Modelling survival and capture probabilities of the mountain brushtail possum (Trichosurus caninus) in the forests of south-eastern Australia using trap-recapture data

Abstract

The results are described of a capture–recapture study of the Australian arboreal marsupial, the mountain brushtail possum (Trichosurus caninus Ogilby). Adult, subadult and juvenile T. caninus were captured during 11 trapping periods and at 70 different trap placement sites between June 1992 and November 1995 in a 40 ha area of mountain ash (Eucalyptus regnans F. Muell.) forest in the central highlands of Victoria, south-eastern Australia. A total of 80 (37 male and 43 female) T. caninus were captured, comprising 45 adults, 14 subadults and 21 juveniles when first trapped. This gave a total of 333 captures of T. caninus from approximately 4600 trap nights.

The Jolly–Seber–Cormack approach was employed to model the frequency of recapture as simple product functions of the survival and capture probabilities for each inter-trapping interval. Using this method, we explored the relationships between survival and capture probabilities of T. caninus and a range of potential explanatory variables including sex, age class, time, and trap location.

The results of our analyses indicated that there was a high probability of recapture of T. caninus – P = 0.80 (0.03 SE – with females significantly (P < 0.05) more likely to be caught than males. There was spatial heterogeneity across our study site in the probability of recapture of T. caninus, and animals inhabiting areas of old growth were more likely to be retrapped than animals in younger forest where potential nest sites within large standing dead trees with hollows are decaying rapidly and are at high risk of collapse. There were no significant differences between the survival rates of different sexes or age classes, and the monthly value for the probability of persistence calculated for T. caninus was 97.7% (0.48%).

Our study demonstrated that T. caninus is long-lived with high levels of survival among individuals of all age classes once young animals have emerged from the pouch. Animals have relatively high site affinity and are readily recaptured, often at exactly the same place, or very close to, where they were first caught. Some juveniles have taken up permanent residence in the natal territory, which may result in opportunities to mate with close relatives.