No abstract

In parts of the world such as the Pacific Islands, Australia, and New Zealand, introduced vertebrate predators have caused the demise of indigenous mammal and bird species. A number of releases for reestablishment of these mammal species in mainland Australia have failed because predators extirpated the new populations. The nature of the decline of both extant populations and reintroduced colonies provides information on the dynamics of predation.Predator-prey theory suggests that the effects of predation are usually inversely dependent on density (depensatory) when the prey are not the primary food supply of exotic predators. Thus, such predators can cause extinction of endemic prey species. Three types of evidence can be deduced from the predator-prey interactions that allow predictions for conservation: (1) whether per capita rates of change for prey increase or decrease with declining prey densities, (2) whether predation is depensatory or density-dependent, and (3) the overall magnitude of predation. If this magnitude is too high for coexistence, then the degree of predator removal required can be predicted. If the magnitude of predation is sufficiently low, then the threshold density of prey that management must achieve to allow predator and prey to coexist can also be predicted. We analyzed published reports of both declining populations and reintroduced colonies of endangered marsupial populations in Australia. The observed predation curves conformed to the predictions of predator-prey theory. Some, such as the black-footed rock-wallaby ( Petrogale lateralis ), were classic alternate prey and were vulnerable below a threshold population size. Others, such as the brush-tailed bettong ( Bettongia penicillata ), have a refuge at low numbers and thus offer the best chance for reintroduction. Our predictions suggest a protocol for an experimental management program for the conservation of sensitive prey species: (1) determination of net rates of change of prey with declining population, (2) improvement of survivorship through habitat manipulation, (3) improvement of survivorship through predator removal, (4) determination of the threshold density above which reintroductions can succeed, and (5) manipulations to change interactions from Type II to Type III. The task in the future is to determine how to change the vulnerability of the prey so that they can have a refuge at low numbers. Predicción de los Efectos de la Depredación en la Conservación de una Presa en Peligro de ExtinciónResumen: En algunas partes del mundo como son las Islas del Pacífico, Australia y Nueva Zelandia, los veretebrados depredadores introducidos han ocasionado la desaparición de especies indígenas de mamíferos y aves. Un gran número de liberaciones para el restablecimiento de estas especies de mamíferos en tierras continentales de Australia han fracasado debido a que los depredadores han extirpado las poblaciones nuevas. La naturaleza de la declinación tanto de las poblaciones existentes como de las colonias reintroducidas provee inform...

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 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.