Recovery plan for the brush-tailed rock-wallaby - Department of ...

Approved NSW Recovery Plan Brush-tailed rock-wallaby
colonies existing in 18 sites in central, southern and western NSW and more than 100 colonies in
15 sites in northern NSW (Dovey et al 1997). The survey found a further 14 sites where the
animals had become extinct. Since 1993, the BTRW has disappeared from another five sites,
meaning it has become extinct at, or vacated, 49 sites since 1990. A National Parks and Wildlife
Service survey in 1995 located 15 colonies of BTRWs in Yengo National Park and Parr State
Recreation Area and surrounding areas (Rummery et al 1995).
Past variations in rock-wallaby distributions and genetic variation are thought to be due to
bioclimatic variables (Eldridge and Browning unpub., Cavanagh unpub.). In particular, modelled
shifts in the climate of the ranges of the BTRW and Herbert’s rock-wallaby (Petrogale herberti)
indicate that oscillations and overlaps in the two species’ ranges over time may be factors in the
development of hybrids between the species. Climate modelling also indicates that the presence
of the BTRW in the Grampians region may have occurred only within the last 20,000 to 10,000
years (Cavanagh unpub.).
5.2 Abundance
During the late 1880s, BTRWs were abundant and widespread across the rocky country of southeastern
Australia from southern Queensland to Victoria. From 1900 until about 1920, hundreds
of thousands were shot as agricultural pests and hunted for fur (Lunney et al 1997). Their
numbers have continued to fall in most localities and it is now estimated that there are between
15,000 and 30,000 animals left. Gaining a more precise estimate of numbers is difficult due to
the inaccessibility of the species’ habitats, particularly in the north where numbers are greater.
Approximately 17% of BTRWs occur in south-eastern Queensland, 82% in NSW, and fewer
than 1% in Victoria. In NSW, as many as 98 % of BTRWs are found north of the Hunter River,
and up to 80 % of the total number of BTRWs in Australia are found in north-eastern NSW (see
Figure 8). Most northern NSW populations are in the Macleay River and Clarence River gorges
(Bayne pers. comm., Dovey et al 1997).
While it is possible that numbers in the north of the species’ range were always higher than in
the south, substantial evidence indicates the relative numbers in the south have been drastically
reduced (e.g. Lunney et al 1997, Short and Milkovits 1990). Accurately estimating abundance
remains one of the challenges in recovery planning.
A recent study by Piggott et al (2006a) explains a promising new technique for estimating the
population size of BTRW colonies. In a study of four BTRW colonies in Wollemi National Park,
individual BTRWs were identified by genetic analysis of scats. DNA fingerprinting of genetic
material from faeces can distinguish individuals and provides a minimum number of animals
alive at the time of sampling. This method can also distinguish between males and females, and
can detect birth and recruitment over time. Repeated sampling in colonies can then provide an
estimated population size. In the Wolgan Valley, this technique determined that there was one
large colony of approximately 67 animals, two smaller colonies of approximately 11–17 animals
and a very small colony of 2–4 animals. In the smallest colony, there was evidence of an
increase in population size over a two-year period due to the birth of a new individual from a
female previously identified in a different sampling time (Piggott et al 2006a).
An added advantage of this method is that the genetic data obtained can also be used to infer
dispersal patterns and population structure.
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