Overbrowsing, and Decline of a Population of the Koala, Phascolarctos cinereus, in Victoria 111. Population Dynamics

Roger W. Martin

Department of Zoology, Monash University, Clayton, Vic. 3168.

The demography of a population of koalas which were severely defoliating their preferred food trees was monitored for three years from August 1977. Population density declined from 3.O animals per hectare in winter 1978 to 0.7 ha-' in winter 1980. Population density also fluctuated seasonally with a maximum in autumn-winter (March-August) and a minimum in the spring (October) of each year; this appeared to be due to localized dispersal of animals into the surrounding forest. Sex ratio favoured females (Pf = 0.55) and there was a high proportion of old animals in the population. Fertility rate was low (11-36%) compared with that of other populations and declined over the period of the study; this was due to depressed fertility among the younger females and substantial infertility among the older females. Mortality was heaviest in the winter and the majority of carcasses found were of old females. A sharp decline in population density, caused by the dispersal of many of the residents, occurred in winter 1980 when the defoliation of the preferred food species, Eucalyptus ovata, was most advanced. Historical information suggests that koala populations have always been subjected to severe disturbances and have experienced declines, but have been able to recover because of the koala's high reproductive rate and powers of dispersal.