NPWS surveys found possums in good numbers in travelling stock routes (TSRs) along the Bogan River, and they were also regularly found near watercourses with tree cover. These areas still retain remnant patches of natural vegetation and tree clearing by and large has not occurred.

These TSRs and riparian zones are often thin and linear remnants, but the resilient brushtail is able to hang on. Ringtails are not as aggressive as brushtails, and when resources get tight it is easy to see how a brushtail could displace a ringtail in order to obtain shelter. Ringtail declines and local extinction in the dry woodlands have been more pronounced than for its larger cousin.

Did foxes kill them off? Well, quite probably, but bear in mind the abovementioned sequence. Ironbarks have been harvested for sleepers and building material in the Pilliga for a long time (see also NPJ December 2000). Ironbark operations still proceed, removing up to 10,000 tonnes of sawlogs per year. The average density of narrow-leaf ironbark in the area has fallen to two large trees per hectare from an estimated pre-European density of 20–30 large trees. This is well below the density of mature trees required by possums.

Some areas of relatively untouched forest remain in the Pil-liga. Areas dominated by 40-year old regrowth are common. If left alone, these areas would be allowed to mature, and in another 40–50 years a mature woodland could have developed. Greater protection of the depleted ironbark woodlands would be the guarantee possums need.

However, logging still proceeds and surveys need to be carried out to assess the presence of possums before significant damage is done to remaining tree-rich areas from ongoing operations. State forests support significant native vegetation areas in the western sheep/wheat belt of NSW. Increased protection through new national parks and other conservation reserves in the western woodlands is essential for the survival of two possum species, species that are important indicators of our woodland health.