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Cats in unpopulated areas

Cats probably arrived in Australia before 1788 due to early European contacts. Since that time they have spread across the continent. Their dispersal was assisted by farmers, who in the past deliberately released cats around burrows as a form of rabbit control (1).

Adult cats maintain discrete home ranges (2). They can breed at any time of the year. There is no specific breeding season, but most kittens are born in spring to late summer/autumn. Females average 2 litters a year, with an average of 4.4 kittens/litter. Even though juvenile survival is limited by the availability of easily caught prey, such as young rabbits (2), a cat population can increase dramatically. For example, in 1949, 5 cats were introduced to Marion Island. After 25 years, the population on the island was over 2000 (3).

Cats and endangered wildlife

It has become fashionable in some conservation circles to blame cats for the perilous position of some native species. However, a more accurate perspective is provided by well-respected conservation biologists (1, p.147)
“Land clearance, mainly for agriculture, is perhaps the single most important cause of environmental degradation, and loss and depletion of species and ecological communities both in Australia and worldwide. Almost 80% of mammals and about 60% of birds listed by the IUCN have declined as a result of habitat loss.”

In terms of the diet of feral cats, it is important to remember the extent to which they prey on rabbits and introduced rodents. On Macquarie Island, 82% of the cat scats and 71% of the stomach contents examined contained rabbit remains (4). Around Lake Burrendong in central eastern NSW, 68% of the volume of cat scats was composed of rabbit, and a further 11% of carrion, that is, kangaroos killed by shooters and sheep that had died or been killed by a larger predator. Scats were collected for 3 years, during which time the rabbit calicivirus arrived in the area. Even though the rabbit population declined by 90%, rabbits were still a major prey species. Ten months later, house mice increased as a component of the diet, maybe due to the decline in rabbits, but possibly just because they were plentiful, and cats are opportunistic predators. Few native mammals were eaten in this study, and birds are seldom an important prey item for mainland cats (5).

Worldwide, most extinctions have occurred on islands. Attention if often focussed on the role of introduced cats, but in fact rats are responsible for twice as many extinctions as cats (6). Rats alone prey on birds more effectively than cats alone in all life stages from eggs to ground-nesting adults. Rats are found on the majority of major island groups. Predation by cats can control their numbers and benefit bird populations, even when the cats themselves lightly prey on the birds. For example, on Stewart Island, remains of the endangered kakapo were found in 5.1% of cat scats, whereas rat remains were found in 93% of scats. Cats benefit the kakapo by reducing rat predation. An attempt to eradicate cats from Amsterdam Island was abandoned after rat and mouse populations increased rapidly (6).

On Raoul Island, rats were the most frequent prey found in both cat stomachs and scats. Bird remains found were those of common songbirds on the island; few seabirds were taken. There are many examples of islands where seabird populations have been reduced by Norway rats, and since rats are the predominant prey of cats, the researchers concluded (7: p.127):
“One management option for Raoul Island is to eradicate cats but this may bring little benefit to bird populations.”.

Unless Norway rats could be eradicated at the same time as cats, “...the effort and expense involved in a cat eradication programme might be better spent on other conservation projects that are more assured of a positive outcome.” (7: p.128)

Cat control methods

For decades, cats have been killed by various methods, such as trapping, poisoning and shooting. Such methods may not only cause suffering to the cats, but in the case of lactating females leaves kittens to slowly starve to death. In addition, these methods have not been effective in achieving a lasting population reduction. Several authors have noted that conventional baiting is not effective because cats are not attracted to carrion (baits) when prey is plentiful (1, 2). Steel-jaw traps are inhumane, broadacre live trapping is impractical, and shooting where there is cover is a waste of time (2). With regard to killing introduced animals generally, it has been observed (8: p.1):
"Many decades of pest control programmes have demonstrated that it is impossible to eradicate such exotic animals on all but the smallest islands..... efforts to remove cats on Macquarie Island show that culling is merely a harvesting operation with the cats compensating by presumably higher reproduction and juvenile survival."

Death rate is only one factor determining population size – other factors include birth rate and immigration/emigration. When some cats are killed, those remaining continue to breed. Because there are now more vacant territories, and fewer cats hunting easily available prey, more juveniles are likely to survive and become established. They in turn will breed, and so the population returns to where it was before killing started.

An alternative strategy to increasing mortality involves reducing fertility, especially through immunocontraception. Animals are induced to produce antibodies to their own reproduction proteins. These antibodies bind to the proteins and prevent fertilisation taking place (6). Research into this approach is well underway for rabbits and mice, and is advocated for foxes. The stimulus for antibody production, the antigen, may be delivered to the target animals either by baits or through other organisms, such as viruses. In Europe, the DNA of a non-infectious virus was altered to include a gene for a rabies protein. When foxes (or other animals) ate baits containing the virus, they produced antibodies against the protein, and so were vaccinated against rabies (9). According to the same principle, genes for species-specific reproductive proteins could be added to viral DNA. Then, even if other species ate the baits, they would not become infertile.

Alternatively, a species-specific infectious virus could be used to deliver the gene for reproductive proteins, and to stimulate an immune response to the proteins. An effectively spreading viruses would increase infertility with less effort that spreading baits.

Leading researchers in the field of rabbit immunocontraception have noted with regard to cats (10: p.33):
“...the delivery of a sterilising agent to a feral cat population poses considerable difficulties as cats are widely dispersed, have little contact with each other, and breed at any time of the year. An infectious, disseminating viral vector would appear to be the only feasible option and yet it is unlikely that a recombinant feline virus would be approved for use because of the threat it would pose to domestic cats."

One area where the least objections to genetically engineered fertility control would be made is on uninhabited islands, where cats (and rats) have the biggest impact on native animals (3). Advantages include that immunocontraception is more humane than conventional control methods, is specific to particular species, and is cheaper to apply because it can potentially spread itself over large and inaccessible areas. Some possible problems are not relevant to the island setting. There is no risk to pet cats, the spread of the virus is limited by the ocean surrounding the islands, and being irreversible doesn't matter because future cat fertility is not desirable anyway. The fact that it reduces the cat population slowly is an advantage because it will not allow an explosion in rat numbers (3).

Controlling cat fertility should be given greater research attention, to not only control cat numbers more humanely, but also more effectively.