A good example of the pest/not-pest duality is the domestic cat on the island continent of Australia. Between 4 million and 18 million feral cats (Felis catus) live wild in Australia. Until recently most of these cats were believed to be descendants of European cats brought to the continent in the late eighteenth century, with a few earlier arrivals via trading ships and shipwrecks. However, Australia's aboriginal people regard cats as native. Genetic analysis indicates that Australian feral cats may have more in common with Asian than European cats, supporting the aboriginal view for an earlier arrival of cats on the continent.
But the debate of more practical consequence is whether feral cats threaten native species such as tammar wallabies (Macropus eugenii). If viewed as an invasive pest, then feral cats need to be hunted down, poisoned, given birth control, or otherwise controlled. If viewed as beneficial predators helping control other pests such as rabbits, rats, and mice, then feral cats should at least be tolerated.
In the late nineteenth century feral cats were viewed as beneficial. Cats were deliberately acclimatized and released into the Australian wild to hunt pestiferous (nuisance) European rabbits (Oryctolagus cuniculus). Indeed, Australia's Rabbit Nuisance Bill of 1883 supported releasing feral cats to help control rabbits that were damaging agricultural grazing lands.
But later in the twentieth century, feral cats were no longer welcomed as rabbit killers. Feral cats began to be viewed as invasive pests, threatening to native birds and mammals. Anti-cat forces pointed to the case of Marion Island, where five domestic cats released in 1949 had become a colony of over 2,000 by 1975. The Marion Island feral cat colony was destroying nearly half a million burrowing petrels per year.
On the Australian continent and on Australian offshore islands, feral cats were blamed for the regional extinction of several native bird and mammal species. The vanishing species were ground dwellers living in open habitats (favorable to cat hunting) and were the right size to be cat prey. The anti-cat forces also suspected that toxoplasmosis, a disease vectored by cats, may have played a role in mammal and carnivorous marsupial population declines many years earlier.
Feral cats were suspects in Western Australia, where a red fox (Vulpes vulpes) removal program did not stop the population decline of native fauna. Feral cats were suspected of filling the niche vacated by the red fox, and adding native species to their predominately rabbit and rodent diet. To determine whether feral cat control is a necessary policy, researchers like Robyn Molsher set up studies in New South Wales to evaluate the ecological relationships among feral cats, red foxes, and other fauna.
Since feral cats are difficult to follow in the wild, their scats (fecal droppings) were analyzed for dietary clues. Rabbits were the primary feral cat prey in New South Wales; rabbit remains were present in 82% of the scats and constituted over 68% of scat volume. The majority of the other prey was carrion, primarily sheep and eastern gray kangaroo (Macropus gi-ganteus). Even after rabbit populations plummeted following application of a biological control agent known as Rabbit Cali-civirus Disease, rabbits remained the dominant prey of feral cats. Feral cats consumed more of the house mouse (Mus mus-culus) following the rabbit population decline.
Foxes and feral cats tracked via radio collars shared similar habitats and prey. Foxes displayed aggression and killed some of the feral cats competing for the same food resources. When foxes were present, feral cats ate mostly rabbits and left the carrion for the foxes. In fox removal experiments, feral cats ate more carrion and hunted more at night in the same prey-rich grassland habitats favored by foxes.
Molsher concluded that integrated rabbit control programs needed to also consider fox and cat control to prevent native fauna from becoming prey in the absence of rabbits. However, rabbits are so well established across such a vast
area that the goal of rabbit eradication in Australia has been abandoned in favor of long-term population suppression. This seems to vindicate a continuing beneficial role for feral cats as rabbit and rodent predators in Australia.
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