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Biodiversity Group Environment Australia, 1999
0 642 2546320
Eradication of foxes is an attractive prospect because, once achieved, it requires no further commitment of resources, however this is not currently considered a viable proposition for mainland Australia. To achieve eradication:
Complete removal of foxes from Australia is well beyond the capacity of available techniques and resources because the species is well established across a vast area. Eradication from an island, or of a localised, newly introduced population, may be feasible provided a sufficiently rapid, well funded and persistent campaign can be mounted.
Saunders et al.et. al. (1995) reviewed current knowledge on techniques for suppressing fox populations including poison baiting, shooting, trapping, hunting with dogs and fumigating dens. The review concluded that, with the exception of broad-scale baiting, the existing control methods are expensive, labour intensive, require continuing management effort and can be effective in only limited areas.
In most situations, poison baiting is the most effective method of reducing fox numbers and impact, although a major drawback is that it may affect native carnivores and scavengers such as dingoes, quolls, goannas and some scavenging birds, and also domestic dogs. The benefits of this control method are confined to the baited area and, unless some barrier prevents reinvasion, last only for as long as baiting is regularly applied.
Aerial baiting of foxes has been demonstrated to be an effective method of control for covering large areas provided the risk of non-target bait uptake is minimal. Currently Western Australia is the only State that uses aircraft to lay bait for fox control and this method is not permitted in New South Wales and Victoria. In Western Australia, fox-baiting programs over large areas (up to three million hectares) have been shown to dramatically reduce fox numbers, to allow populations of rare species to increase and have minimal impact on non-target species. This is largely due to the native fauna having a higher resistance to the naturally occurring 1080 poison found in native plants.
Where broad- scale baiting is not feasible and where the risk of bait uptake by non-target species is high, fox suppression at a local scale can be effective. With any small scale baiting program (less than 1000 hectares) there is the need to consider establishing and maintaining an effective buffer zone. Protection of endangered species may require developing buffer zones of up to 20 kilometres where foxes are held at a low density to decrease the risk of inward migration rapidly replacing foxes killed (Saunders et al.et. al. 1995).
Tradition and convenience have usually determined the selection of bait materials: injected eggs, dried meat baits, fresh meat, and commercial products (eg Foxoff baits) have all been used. The effectiveness of the different baits and the conditions which determine their acceptability to foxes need to be determined to ensure that the most cost-effective delivery systems are used. It is possible that some baits which work well in one environment may not be as effective in other environments.
One of the consequences of prolonged baiting is that animals may develop bait shyness or an aversion to the poisons being used. Repeated use of poisons over a long period of time may lead to evolutionary selection for animals which are genetically bait shy, a condition which has been reported in rabbits in Western Australia and New Zealand. The risk of bait shyness developing in foxes can be minimised by appropriate selection of bait types and dosage of poison per bait. Further work is needed to identify the conditions and baiting methods to minimise the risk.
Some form of pathogen could conceivably affect foxes on a continental scale, but currently none is known to be virulent, humane, specific to foxes and not transferable to other species.
Like most other pests, foxes have a high rate of reproduction but they only breed once a year over a period of two to three weeks in early winter. This feature of their biology provides an opportunity for management based upon the application of fertility control over a short period of time each year. Targeting their fertility may yield an effective long-term approach to reducing their numbers and hence lower the need to use poison baits.
Fertility control will not provide any significant level of amelioration of the threat to endangered species until fox numbers have been reduced by natural attrition or by other means. Lethal control will be needed to achieve a rapid reduction in fox numbers . Fertility control could then be used strategically to maintain reduced fox numbers to provide long-term protection for threatened species.
Fertility control is still at an experimental stage of development. In practice, fertility control of wild vertebrates has been achieved on only a very limited scale using expensive, labour-intensive methods (Bomford, 1990). It has not been successfully applied to a free-ranging population of wild vertebrates over a large area nor has it been attempted as a method of reducing the impacts of predation on an endangered or vulnerable species.
Methods of fertility control include hormone treatment and the use of abortifacients such as Cabergoline. Hormone treatment is not considered a viable option for managing populations of wild foxes as there are no practical methods of ensuring effective treatment of unrestrained animals. Research has, however, indicated that Cabergoline may have the potential to control fox fertility in areas where poison baiting cannot be undertaken (Marks et al.et. al. 1995a, 1995b).
As an alternative, a technique based on developing sterility through an auto-immune response to reproductive proteins or hormones (immunocontraception) has been proposed. This technique has the potential to provide a target specific form of fertility control which can be used on wild populations. The Cooperative Research Centre for Biological Control of Vertebrate Pest Populations began a major program of research on this subject in 1992. Work has concentrated on the fox and the rabbit, although mouse fertility has now been included. Funding support for this work has been provided through the National Feral Animal Control Program of the Natural Heritage Trust.
Tyndale-Biscoe (1994) argued that if the immunocontraceptive technology can be made to work, it may provide a cheap, easily disseminated method for reducing fox fertility and populations on a continental scale. Some scientists and wildlife managers remain sceptical about the likely success and effectiveness of this approach (Carter, 1995). The obstacles to achieving a workable method are formidable and include:
Although the early discussions about using an immunocontraceptive control method contemplated the possible use of a self-disseminating viral vector, the research emphasis has now shifted to a bait delivery system. This change in research emphasis is directed at delivering an immunocontraceptive agent to foxes by a non-disseminating organism in a suitable bait. This will allow for a controlled release of an anti-fertility agent which will both initiate the reduction of fox numbers through sterilisation and enable field testing of its efficacy as an anti-fertility strategy.
Despite the acknowledged difficulties, the need to develop a range of methods to provide flexibility in control options justifies continuing investigations on fertility control methods including abortifacients such as Cabergoline and immunocontraception. In both cases, the aim is to develop a safe, effective fertility control agent that may be delivered in a bait.
Barriers to Fox Invasion
Australia has a long history of fencing to control pest animals extending over more than a hundred years. The early fences were designed to control rabbits or dingos. More recently, fences have been proposed as a component in conservation management programs to protect endangered species from predators such as foxes and cats.
A large range of fence designs has been used to exclude foxes from particular areas but there is little information on the effectiveness of particular designs. A recent review of predator-proof fencing in Australia (Coman and McCutchan, 1994) found that although fences can be a significant barrier to foxes, even the most elaborate can be breached. Frequent monitoring for the presence of foxes inside the fence is an essential precaution as considerable damage can be caused by a single fox breaching the fence. While fences may restrict movement of foxes they may also pose a hazard to non-target wildlife as well as placing limits on the natural ability of native animals to disperse.
The high cost of establishing predator-proof fencing, and the ongoing maintenance costs involved, mean that it is likely to be useful only for small areas (Aviss and Roberts, 1994). However, recent studies at Shark Bay, Western Australia have investigated integrating fencing with baiting and trapping to reduce the frequency of challenge to the fence by incoming predators. These studies have focused on using a combination of natural water barriers, fencing and baiting to create large predator-free reserves on peninsulas (Department of Conservation and Land Management, 1994).
The review by Coman and McCutchan (1994) highlighted the need for a comprehensive evaluation of the cost-effectiveness of different fence designs for comparison with other methods of controlling foxes.
Eradication of predators on islands and subsequent translocation of threatened species has been an important strategy for wildlife conservation. Some of the most successful examples of endangered species conservation have been on such islands or within enclosures from which feral predators are excluded.
Tasmania, Kangaroo Island and a number of small islands off the coasts of South Australia and Western Australia are the primary refuges of mammal species that are extinct or very rare on the mainland (Burbidge, 1989). Given the threat that foxes pose to wildlife, it is essential that foxes continue to be excluded from the significant islands where they do not occur. The recent incident in which a fox was transported by ship from Victoria to Tasmania highlights the risk of such incursion and the need for rapid action once an incursion is detected.
Preventing the introduction of foxes to islands of conservation value requires identification of potential routes of invasion, a risk analysis to determine the probability of such an event, and procedures to manage and minimise the risk. There must also be the ability to detect incursions before fox populations have a chance to become established, contingency plans which identify the most appropriate control measures and funding sources to implement the required control.
In environments with dense vegetation, steep topography, rocky crevices or extensive wetlands, prey are less likely to be caught by foxes (Saunders et al.et. al. 1995). The foraging efficiency of foxes seems to be maximal in open habitats where they are able to range widely and freely. They readily use roads, tracks and other cleared access ways through denser vegetation or complex topography. One option to minimise fox impacts on endangered species is to reduce such access points to a minimum and to maintain bait stations along those access paths which are retained.
Arboreal marsupials become vulnerable when they descend to the ground to move between trees. A continuous canopy and a thick understorey of shrubs enable them to move about in the trees where they are not at risk from fox predation. An important conservation strategy for some situations will be to minimise habitat fragmentation and to investigate options for fire, grazing or other management practices which do not destroy ground habitat.
Reviews of the history of vertebrate pest management in general, and fox management in particular (Braysher, 1993; Saunders et al.et. al. 1995), conclude that subsidies and bounties have rarely been effective in reducing the damage caused by pest animals. Similarly, there is little evidence that, except occasionally and in small areas, hunting of foxes has a significant or lasting impact on fox numbers or the damage they cause.
Although there may be benefits to wildlife from widespread fox control on rural properties, it is not cost-effective, as a general policy, to seek to raise the level of recreational or professional hunting or trapping of foxes on a broad scale by payment of bounties, subsidies or other similar artificial market incentives. Where private land adjoins or contains important wildlife habitat, assistance or encouragement to landholders and the development of incentives to promote fox control on private land may be appropriate, especially if the property forms part of a buffer zone to protect threatened species populations.
Published June 1999 by Environment Australia under the Natural Heritage Trust.
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