Australian Journal of Ecology {I9i&) t3, 445-450 The impact of predation by the powerful owl, Ninox strenua, on a population of the greater glider, Petauroides volans RODNEY P. KAVANAGH Forestry Commission of NSW, Wood Tech- nology and Forest Research Division, PO Box 100. Beecroft, NSW 2119, Australia Abstract Powerful owls werefrequently observed during a study ofthe ecology of a community of arboreal marsupials in south-eastern New South Wales. For about 17 months the population of greater gliders in the 100 ha study area appeared to remain 'stable' at more than 80 individuals. In the following 46 months, the population declined to about one-tenth of its previous level. The forest in the study area was unlogged and remained undisturbed during this period. The frequency of sightings of powerful owls holding captured greater gliders, and of observations on the ground of tails and bodies belonging to greater gliders, and the unaccountable disap- pearance of 9 out of 11 individually marked greater gliders in the study area, suggest that the observed decline in the population of greater glitiers was due to predation by the powerful owl. Powerful owls were not detected in the study area during the first 12 months or the last 21 months of the study. It is suggested that powerful owls forage by concentrating their activities in pockets of their large home range until they reduce the populations of their pre- ferred prey below limits where it becomes diff- cult to catch the remaining animals. If preferred prey are available elsewhere, powerful owls probably move their centres of foraging activity to these pockets and harvest them before moving on to the next pocket of their range. Introduction The powerful owl Ninox strenua (Gould) and the greater glider Petauroides volans (Kerr) are two of the largest species of forest-dependent fauna in south-eastern Australia. The greater glider, weighing 1000-1700 g, is a sedentary, slow-moving folivore which occupies a small home range of about 2 ha (Henry 1984; Kehl & Borsboom 1984). The abundance of this species may exceed 84 individuals per 100 ha in good habitat (Kavanagh 1984). The powerful owl is of similar size (1050-1700 g) and is also sedentary, but the minimum size of the territory for a pair of birds in good habitat is estimated to be at least 800-1000 ha (Fleay 1968; Seebeck 1976; Schodde& Mason 1980). Powerful owls forage opportunistically. They select the largest available but manageable prey, which is usually mammalian (Fleay 1944, 1968; Seebeck 1976; Hyem 1979; James 1980; Van Dyck & Gibbons 1980; Tilley 1982). When they are available, the greater glider and the ringtail possum, Pseudocheirus peregrinus, are the most important prey to the powerful owl. Nothing is known about the impact of predation by the powerful owl on populations of its major prey species, nor about the way in which these owls utilize their large home range. Populations of greater gliders are thought to be regulated in stable environments by factors affecting the birth rate, as well as by age- specific mortality (Tyndale-Biscoe & Smith 1969a). Powerful owls were frequently observed dur- ing a study of the ecology of a community of arboreal marsupials in south-eastern New South Wales. This study provided an oppor- tunity to assess the impact of predation by powerful owls since the populations of all species of arboreal marsupials, including the greater glider, were regularly monitored (Kavanagh 1984). The frequency of observa- tions of powerful owls hunting in the study area provided an indication of the way these birds use their home range. Methods The study area was situated in the Waratah Creek catchment of the Coolangubra State