The decline of the Brushtail Possum, Trichosurus vulpecula (Kerr 1798), in arid Australia.

Abstract

That the brushtail possum was once common and widespread in the arid zone is confirmed by a collation of historical information. Although possums were widespread, detailed records from the Northern Territory and South Australia show that they were most abundant in rocky ranges and outcrops and along watercourses. Possums are now rare in the arid zone. In considering the reasons for the decline of this apparently robust species we have further developed Morton's (1990) model for mammal decline in the arid zone. Our hypothesis is based on the premise that disturbance of refuge habitat patches critical for the survival of the species was occurring at the same time as the country was subjected to a below average water balance. During the period between 1920 and 1970 rainfall was either average or markedly below average with no exceptional rainfalls recorded. Analysis of the rainfall data using a cumulative residual mass curve suggests that sub-surface waters were probably not fully recharged, placing a natural stress on the refuge habitats. At the same time, possum populations were being affected by many disturbing factors introduced by European settlement. Once the populations were reduced, they fell into a 'predator- pit' through depredation by dingoes and introduced predators and were unable to increase in numbers even with the advent of improved conditions. In this paper we identify the need to accurately determine the refuge habitats of a species and then utilise tools such as the cumulative mass residual curve to predict when these habitats will be under most stress. Specific protection measures can then be developed in association with land managers in order to reduce disturbance at the most critical times.