Wild Rabbits

WILD RABBITS have been causing widespread devastation in Australia for over 100 years. Rabbits have found conditions in Australia and drier areas of New Zealand to be ideal. Both countries have attempted to control the rabbit population with a combination of measures including: 

  • destroying warrens through ripping, ploughing, blasting, and fumigating; 
  • poison baiting; 
  • shooting and hunting with dogs and ferrets; 
  • releasing predators (such as cats and foxes); 
  • biological control (for example, the myxoma virus and rabbit fleas); 
  • rabbit proof fencing. 

These methods have impacted on the rabbit population. However, the rabbit's breeding vigour and adaptability have made effective control, particularly in some remote and less productive areas, impossible. Experience clearly shows that no one control method can solve Australia's rabbit problem. A combination of biological control and conventional methods is needed. 

Myxomatosis
When first introduced in the 1950s, myxomatosis killed 99 per cent of the rabbit population: the kill rate today is often less than 50 per cent. After release of the myxoma virus, insufficient attention was given to the need for conventional rabbit control methods to cull those rabbits which survived myxomatosis. The virus evolved into less virulent forms that allowed both the rabbit and virus to survive. At the same time, rabbits developed greater resistance to the disease. These factors allowed rabbit numbers to increase again to plague proportions in several areas, and the opportunity to keep rabbit numbers low was lost. 

Commercial harvesting
Commercial harvesting, including hunting, has not proved effective in controlling wild rabbits to low enough numbers to protect natural resources. Shooting may remove a portion of the population but these are quickly replaced by reproduction. 

Harvesting rabbits is a relatively small industry in Australia. The rabbit industry is unlikely to kill enough rabbits to control them and maintain numbers at low levels. Rabbit shooters harvest a small percentage (one to two per cent) of the population and even after a 70% reduction in population size, rabbits can regain their former numbers within a single year. Based on 1991 figures, shooters would need to take approximately 150 million rabbits annually to begin to reduce the size of the rabbit population. 

In 1991, two to three million rabbits were harvested annually in Australia with a wholesale value of about $5.2 million to $5.8 million (Ramsay 1991). The total rabbit population in Australia is estimated between 200 million to 300 million. 

Exported rabbit products were worth approximately $2.5 million in 1990/91. However, as noted by Williams et al. (1995) "This needs to be put into perspective with other industries. For example, most harvested rabbits came from the Lake Eyre Basin. Cattle production from this area is estimated at $290 million annually (Ramsay 1991)." 

Cooke (1991) also notes the following: 

Overseas markets would be incapable of absorbing a 50-fold increase in rabbit harvest rates in Australia. 
The vast majority of rabbit meat eaten in Europe is from domestic rabbits and Europeans would not allow the Australian rabbit meat industry to take over traditional European markets. 

In arid areas, rabbit populations fluctuate dramatically in relation to food supply. Commercial shooting has virtually no effect on these seasonal changes in numbers. 

The rabbit meat industry in Australia is small in relation to the wool and livestock industries but the damage which rabbits cause to those industries is very serious. For example, in 1991, one station sent off 7 000 head of cattle which, at $450 each, have a value equivalent to that of all the rabbits shot in Australia over the previous year. 

Integrated control
Rabbit control is currently dependent on conventional control measures. These include poisoning, warren ripping, and fumigating. A study by CSIRO's Division of Wildlife and Ecology investigated which combinations of these measures had the greatest impact on rabbits in rangelands. Warren-ripping was found to be the most effective method and recolonisation can be prevented by a co-operative effort among landholders. 

It is important that new controls are complementary to existing techniques. Only carefully designed control programs, incorporating conventional and new control methods, will maximise the reduction of rabbit damage. The RCD Program is one of several projects developing new tools to combat the rabbit problem. 

The Spanish rabbit flea
For many years, scientists have been evaluating means to enhance the effectiveness of myxomatosis to increase rabbit mortality in arid areas. In 1990 a team from the South Australian Animal and Plant Control Commission imported a new species of rabbit flea from Spain. Two years of tests demonstrated that the flea cannot survive on native and domestic animals and was therefore safe to release from quarantine for field testing. 

The flea has been released into rangelands in South Australia, New South Wales, Queensland, and the Northern Territory. The Commission anticipates that the flea will assist the spread of the myxoma virus in drier areas of the continent. 

Immunocontraception
To date, the myxoma virus has only been used as a direct form of rabbit control. When first introduced into Australia in the 1950s, it wiped out a large percentage of the wild rabbit population. However, its effectiveness diminished due to increased rabbit resistance and because the virus evolved into less virulent strains. 

Scientists now believe that it is possible to modify the myxoma virus to include genes that will prevent conception in rabbits. This approach directly attacks the feature which gives the rabbit its greatest advantage: a high reproductive capacity. Such genetic manipulation should impede natural selection by ensuring that those rabbits which survive myxomatosis do not breed and produce more rabbits that are resistant to myxomatosis. 

This research is being conducted by scientists in the Cooperative Research Centre for Biological Control of Vertebrate Pest Populations. They are also investigating the strain differentiation and epidemiology of the myxoma virus, and exploring ways to apply this new technology to other feral pests, especially foxes. This is long term research, expected to take eight to ten years. 

Scientific research being conducted on rabbit control techniques is encouraging, but its success cannot be guaranteed. Conventional control methods will continue to play an important role in any rabbit control program. New tools being developed will enhance the effectiveness of current control methods and provide the opportunity to reduce the rabbit population to a manageable level on a long-term basis. 

Why do we need rabbit calicivirus?
The rangelands present the greatest problem for rabbit control. Some areas contain 100 warrens per square kilometre. Working on an average of 30 rabbits per warren this adds up to 3,000 rabbits per square kilometre, a very high level of unmanaged grazing pressure. 

The vast area and absence of reliable myxomatosis carriers makes control especially difficult in arid areas. Falling primary production commodity prices and fewer farm workers means that less resources can be put into rabbit control. Australian farmers and the environment suffer the consequences as rabbit damage continues. Rabbit calicivirus disease could provide the incentive to turn this around. 

In New Zealand, thousands of hectares in the South Island have been degraded by a growing population of rabbits. Rabbit control is also necessary over extensive parts of the North Island's east coast and other areas of free-draining soils. Rabbits cause severe ecological damage even at low densities. New approaches are needed to complement traditional control measures. Myxomatosis was never introduced into New Zealand so other biological controls are needed to complement current control methods.

 

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