Australian fauna have suffered serious declines since European settlement, with small- and medium-sized mammals being the worst affected. Feral cats depredate native birds, mammals and reptiles and are listed as a Key Threatening Process under the Commonwealth EPBC Act. Reducing the harmful impact of feral cats on native fauna presents wildlife managers with a formidable challenge.
Domestic cats came to Australia with European settlers in the late 18th century and spread into the natural environment from coastal locations in the period 1824-86. Rabbits provided cats with a reliable food source during their dispersal. The intentional release of large numbers of cats to control rabbit plagues also furthered their spread.
By 1890 the feral cat had colonised the entire continent, including Tasmania and a number of offshore islands. Their successful invasion was aided by the species' flexible habitat use, adaptive diet and no requirement for free water. Moreover, feral cats have a high reproductive output and are threatened by few higher order predators in Australia.
Cats were valued as pest control agents and from 1918-21 it was illegal to kill feral cats in Western Australia. Their negative environmental impacts did not receive significant attention until at least 50 years later.
Fauna decline and extinction
Feral cats have been implicated in the decline and extinction of some 22 mammal species in the past 200 years. It is difficult to assess the relative contribution of feral cats to fauna declines as altered fire regimes, introduced herbivores, fox predation and climate change have also occurred in this time, and their impacts may be synergistic in nature - see here, here and here.
The first experimental evidence illustrating their impact was provided by research at Shark Bay, Western Australia. In that study, small mammals showed a positive population response where both cats and foxes were removed, but showed an 80% decline where foxes were removed, but cats remained.
Further evidence can be found in the failure of numerous reintroduction attempts of threatened mammal species.
In 1992, 40 burrowing bettongs were released into the Gibson Desert Nature Reserve. Sixty days later no live animals could be located and of the 11 carcasses that were found, all showed clear evidence of cat predation.
More recently, preliminary results from northern Australia have shown that cats decimated an unfenced population of reintroduced pale field rats, while a fenced population survived and successfully reproduced.
Protecting threatened species
The recognition of these impacts has led to a strong focus on control measures. Predator-free islands are an important nature conservation tool. A combination of trapping, shooting, baiting and sterilisation has been used to eradicate cats from islands.
Eradication of cats on Hermite Island allowed the establishment of insurance populations of the spectacled hare-wallaby and golden bandicoot. In similar fashion, four other threatened mammals have been translocated to Faure Island following removal of cats.
The remaining challenge now lies in protecting mainland fauna populations from an invasive predator that has high dispersal ability and an aversion to conventional baiting methods.
Islands on the mainland
Predator-proof fences have become an invaluable tool for protecting threatened fauna. Fenced conservation properties, some greater than 5000ha, now protect at least 15 threatened mammal species in self-sustaining populations. These populations are a source of individuals for translocation to additional insurance populations, further strengthening their value.
Fencing, however, comes at a price; Long and Robley costed cat, fox and rabbit exclusion fencing at $8,000-$11,400 per km for materials only. The authors estimate that labour for planning and construction may make up 50% of the budget. The high cost of fencing and potential consequences of overabundance illustrate the need for sustained predator control at the landscape scale.
Baiting as a control measure
Eradicat and Curiosity baits were developed in response to the feral cat’s unwillingness to take conventional dog and fox baits. These cat baits are a sausage type medium of kangaroo meat mince, chicken fat, digests and flavour enhancers.
Broad-scale aerial application of Eradicat 1080 baits in semi-arid WA is used for the ongoing control of foxes and feral cats. The baiting targets high-value conservation areas such as Cape Arid National Park, the final stronghold for the critically endangered western ground parrot.
The Curiosity bait has been developed for areas outside of south western Australia, where native fauna do not have a natural tolerance to 1080 poison. Curiosity contains the toxicant para-aminopropiophenone encapsulated in a “hard-shell delivery vehicle”, which may increase the bait’s target specificity.
Successful reduction in cat populations using Curiosity has been achieved on Christmas, French and Dirk Hartog Islands. Mainland trials at Flinders Ranges and Wilsons Promontory National Parks achieved some reduction in cat numbers, but may have been compromised by excess food availability and heavy rainfall respectively.
Eruptions of native and introduced mammals can compromise baiting operations. Christensen et al. have recently used a 14 year data set to show that a predator-prey index is potentially useful to land managers as an a priori predictor of the efficacy of planned baiting operations.
Control methods for feral cats are still in their infancy and will benefit from an improved understanding of feral cat ecology. Additional research will reveal whether habitat structure and higher-order predators can mediate feral cat impacts. Further investment in research, exclusion fencing and baiting will improve protection of Australian fauna from cat predation.