The dynamics of hunted populations of brushtail possums Trichosurus vulpecula in Tasmania : a technique for population analysis

A study of the population dynamics of four hunted populations of the‚ÄöBrushtail Possum Trichosurus vulpecula in Tasmania was carried out. Efforts by the management authority, the National Parks and Wildlife Service to monitor and census the species have been hampered by a lack ‚Äöof suitable methods of population assessment. The principal aim of the study was to describe a methodology for timely monitoring of changes in local populations. ‚ÄöThe approach taken was distinctive in that the collection of the sample and the analysis were carried out by separate parties. The analysis of the populations was made from material salvaged from commercial hunters in the course of their normal operations. This division of labour reduced the field time required to virtually zero. The material salvaged from the four populations has been used to.give reliable estimates of some population parameters including age structure, sex ratios, natality and recruitment rates. Predictions of the rate and direction of change in the populations were made on the basis of these estimates. The secondary age ratio was used as an estimate of the rate of recruit-ment. The effects of hunting could be seen in the high rates of recruit-ment in these as compared to unhunted populations. The secondary age ratio gave a measure of the intensity of hunting. Where the estimated production of juveniles was in excess of that needed to maintain the population, characteristics observed in unhunted populations were displayed. These were an age dependent survival between year classes for adults and an adult sex ratio near unity. For populations where the production of juveniles is insufficient to maintain the population these characteristics are altered. Hunting is unselective with respect to age and sex and when this is intense survival between age classes is approximately constant. Social factors mediate dispersal in juveniles so that the bulk of dispersing individuals Are male. A population dependent on immigration to maintain numbers thus tends to recruit males, depressing the proportion of adult females and thus the capacity of the population to produce young. An excess of adult males was found where the production of juveniles was not sufficient to maintain numbers. The status of the populations can be examined in some rletail,\ Differ-ences between populations are readily apparent and the effects of hunting can be gauged. The results are compatible with those from other studies of the species. The method of assessment allows estimates of changes to be made and provides an appropriate means to spot check the status of local populations of T. vulpecula."