Since its introduction to Australia during the mid-1800s, the Red fox has been implicated in the decline of several native vertebrate species and considerable time, effort and funding have been diverted into trying to eradicate them from the continent. Indeed, the way foxes have colonised Australia has led the IUCN's Species Survival Commission to rank them in their top 100 worst exotic invasive species. The Global Invasive Species Database states:
“The damage to Australian wildlife since European settlement has been catastrophic. At least 20 species of Australian mammals have become extinct. This represents about one half of the world's mammal extinctions in the last 200 years; a further 43 species are judged to be either endangered or vulnerable.”
Foxes seem to have had the biggest impact on small to medium-sized birds and mammals (i.e. those in the 35g/1.2oz. to 5.5kg/12lbs range), and this predation seems to have severely limited both their abundance and distribution.
In 1975, Hans Brunner and colleagues of the Vermin and Noxious Weeds Destruction Board published their analysis of almost two thousand fox scats collected from a small forest in south-eastern Australia. They found that almost 70% of the diet was composed of mammals, particularly rabbits, rats, pouched mice (Antechinus spp.), ring-tailed possum (Pseudocheirus peregrinus), and the brush-tailed possum (Trichosurus vulpecula). The remains of some other indigenous mammals—e.g. broad-toothed mice (Mastacomys fuscus), other possums and wallabies—were also found, but these were uncommon.
Obviously, as we have already discussed (see: Food and Feeding), presence in the diet doesn't necessarily confirm predation – the meat could equally well have been scavenged. Nonetheless, there are data suggesting that where foxes are abundant, native mammals are rarer, and fox predation in Australia is believed to have contributed to the decline of several native mammal species, including the eastern barred bandicoot (Perameles gunni), the long-footed potroo (Potorous longipes), the little penguin (Eudyptula minor) and the bush-tailed rock wallaby (Petrogale penicillata).
It seems that foxes can have an impact on larger marsupial species too; with data suggesting that predation may limit the populations of Australia’s eastern grey kangaroos (Macropus giganteus). Between 1993 and 1995 a team of biologists at the University of Sydney, headed up by Peter Banks, studied the impact of foxes on the kangaroo population in south-eastern Australia’s Namadgi National Park. Banks and his colleagues found that there were more juvenile kangaroos in areas where fox numbers were controlled than in those where foxes were left alone. This dataset indicates that foxes were responsible for taking 25% to 35% of the juvenile kangaroos in their first year and, although not conclusive, suggests that foxes were limiting population recruitment.
Finally, in a paper to Australian Mammalogy during 2018, University of Sydney biologist Valentina Mella and colleagues argue that their arboreal behaviour makes them a potential threat to several species of bird and tree-living mammals. Late one night in April 2016, the researchers' captured footage of a fox visiting a drinking station, situated nearly 1.4m (4.6 ft.) up in the fork of a eucalyptus tree, on their trailcam. The fox climbed beyond the reach of the camera, appearing to follow the trail of a koala (Phascolarctos cinereus) that had visited the same tree a few nights before. Foxes were subsequently recorded visiting and climbing trees in a neighbouring plantation twice during July— including one frequented by sugar gliders (Petaurus breviceps), koalas and various native birds— to investigate the branches. Mella and her team note that no further visits by sugar gliders were recorded after the latest fox visit, although while antecedent these two events may not necessarily be correlated.
A study conducted in Kichenga National Park in New South Wales (NSW) during the 1970s found the remains of bearded dragons (Pogona vitticeps) in fox scats, although reptiles in general were rare. More recently, in a 2005 paper to Austral Ecology, Mats Olsson and his colleagues reported that fox removal increased the number of small, day-active lizards in open grassland habitats; they also observed a five-fold increase in the population of the sand goanna (Varanus gouldii). Olsson and his co-workers concluded that removing foxes—which compete with, and prey on, the goanna—may allow the goanna to take over the role of top predator in the ecosystem.
The excavation of turtle nests by foxes has been discussed elsewhere, because it is a problem observed more widely than just Australia. It is, however, a significant problem in Australia and, along with direct predation of the adult female turtles when they come ashore to lay their eggs, is considered to detrimentally impact turtle populations here. The situation is currently being studied by Stuart Dawson and colleagues of the “westernWEB” research coalition.
Part of the problem for Australian conservationists is that foxes are not easy to control. A recent study in the Goonoo forests of NSW, for example, used camera traps to assess the impact of baiting (i.e. poisoning) on fox abundance. The biologists noted some impacts locally, but found that baiting had no effect on fox activity or abundance at the landscape scale; nor was there any clear effect on the populations of the prey species (i.e. there wasn’t increased activity or abundance of prey species where foxes were baited).
Generally speaking, the impact of a baiting scheme is highly dependent upon the habitat and baiting strategy used. Indeed, an earlier study looking at the survival of malleefowl (Leipoa ocellata)—a member of the partridge/pheasant family—in the Yathong Nature Reserve, also in NSW, suggested that intensive and widespread baiting was necessary in order to reduce fox numbers sufficiently for this bird’s population to recover. There are many reasons why this is the case, including that foxes rapidly become bait-shy and dispersing individuals recolonise vacant areas rapidly, but it means biologists are faced with a daunting task.
Despite the foregoing, however, there have been some successes and a five-year study (2001-2006) in the eucalyptus forests of western Australia found that baiting foxes did have a significant impact on prey species in the area. In a paper to the journal Australian Forestry, Adrian Wayne and his colleagues reported that fox-baited areas had significantly more (in some cases, three-times more) prey species than un-baited areas, with the brush-tailed possum (T. vulpecula), wyolie (Bettongia penicillata), southwestern pygmy possum (Cercartetus concinnus), western quoll (Dasyurus geoffroii), and the shingleback skink (Tiliqua rugosa) being particularly abundant. It seems the areas regularly baited supported a greater number of species, as well as a higher abundance of individual species. Furthermore, studies by Andrew Hayes and colleagues at the Queensland University of Technology have suggested that some native Australian rodents are learning to recognise and avoid fox scent.
While some native mammals learning to recognise the non-native fox as a predator has the potential to bring benefits to the population, recent research by University of Sydney biologist Jenna Bytheway and colleagues suggests foxes may be specifically attracted to unfamiliar native prey. The study, published in Scientific Reports during July 2016, found that foxes living on island of North Head in Sydney harbour were significantly more interested in the odour of long-nosed bandicoots (Perameles nasuta) than black rats (Rattus rattus), despite never having encountered one before. We know that investigating new potential prey (neophilia) doesn’t carry any real risk for the fox – either it leaves with a meal or it doesn’t – but running from something that’s not a predator is energetically costly for prey (bandicoots), which is why they tend only to flee from things they know are dangerous. In the paper, Bytheway and her colleagues conclude:
“It is also possible that attraction to the evolutionary novel prey may reflect a general inherent preference for novel prey by alien predators.”
In other words, foxes may actively opt for novel prey because they’re more likely to be naïve and therefore easier targets.