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Biodiversity Group Environment Australia, 1999
0 642 2546355
Before 1950 there was a substantial wild harvest rabbit industry trading products in both the domestic and export markets. Exports peaked in 1948-49 when 50 million rabbits were exported with a value equal to the total of mutton and lamb shipped in the same year (Ramsay 1994). Following the introduction of myxomatosis in the 1950s rabbit populations collapsed, as did the supply to the commercial trade. Current commercial interest in rabbits in Australia is based on both a domestic farmed rabbit industry that commenced in 1987 and a small wild harvest industry. Commercial use of wild rabbits takes the form of field shot animals to supply game meat and pet meat markets and skins, which are mostly used in the felt hat industry. Ramsay (1994) noted that the wild harvest industry was based in regions where rabbit population density was high and rainfall low as these areas produced rabbits with lean white meat and white body fat preferred by consumers.
The production of large quantities of meat and fur from domestic rabbits at prices that are very competitive with wild harvested animals has occurred as a result of developments in the husbandry of domestic rabbits. This has led to a steady decline in the commercial harvest of wild rabbits (Ramsay 1994). Further decline has occurred since the release of RCD as a new biological control agent. This disease is most effective in the low rainfall areas where the wild harvest industry has been based, and where myxomatosis has been less successful.
While it has been suggested that commercial use may be a socially acceptable method of controlling rabbits, it has limited capacity to mitigate the damage they cause. Commercial harvesting of rabbits is restricted to areas of low rainfall with high population densities (Ramsay, 1994). Commercial operators harvest rabbits only while they are present at densities substantially above the level needed to achieve effective management of rabbit damage. There are no Australian data on the impact of commercial rabbit harvesting on rabbit populations, but evidence suggests that the reductions are too small and too localised to aid conservation management.
The annual wholesale value of Australian wild rabbit products, including exports was estimated at about $ 9 million in 1991-92. In addition, about $2.5 million is invested in capital equipment, and over 300 processing staff and shooters are employed (Ramsey, 1994). In comparison, rabbit damage is reported to cost Australian agriculture between $150 million (Morris, 1996) and $600 million per year (ACIL, 1995). If the non-monetary costs of damage to the environment are added to this equation it becomes clear that the value of the commercial rabbit industry is trivial in comparison to the costs wild rabbits impose on the environment, agriculture, and forestry.
Where an industry is based upon a wild harvest of a feral animal and this is compromised by improved control measures, it can result in calls for compensation. The Standing Committee on Agricultural Resource Management, at its meeting in October 1994, recognised this problem when it supported the development of industries based on the use of feral animals but agreed that such development should include the objective of eliminating rather than encouraging the propagation of those species in the wild. The presence of an industry dependent upon a regular supply of wild harvested rabbits was compromised by the introduction of myxomatosis during the 1950s. This was further compounded by the recent release of RCD which resulted in substantial declines in rabbits in the arid and semi-arid zones where myxomatosis had been less successful as a population control measure due to the absence of a suitable vector.
Rabbit control operations can impact both directly and indirectly on a wide range of other species. Direct impacts occur when the control technique used kills species other than rabbits. Indirect impacts occur when the reduction of rabbit numbers affects species which rely on rabbits as a source of food or when the decline in grazing pressure as a consequence of reduced rabbit numbers leads to the regeneration or return of plants that have been suppressed by rabbits. Some of the vegetation changes associated with a reduction in rabbit impacts may be seen as incompatible with other land uses for example the regeneration of species which are not palatable to domestic livestock on pastoral properties.
The potential impact of poisoning on native non-target species must be carefully considered, particularly in areas of high conservation value. Non-target species may be exposed to poison following direct ingestion of the bait (Brunner, 1983; McIlroy and Gifford, 1991) or by scavenging rabbit carcasses following a poisoning program (McIlroy and Gifford, 1992).
The sensitivity of native animals to 1080 varies greatly and appears to depend partly on the level of natural exposure to this compound. Animals of the same species occurring in different parts of Australia show different levels of sensitivity. For example, individuals from an ACT population of Rattus fuscipes were much more sensitive to 1080 than individuals from a population in Western Australia. Twigg and King (1991) demonstrated that many native animals that coexisted with vegetation containing this toxin have evolved to have a marked tolerance to 1080. The majority of less sensitive species occur in south-west Western Australia since this is where vegetation containing the toxin predominates.
Risks of poisoning to non-target species can be minimised by pre-baiting the target area to ensure that rabbits are the main bait-takers, using baits which are attractive only to rabbits, and using the minimum concentration needed to kill rabbits. Collecting the carcasses of poisoned rabbits will also assist in minimising the risks of secondary poisoning (Williams et. al. 1995).
The potential impacts on native fauna arising from the release of RCD and other rabbit control programs have been comprehensively reviewed (Newsome et. al. 1997). Foxes, dingoes, feral cats, quolls, some large reptiles and several species of diurnal raptors all prey on rabbits (Baker-Gabb, 1983; Newsome et. al. 1983; Catling, 1988; Belcher, 1995). The level of dependence on rabbits may vary according to rabbit density and availability relative to other prey (Newsome et. al. 1983; Catling, 1988). Feral rabbits have been demonstrated to be an important component of the diet of 11 of 24 species of Faliconiformes found in Australia (Baker-Gabb and Steele, 1996). Anecdotal evidence suggests that following rabbit control through myxomatosis the clutch sizes of raptors relying on rabbits as a main source of prey declined, while those of species which did not rely on rabbits remained unchanged (Olsen and Marples, 1992).
All of these predators are capable of subsisting on other prey when rabbits are scarce or absent. There is speculation that a catastrophic decline in rabbit abundance, such as that following an RCD epizootic, could lead to increased predation on threatened fauna (Williams and Munro, 1994). Whether such a change is likely to occur as a result of prey-switching per se or as a result of predators taking supplementary or opportunistic prey items is unknown (Newsome et. al. 1997). Alternatively, a reduction in rabbit numbers may be sufficient to reduce densities of introduced predators such as the fox and feral cat. There is already evidence that this has occurred in northern South Australia, where numbers of both foxes and cats have been reduced by over 70 per cent since RCD arrived (Anon, 1998). This could lead to decreased predation on threatened fauna. Clearly, these issues must be carefully considered in planning rabbit control programs in areas where native or introduced predators rely on rabbits as their main source of food. In some areas it might be deemed prudent to conduct concurrent control programs targeting introduced predators during any rabbit control program, so that encounters between threatened fauna and introduced predators are minimised.
In some regions of Australia certain tree and shrub species are considered weeds, because they are thought to suppress the growth of grasses and herbs (Tatnell and March, 1991). Grazing by rabbits prevents the regeneration of trees and shrubs, and there is concern that reduced rabbit numbers could increase the growth and spread of these weeds (Bureau of Resource Sciences, 1996). The relationship between rabbits and the occurrence of these weeds is not clear (Williams et. al. 1995). Nevertheless, it would be more prudent for land managers to control the spread of weeds using appropriate techniques, rather than to rely on rabbits for this control (Bureau of Resource Sciences, 1996).
Surface dwelling rabbits pose a special problem to managers of conservation reserves. Poisoning and destruction of surface harbour are the only effective techniques for control of surface dwelling rabbits (Williams et. al. 1995) yet the removal of surface harbour is likely to adversely affect native species. With due caution, poisons can be used with minimal risk of adversely affecting non-target species.
Interaction with other herbivores
Too many herbivores in an area can lead to overgrazing and land degradation. Whereas domestic livestock numbers can be actively controlled by land managers, there are a range of other herbivores, rabbits and feral goats in particular, that may be significant contributors to total grazing pressure but are not as easy to control. These species are not normally considered in determining total stocking rates on an area but their numbers, combined with domestic livestock numbers, may exceed the safe stocking rates for the land. The impacts of these species will be most pronounced during drought when animals will be competing for declining food and water resources. Studies in the Broken Hill district by Tatnell (1991) showed that rabbits were responsible for 5 to 50 per cent of the total grazing pressure. Mutze (1991) estimated that the grazing pressure due to rabbits was seven times the average stocking rate for his study site in South Australia. Herbivores are also known to selectively feed upon species that are highly palatable to them. Studies by Auld (1990), Cooke (1991a) and Henzell (1991) indicate that rabbits by themselves are capable of preventing the regeneration of a range of native trees and shrubs. Where conservation of a particular native species is the objective of management, it will be important to determine the relative contribution to the damage caused by each species of herbivore.
Decisions on the effective allocation of resources to control feral herbivores require a more detailed understanding of the interactions between the individual species in an area.
Foxes and cats
Rabbits and foxes are thought to have the same distribution on mainland Australia and rabbits are one of the preferred foods for foxes. Where rabbit numbers are high, fox populations generally thrive. When rabbit numbers drop, fox populations often decline (Williams et. al. 1995). Rabbit control is therefore of critical importance to any operation aiming to achieve long-term suppression of fox numbers (Newsome, 1990).
Although all feral cats prey on native animals, in areas where rabbits occur these are the main prey item. Feral cats show a preference for young rabbits but are able to survive on alternative prey when rabbit populations decline (Catling 1988). In rabbit areas feral cats will prey upon native animals opportunistically, particularly when rabbit numbers decline (Williams et. al.. 1995; Newsome et. al.. 1996).
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).
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.
Given the high level of interaction between rabbits, cats and foxes, activities identified in this plan must be integrated wherever possible with those detailed in the threat abatement plans for the cat and fox.
There is an increasing community expectation that all animals, including pests, are to be treated humanely (Braysher, 1993). Animal welfare issues must therefore be an important consideration when planning a rabbit control operation.
In general, the National Consultative Committee on Animal Welfare (NCCAW) (1992) has advocated the use of techniques which result in high level and long lasting control, therefore reducing the need to frequently apply controls. The techniques used however must still be as humane as possible. Techniques that are generally considered inhumane are steel jaw traps and fumigation using chloropicrin (Williams et. al. 1995) and their use is not recommended.
There is still some debate as to the humaneness of 1080 poisoning. Williams et. al. (1995) concluded that there is no evidence to suggest that rabbits suffer severe or prolonged pain. Gregory (1991) outlined the case that 1080 poisoning is humane and reported that the majority of humans who have ingested a sub-lethal dose of 1080 report no pain. Kill rates with 1080 can be in excess of 90 per cent and when combined with some form of harbour destruction, can result in long lasting control. The NCCAW (1992) suggest that licences to use 1080 be made available on the condition that follow-up control, such as ripping, is used. This would limit the over-use of 1080 but in some areas, such as much of Western Australia, this may not be feasible.
Other poisons used to control rabbits include pindone as an oral bait and phosphine as a fumigant. Pindone, an anticoagulant, appears to be less humane than 1080 since bleeding into joints can occur resulting in severe pain. Phosphine is more humane than chloropicrin, but the animals’ length of suffering depends on the structure of the warren and environmental conditions (Williams et. al. 1995).
The humaneness of ripping depends on the depth of warrens and population density. Where warrens are deeper than the tines of the ripper, rabbits will be trapped in tunnels where they suffocate or starve slowly. This type of death is less acceptable than rapid asphyxiation that kills rabbits in tunnels that are collapsed by the tines. The humaneness of this technique could be improved by conducting a poisoning campaign using 1080 prior to ripping, reducing the number of rabbits that will suffer a slow death. The NCCAW (1992) considers this is more humane because it achieves longer lasting control.
Lenghaus (1993) observed that rabbits inoculated with RCD died quietly within about 36 hours. In this acute form RCD appears to be a very humane form of control, which compares favourably with myxomatosis and other rabbit control techniques. Myxomatosis can be acute or chronic and the latter form may cause prolonged suffering (NCCAW, 1992). When rabbit populations are deliberately inoculated with the myxoma virus the animal welfare implications of the particular strain of the virus must be considered (Williams et. al. 1995).
Regardless of the technique used, any significant rabbit control program should be evaluated to ensure it meets appropriate animal welfare standards.
Many Aboriginal people regard exotic species such as the rabbit as having just as much right to inhabit the land as the indigenous fauna, and are generally opposed to control programs which do not make use of the animals destroyed (Rose, 1995). The decline and extinction of many of Australia’s small mammal species, particularly in the arid and semi-arid zones, was associated with the appearance of the rabbit (Calaby, 1969). Many of the mammal species which were used by Aboriginal communities in central Australia for sustenance either disappeared altogether or became exceedingly scarce. The rabbit however was easily obtained and became a favoured food item for many central Australian Aboriginal communities (Burbidge et. al. 1988; Reid et. al. 1993). Rabbits are also commercially exploited by some of these communities, providing economic and employment benefits (Williams et. al. 1995).
Rabbit hunting for sport and game meat is a relatively important pastime for some Australians, although many hunters perceive rabbits to have little sporting merit. Rabbits will not be eradicated in the foreseeable future, so hunting opportunities for rabbits remain available. Nevertheless effective rabbit control campaigns should reduce the available opportunities.
In order to minimise potential conflict and misunderstanding, the involvement of all groups with an interest in the planning and implementation of rabbit control programs should be encouraged wherever possible.
Under the Income Tax Incentives Act 1997 (Subdivision 387-A) expenditure on preventing and treating land degradation is eligible for a rebate or deduction. This includes:
The availability of these incentives should encourage landholders to undertake rabbit control programs and simplify the coordination of control programs across lands of different tenure.
Published June 1999 by Environment Australia under the Natural Heritage Trust.
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