Predator-prey studies in semi-arid eastern Australia demonstrated that populations of rabbits (Oryctolagus cuniculus) could be regulated by predators. The functional, numerical and total responses of foxes (Vulpes vulpes) to rabbits and the numerical response of feral cats (Felis catus) to rabbits, are described. Measurement of the rabbit component of foxes' stomach contents indicates a Type III functional response. The size of the fox population in summer was dependent on the availability of rabbits over the immediately preceding rabbit breeding season but there appeared to be no density-dependent aggregation of young foxes in areas of surplus food. The total response of foxes, estimated using the short-term numerical response of dispersing foxes, was directly density-dependent for low rabbit densities and inversely density-dependent for high rabbit densities. Two states are possible with this form of total response: a state with low rabbit densities regulated by predators and a state with high rabbit densities which occurs when rabbits escape predator regulation. The boundary between regulation and non-regulation by predators was demonstrated by a predator-removal experiment. In the treated areas, predators were initially culled and rabbits increased to higher densities than in an untreated area where predators were always present. When predators were allowed back into the treated areas, rabbit populations continued to increase and did not decline to the density in the untreated area. This is the critical evidence for a two-state system. When predators were present, rabbits could be maintained at low densities which were in the density-dependent part of the total response curve for foxes. Exceptionally high rabbit recruitment, or artificially reduced predation, could result in rabbits escaping predator-regulation. Under these circumstances, rabbits could move into the inversely density-dependent region of the total response curve for foxes.

This paper challenges conclusions of Caughley et al. (1980) that the abundance of red kangaroos (Macropus rufus) in western New South Wales is solely due to lack of dingoes (Canis lupus dingo), and vice versa for neighbouring South Australia. A Dingo Barrier Fence divides the two different ecological systems, which have sheep in New South Wales and cattle in South Australia. This paper re-examines in particular whether there is an environmental gradient across the Fence that was dismissed by Caughley et al. This paper concludes to the contrary, that there is a strong environmental gradient. Our aerial surveys demonstrate significantly that habitats favouring red kangaroos are prevalent in New South Wales today, but are very scarce or absent in South Australian landscapes. Aerial surveys were used in both studies, but designs differed. Caughley et al. flew at right angles across the Fence on paths 28 km apart. Flights would have crossed the south-westerly streamlines rarely. Our flight lanes followed streamlines looking for floodouts, the favourite habitat of red kangaroos. Return lanes went between streamlines sampling other habitats. Counts of red kangaroos seen were made every 1.75 km, with the specific habitat also identified. Three extra factors are invoked in our study. One is that the low annual rainfalls translate into intrinsically low survival rates of pouch-young of red kangaroos, contrary to their abundance in New South Wales today. The other two are related to that current abundance also. There is now evidence for greatly increased run-off of rainfall from catchments onto the open plains in New South Wales. Also present is a very large shallow basin lying between catchments and the Dingo Barrier Fence. Streamlines enter it but none flow past its western rim. The above conclusions were confirmed during subsequent ground surveys over three years. Of eleven species of medium and large vertebrates seen in New South Wales, five were absent in South Australia. Three were kangaroos, and the others were feral pigs and goats. Emus are more abundant in New South Wales also. All of those species would be targets for dingoes, especially as alternate prey to rabbits that generate huge eruptions every decade or so. Red foxes (Vulpes vulpes) were in lower abundance in South Australia with dingoes present, as expected with meso-predator interactions. Feral cats (Felis catus) were in similar numbers on both sides of the Fence for unknown reasons. Competition between rabbits (Oryctolagus cuniculus) and sheep for food in New South Wales was shown to significantly reduce rabbit numbers in drought. That rabbits are perennially in lower densities there than in South Australia may be due to the higher densities of foxes than in South Australia. Historically, red kangaroos were rare in the region in the mid-1800s. Their abundance has arisen since European occupation. Thc species was rare on those open plains, and permanent water was scarce. Rabbit Haemorrhagic Disease reached the study-area in 1995. Its impact reduced rabbit populations to a rarity that prevails today on both sides of the Dingo Fence. Predation from dingoes, foxes and feral cats may assist continuance of low numbers of rabbits. Pastures, seedling trees and livestock will benefit, as will the kangaroos.

The diets of the fox, Vulpes vulpes, and feral cat, Felis catus, were studied at Yathong Nature Reserve in semi-arid western New South Wales. The overall occurrence of rabbit was 45.1% in stomachs of foxes and 54.0% in cats, representing 51.3 and 82.6% respectively of the weight of stomach contents. Both predators exhibited a functional response to rabbits, Oryctolagus cuniculus, (their staple prey) during the rabbit breeding season. Predation on rabbits was greatest on an increasing prey population during good pasture conditions and a decreasing population during drought. After the rabbit breeding season, diet changed to other prey and resulted in an annual prey cycle which was similar for foxes and cats. Both predators successfully co-exist in the semi-arid environment by primarily utilising different age groups of the same staple prey and to some extent different supplementary prey. Foxes mainly ate adult rabbits and cats young rabbits. During the drought foxes preyed heavily on adult rabbits; cats ate some rabbits but relied heavily on other food sources. The supplementary prey of foxes were invertebrates, birds, reptiles and carrion; small mammals and fruits opportunely eaten. Invertebrates, birds, reptiles and small mammals were supplementary prey for cats with carrion opportunely eaten.

The hypothesis that carnivores can significantly suppress prey populations after they collapse during drought was tested by predator-removal experiments. Low populations of rabbits (Oryctolagus cuniculus) responded with significantly accelerated growth where foxes (Vulpes vulpes) and feral cats (Felis catus) were continually shot. Experiments in years of good pasture and poor were confirmatory. After only 14 months, the rabbits were well on their way to another eruption whereas untreated populations had remained low for 2.5 yrs until a second drought. These studies confirm the impact of carnivores found for low populations of cyclical prey but there was no measurable effect of predator-removal on the population declines in our studies. They were due to aridity and poor pastures. The concept of Environmentally Modulated Predation is presented. Only after the intervention of a widespread environmental event is such limiting predation possible. Drought is also the cause in arid Australia for dingoes (Canis familiaris dingo) preying seqenntially on rodents, rabbits and red kangaroos, while wildfire was the cause in temperate forests. Such environmental intervention may be more widespread than usually considered, triggering some apparent predator-prey cycles. The major factors limiting rabbits in inland Australia are: adequacy of green herbage during breeding, food scarcity during average summers, critical shortages of food and its low quality (including moisture content) during 'crashes' in drought, followed by limiting predation. Contrasting life-histories are one cause for the ultimate escape of rabbit populations from limiting predation as rabbits can breed continuously but carnivores seasonally only. Patchy predation and alternate prey may also play a part.

The dietary and predator-prey relationships of Canis familiaris dingo were studied for 9 yr at a coastal site and for 1.5 yr at a montane site in south-eastern Australia. The percentage occurrences of items eaten were obtained from faeces, and the abundances of prey by counting waterbirds, trapping small mammals, and tracking large and medium-sized mammals on specially prepared soil plots. Dingoes were also estimated by tracking.The diet was broad but predominantly mammalian (23 species). Dietary frequencies were grouped around three weight modes, 0.1, ] .25 and 16 kg, corresponding with bush rat (Rattus fuscipes), ringtail possum (Pseudocheirus peregrinus) and swamp wallaby (Wallabia bicolor). The amplitudes of those frequencies approximately doubled from modetomode, viz. 4,13.5and28.3%, respectively. Medium-sized mammals have been recognized as the staple prey because of their dependability. Their density estimates fluctuated least among preygroups, and track records of dingoes in the mountains were significantly related to them. Other categories of prey were supplementary (large mammals), opportune (small mammals) and scavenged.Although there were general tendencies for dietary frequencies to follow prey abundances, significant functional and numerical responses were obtained only for water-birds (coot and swan). Their super-abundance in the mid-years of the coastal study and their highly clumped distribution were the likely causes.Predation was disproportionately severe on mammalian prey-classes after an extensive wildfire at the coastal site. Such predation may have suppressed populations of wallabies and kangaroo for 2 yr until the water-birds became super-abundant. The prevalence of wombats in the mountains may have induced heavy predation upon other less numerous large prey. Concepts of profitability in feeding appear to apply to the dingo more than those of optimization of time or energy.The decline in dingoes was correlated most with long-term declines in water-birds and medium-sized mammals. At the time, abundance estimates of wallabies and kangaroos were increasing and those species increasingly predominated in the diet. The ability to hunt co-operatively was apparently ineffectual in preventing decline in dingo numbers.It is suggested that pack size is related inversely to the level of temporal instability in the environment. Frequent wildfires may prevent staple (mediumsized) and supplementary (large) prey from being abundant simultaneously, a combination thought necessary for large pack size.