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Biodiversity Group Environment Australia, 1999
0 642 2546339
In developing an agreed approach to the management of the impacts of feral cats on threatened species or other native wildlife, a range of issues and constraints need to be considered.
Convincing evidence that feral cats exert a significant effect on native wildlife on the mainland is scarce (Dickman 1996; Jones 1989; Wilson et. al. 1992). Feral cats have occupied tropical Australia, Tasmania and Kangaroo Island for at least a century. Although there are a number of vulnerable and endangered species which are susceptible to feral cat predation in these areas, there have been virtually no extinctions or none that could be attributed to feral cat predation.
The situation is complicated by land-use changes since European settlement, in particular the spread of pastoralism, land clearing and the removal of Aboriginal people and their fire regimes from arid and semi-arid desert, grasslands, shrublands and woodlands. Paltridge et. al. (1997) observed that watering points were used as daytime shelter for feral cats as a consequence of the taller vegetation behind fenced-off bores. Furthermore Paltridge et. al. (1997) noted that many species of birds taken by feral cats in central Australia were those that regularly required free water, and that during drought feral cats consumed carrion from dingo kills near watering points. Landsberg et. al. (1997) stated that little is known of the effects of feral cats on native animals that require free water. Anecdotal information confirms that predation occurs, but the impact on species or faunal assemblages has not been quantified.
In addition, other introduced animals that have become feral have had significant impacts both on habitats and on native animals. As an example of the complexity of these interactions, the spread of rabbits has provided a ready food source for introduced predators such as feral cats and foxes, although some native species such as Wedge-tailed Eagles have also benefited.
Predation is a feature of virtually all ecological systems. Raw estimates of the total number of prey animals taken by cats (eg Paton 1990; Reark 1994) and studies of diet are of limited value in determining the ecological impact of the predation. Cat predation becomes a significant threat to native species only where the level of predation by cats exceeds the capacity of individual populations to replace themselves. Australia's pre-European fauna included a suite of native predators including large reptiles, raptorial birds, quolls and dingoes. The degree of threat posed by cat predation hinges upon:
From a wildlife management point of view, the key question is: "Will the removal of feral cats result in significant increases in the populations and/or the distribution of particular native species?"
Dramatic recoveries of species on islands after the removal of feral cats are evidence of their impact (Dickman 1996). Some comparable work has commenced on the mainland, but the results are yet to be fully analysed (Dickman 1996). A significant impediment to answering this question is the technical difficulty of measuring and manipulating the numbers of feral cats. They are very difficult to detect and count and at present there are no effective techniques for their broadscale control.
Although the damage to conservation values is unclear, there is sufficient concern among scientists, wildlife managers and the community to warrant further research and field trials to develop and improve methods for controlling feral cats in conjunction with measuring the benefits to wildlife.
Interactions with other introduced species
Although all feral cats prey on native animals, in areas where rabbits occur, rabbits are the main prey item. In rabbit areas feral cats will prey upon native animals opportunistically, particularly when rabbit numbers decline (Williams et. al. 1995; Newsome et. al. 1996), but only in areas without rabbits do native species form the staple diet of feral cats.
There is good evidence that environments are made more suitable for feral cats by the presence of rabbits (Taylor 1979; Newsome 1990), as they are a preferred food item and create burrows that provide shelter for feral cats. Feral cat numbers have been observed to rise and fall with fluctuations in rabbit numbers (Williams et. al. 1995), and control of rabbits, particularly in rangelands and on islands, is considered important in lowering feral cat numbers and possibly reducing feral cat damage to conservation values (Newsome 1990).
Feral cats show a preference for young rabbits but are able to survive on alternative prey when rabbit populations decline (Catling 1988). Impacts on alternative prey can be particularly severe in island situations such as Macquarie Island, where it has been suggested that winter nesting petrels are preyed upon when rabbit numbers are low (Brothers 1984).
In semi-arid New South Wales, Newsome et. al. (1989) found that rabbit numbers increased significantly in areas where feral cats and foxes were systematically removed, compared to areas where the predators were left alone. The response was most marked after about 14 months of continual predator removal. The study showed that, in semi-arid habitats, shooting predators can effectively release an introduced prey population from the suppressive effects of introduced predators. There are no comparable studies showing such a response in native prey populations and the design of this study did not allow the relative roles of feral cats and foxes to be differentiated.
Dingoes and rabbits have been shown to influence the abundance of feral cats (Williams et. al. 1995; Corbett 1995). Dingoes are common in the northern and central parts of Australia, but have been effectively controlled in the south-east and far south-west of the mainland (Corbett 1995). Dingoes may protect a range of native species by controlling exotic predators like feral cats through direct predation (Palmer 1996a & 1996b) and by excluding them from carrion during droughts (Pettigrew 1993; Corbett 1995). The potential to introduce dingoes to control exotic predators in areas where they are now absent is remote, due to their potential impact on agricultural interests. Foxes and rabbits are absent from tropical northern Australia and from Kangaroo Island, but both species occur in the semi-arid lands where most extinctions and severe declines have occurred. Tasmania is free of dingoes and foxes, but has rabbits.
The magnitude of competition between foxes and feral cats for food is not fully known (see Kinnear et. al. 1984 & 1988), nor is the spatial overlap in their habitat known. Both feral cats and foxes are known to prey heavily on rabbits, but have the capacity to eat a wide variety of foods (eg Catling 1988). The degree of dependency of foxes and feral cats on rabbits is indicated by the decline in their populations after a crash in rabbit numbers (Newsome et. al. 1989). Recent CSIRO studies suggest that foxes are dominant to cats and may restrict their full use of resources in an area as well as possibly killing them (Molsher unpub. 1996).
Smith and Quin (1996) modelled the decline and extinction of a diverse range of native rodents, finding that introduced predators, and the presence of rabbits and house mice explained much of the decline. In particular, the presence of foxes and rabbits led to the most severe declines, but declines were less severe where dingoes were present. The abundance of feral cats explained the loss of conilurine rodents less than 35 grams. Where all predators were sustained by mice and rabbits, the declines in native species were uniformly severe (Smith and Quin 1996). What is not fully understood is the relationship between feral cats and raptors or wild dogs.
Although amensal impacts are not listed as a key threatening process, native species may also be deleteriously affected through parasites and diseases transmitted from cats. Toxoplasmosis gondii is known to be able to infect a range of marsupial and other mammalian hosts including man (Jones 1989). Exposure to this disease may be detectable through disease symptoms or blood tests but significant or sustained damage to the faunal assemblage from Toxoplasmosis may have already occurred historically and the impact can only be guessed at (Dickman 1996). Future action may be necessary to examine the possibility for control or management of the impact of disease on vulnerable populations of endangered species.
The cultural value placed on feral cats varies according to the observer’s own value system. Australia’s unique fauna is widely valued by society and many perceive feral cats to be a threat to the native fauna. Nevertheless, there are concerns that domestic cats may be threatened by actions taken to control the impacts of feral cats.
Aboriginal people recognise introduced animals as part of the landscape and see them as newcomers rather than feral (Rose 1995). In particular, some Aboriginal people argue that the feral cat pre-dated European settlement and is part of Aboriginal law (Rose 1995). With the decline and extinction of many arid zone mammals with a mean adult body mass of between 35 grams and 4200 grams (defined by Burbidge and McKenzie 1989 as the critical weight range mammals) introduced mammals are viewed by Aboriginal people as a welcome addition to their diet.
Consideration of the differing cultural values attached to domestic and feral cats must be an important component of any control program.
Animal welfare issues related to feral cat control were thoroughly discussed at a workshop that preceded the preparation of this plan (Carter 1994). Organisations were invited to submit position statements covering animal welfare issues, with the RSPCA and Australian and New Zealand Federation of Animal Societies (ANZFAS) providing such statements (Carter 1994). Both these organisations accepted that there is a need to control feral cats to protect environmental values and wildlife. Both strongly emphasised the need for control methods to be humane. Neither organisation supported the use of mechanical hold fast traps, such as steel-jawed leg-hold traps, that are widely considered to be inhumane, however the recent development of the Victor Soft-catch trap™ may overcome this concern (Meek et. al. 1995)
ANZFAS was prepared to support research and development into biological reproductive control of feral cats, as long as it was humane and environmentally safe. The RSPCA was prepared to review its position on this issue if the technique could be shown to be humane, and environmentally safe, and if the safety of owned cats could be guaranteed.
Both organisations recognised the need to control feral cats from time to time. ANZFAS argued for live trapping and humane dispatch, while the RSPCA accepted that lethal baits could be used, under appropriate supervision, in the absence of an alternative. ANZFAS did not support any ‘method of repetitive, lethal control indefinitely...without concurrent research into alternative methods which assure long-term humane control’. In recent comments ANZFAS stressed the need to clearly define the problem caused by cat predation. It is accepted that where the removal of cats would contribute to the balance of the environment, humane control methods must be developed.
Published June 1999 by Environment Australia under the Natural Heritage Trust.
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