I’ve been waiting with baited breath (badum ching!) for this report to come out; a ‘Review of cat ecology and management strategies in Australia’ by the eradication happy Invasive Animals Cooperative Research Centre. And while the topic of how to best dispatch cats is a bit pithy, it does shed some light on the situation for these animals in Australia.
First up, why eradication won’t work…
A nationally co-ordinated program of feral cat control across Australia is not feasible, as it is with other introduced species, and control efforts are best targeted at protecting threatened species or habitats. All successful cat eradication programs in Australia have been conducted on islands or within areas bounded by predator-proof fencing, and most have required the use of more than one control method. Successful techniques for the control or eradication of cats on islands have proved largely impractical on the mainland.
Next, a reminder that we humans are the primary disruptor of the environment and cats are simply living on the fringes of our waste-driven society. Cats live where people live…
The highest densities of cats have been recorded from areas associated with humans, where the animals exploit resource-rich habitats such as food dumps (Izawa et al 1982; Izawa 1983; Mirmovitch 1995) and farms (Macdonald and Apps 1978; Liberg 1980; Liberg 1984a and b), or are intentionally provided with food by humans (Dards 1978, 1983; Natoli and de Vito 1988, 1991). Densities reported in these studies range from the equivalent of 200 cats km2 to 2800 cats km2.
High cat densities have been recorded in Australia by Wilson et al (1994) at three rubbish dumps (Mac’s Reef Road, Mugga Lane and Belconnen) on the outskirts of Canberra. The cat densities were assessed as equivalent to 90 cats km2, 19 cats km2 and 38 cats km2 respectively. Hutchings (2003) estimated that 30-50 cats were exploiting a tip site in Victoria and Hale (2003) reported 51 cats exploiting the central holding sheds on a 12 ha pig farm in Menangle, NSW. Denny (2005) reported densities at tip sites at Oberon and Tibooburra in NSW higher than those found by Wilson et al (1994) and equivalent to those of Hutchings (2003) and Hale (2003).
Markedly lower densities of cats have been reported (<1-7 km2) where cats rely on hunting with very little or no food subsidy (Liberg et al 2000).
Also, supporting the ‘nature abhors a vacuum’ idiom that gets bandied about by groups watching cats be killed and re-killed, cat colonies really do come back when you kill them…
Wilson et al (1994) found that cats recolonised tip sites after removal, and suggested that control in perpetuity would be needed to prevent the re-establishment of high density colonies. Wilson et al (1994) also proposed the control of introduced rodents at tip sites to keep cat numbers at lower levels. Further investigations are needed to determine whether rodent control would substantially reduce cat numbers; whether tip cats would be able to survive without the rodent component of their diet; or whether rodent removal would encourage the cats to move away from the tip sites to become established in surrounding habitats.
And cats probably are just one factor amongst lots of really serious factors in native fauna extinction…
The difficulty in assessing the impact of cats on populations of native species, as opposed to predation on prey individuals, is the teasing out of the relative contributions of all the variables that can lead to reductions in the abundance, distribution and densities of species. These variables include climatic events (drought, fire, flood, etc), habitat modification, disease, and food resource distribution and density. It is not sufficient to simply document the diet of cats and assume that this equates to impact at the population level, although several studies have attempted to do this. Such extrapolations are inappropriate for several reasons. First, cats may prey most readily upon the ‘doomed surplus’; that is, the individuals in a prey population that are too young, old, or weak to reproduce (Banks 1999). Their removal clearly makes no difference to a population’s rate of increase. Second, remaining prey individuals may respond to the removal of conspecifics by showing improved survival and increased reproductive output, thereby compensating for individual losses at the population level.
… they might even eat the things that compete with native animals.
Third, cats may eat individuals of a particular prey species but have positive effects on prey population growth and size if they depress other species that would otherwise impact on the prey species more strongly. Such indirect interactions are pervasive in natural systems, and are likely to have particularly strong effects in predator-prey systems (Glen and Dickman 2005; Dickman 2007).
In short, cats are eating lots of non-native animals…
Although native species are well represented in the diet of cats in Australia, most studies of cat predation indicate that introduced species form the bulk of the diet.
In a semi-arid environment, for example, Catling (1988) found that the introduced rabbit (Oryctolagus cuniculus) formed the greatest prey component in cat stomachs by both occurrence and weight. Similarly, Bayly (1978) found that rabbits formed the bulk of the diet of cats in an arid environment. Rabbits also formed the bulk of the diet of cats at Lake Burrendong in central eastern New South Wales, although house mice (Mus musculus) increased in importance after rabbit numbers were reduced by the appearance of rabbit haemorrhagic disease (RHD) (Molsher et al 1999).
Several studies in Australia have reported high frequencies of occurrence of rabbit in the diet of cats in arid and semi-arid zones (eg Bayly 1978; Jones and Coman 1981; Catling 1988; Martin et al 1996; Risbey et al 1999; Read and Bowen 2001) as well as in more temperate regions (eg Coman and Brunner 1972; Jones and Coman 1981; Barratt 1997; Molsher et al 1999; Kirkwood et al 2005). Catling (1988) reported the frequency of occurrence of rabbit in the diet of cats in western New South Wales as varying from 72 per cent to 90 per cent. Read and Bowen (2001) recorded a similar frequency of occurrence of rabbits (>60 per cent) in the diet of cats in arid South Australia, while Risbey et al (2000) found that the frequency of occurrence of rabbit varied from 19.4 per cent in the diet of semi-feral cats, to 66.6 per cent in the diet of feral cats in Western Australia. Jones and Horton (1982) found that rabbits constituted the bulk of cats’ diets on Macquarie Island, and Jones and Coman (1981) found that rabbits and house mice contributed most to the diet of free-living cats in Victorian mallee areas.
Holden and Mutze (2002) reported a substantial reduction in the population of cats in the Flinders Rangers, South Australia, following a rapid decline in rabbit numbers following the release of RHD.
And that cat eradication programs, by their nature, are incredibly inhumane…
Veitch (2001) described a cat eradication program in New Zealand that commenced in 1977 and concluded in 1980 using cage traps, leghold traps, dogs and 1080 poison. The author found that leghold traps and 1080 poison were the only effective methods.
And they don’t really work anyway…
Feral cats currently cannot be controlled effectively in large remote areas, so management should aim to prioritise sites, areas and regions that would be most likely to benefit from intensive cat control.
And we’re gonna have to do it, pretty much forever…
Programs for the control in perpetuity of cat colonies exploiting resource rich, artificial habitats, such as rubbish tips, should be a management requirement. Cat eradication should be conducted regularly at such sites at six-12 month intervals.
And then we’re gonna have to kill other animals too…
As cat control becomes more target specific, measurable and effective, it will be important to measure the responses of not just the target species or systems that are the intended beneficiaries of cat management, but also the responses of other species with which cats may interact. Populations of rabbits and introduced rodents will be the most obvious targets for monitoring in most situations.
So yeah. Welcome to the future of cat control according to the Invasive Animals Cooperative Research Centre – The Hands-free Robotic Killing Tube!
Professor Steven Lapidge is trialling new traps which arouse the natural curiosity of cats.
“This is a tunnel that emits a phonic of a sound and bright features that attract cats to it,” he said.
“It requires them to walk through a tunnel and if they set off certain sensors in a certain configuration, then it detects the shape of the animal.
“If it is a cat then it will deliver a short spray onto its belly of a toxic substance that puts them to sleep.”
The trial will begin on South Australia’s Kangaroo Island next month.
