Since European settlement, many Australian native mammal species have become extinct. The majority of these extinctions have occurred within the arid and semi-arid regions, amongst small and medium sized species. Explanatory hypotheses include habitat degradation by introduced herbivores and predation by feral predators. Yellow-footed rock-wallabies (Petrogale xanthopus)
are a medium sized macropod. existing in disjunct populations across the semi-arid zone. The New South Wales (NSW) wallaby population is currently listed as endangered. Annual monitoring of the NSW population documented its gradual decline and, by 1992. it had reached dangerously low levels (< 100 individuals). In contrast, the Queensland population is ostensibly stable, with the species listed as common.
The initial aims of this study were to examine the space use patterns and demographic characteristics of a single large colony of wallabies (>100 individuals) from the Queensland population. Later, the study expanded to encompass an examination of the cause of suppression of the NSW wallaby population, the main focus being a fox removal experiment. Demographic data from the Queensland colony provided a comparative benchmark for the declining NSW population.
A telemetry study was conducted at the Queensland study site, between winter 1992 and winter 1994. The wallabies moved up to 1.5 km away from their refuges to forage and drink. Foraging range size was dependent on both sex and the direction in which the animals foraged. Although foraging ranges were larger during the drier seasons, no significant seasonal or time effects were observed. The estimated mean home range size was 23.5 ha, which is considerably smaller than that reported by Lim (1987; 134 ha to 202 ha) in South Australia. The large distances moved by the wallabies away from their refuge sites while foraging and their preference for feeding within open herbfield habitat potentially placed them at a high risk from predation.
The size of the Queensland colony was estimated to range between 140 and 180 individuals. Between winter 1992 and winter 1994, the colony underwent a 48% decline in size. Annual exponential rates of population change were comparable to those observed during the slow decline phase of the NSW wallaby population. Reproductive output was consistently high, but very few juveniles were recorded in the population. It is probable that the high reproductive output were offset by high rates of juvenile mortality, as dispersal was low (<1%). Pouch young and juvenile survival ranged between 0.50 to 0.64 and 0.34 to 0.47. respectively. Signs of possible fox predation were found on 70% of the fresh juvenile carcasses found. A general aging of the population was observed, suggesting that the colony was in a state of slow decline following a pulse of recruitment during the above average conditions of 1990. Annual adult survival was relatively high and constant (0.76). suggesting that management actions in floundering colonies should focus on increasing juvenile survival rates.
An (un-replicated) predator removal experiment was conducted within the NSW wallaby population, between June 1995 and June 1999. 1080 baits were applied around the Coturaundee Range sub-population, while the Gap Range colonies were left as an un-baited experimental control. Prior to the commencement of baiting (1992 to 1995). both sub-populations had similar dynamics. Following baiting, the Coturaundee Range sub-population underwent a six-fold increase in size, while no change was observed in the Gap Range sub-population. Juveniles were common at the baited site, but rare at the un-baited site. This suggested that fox predation was primarily focused on juvenile wallabies and was acting to limit the recovery of the wallaby population. The existence of only two extant NSW wallaby sub-populations, severely limited the experimental design. To counter this inherent flaw, a number of other variables (feral goat, feral cat and wedge-tailed eagle abundance, wedge-tailed eagle diet and pasture cover) were monitored. No evidence was found to suggest that any alternative treatment effect influenced the outcome of predator removal experiment.
No evidence was found to suggest that fox predation was acting in a regulatory fashion or was the causal agent behind the NSW populations" initial decline. Such conclusions can only be drawn from a two phase experimental procedure. whereby treatment effects are reversed and the effects on each prey sub-population observed. The experimental confirmation of population regulation is important because the determination of agents of decline is crucial in endangered species management and because it would provide valuable insights into Australian arid zone extinctions.