Discuss the history, geography and ecology of the rabbit invasion in Australia
Numerous studies have been contacted on the Rabbit in Australia (Parer,I.(1977),Twigg, E.L., Lowe, J.T., Wheeler, G.A., Gray, S.G., Martin, R.G. & Barker,W.(1998), Wheeler,H.S. &King,R.D. (1985), Rolls,E.C. (1969)), covering issues such as it’s population ecology, dispersal, survival and the efficiencies of the various control methods that have been used up to date. In the early stages of the rabbit plague, fences were erected to prevent dispersal or slow the rate of dispersal, but these proved to costly and ineffective. The fifties saw the introduction of the biological control agent, myxoma virus. This had great success initially but unfortunately the government failed to capitalize on the success, with continued control. The Rabbit Calcivirus Disease (RCD) was introduced (albeit accidentally) in the early 1990’s. A highly infectious disease, spread by direct contact or by vectors (mosquito) with a mortality rate between 50-90%. However young kittens are not as susceptible as older rabbits.
(Linton 2001) and when the female goes on to breed they are able to pass on maternal antibodies to their young.
In determining whether or not the complete eradication of the rabbit in Australia is a feasible concept, one needs to study or be aware of certain aspects of their ecology. Such as their breeding and dispersal patterns and from this weaknesses might become evident, which would then aide in the eradication of the rabbit.
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The Rabbit made its’ first appearance is the Eocene in Asia and North America, arriving in Europe during the Miocene period. Two sub species from Southern France and Spain were identified O.cuniculus cuniculus and O.cunniculus buxteyi. The first named sub species from France was frequently released on islands as a food source for sailors that might become shipwrecked. The image of the rabbit had some bearing on its’ dispersal and protection. In that it provided the people with their main source of food during times of hardship or depression and therefore they attained a rather prestigious image. This in turn further aided their rapid dispersal.
The arrival of the 1st fleet in 1788 saw the introduction of the rabbit in Australia. However, it was Tasmania that had the first recording of a feral rabbit population in 1827. Mainland Australia remained rabbit free until 1859, when a grazier and sportsman arranged for the shipment of twenty-four rabbits from England. Thomas Austin released the rabbits on his property in Geelong, Victoria around Christmas of 1859. Ten years from this initial release, 14,253 rabbits were shot for sport on this same property. This illustrates the high fertility and dispersal rates of the rabbit. By the year 1910, two-thirds of Australia was inhabited by the rabbit (Parer (1982) for Ratcliffe 1959). With a dispersal rate of approximately 70km/yr (Parer 1982), the rabbit went to colonise Queensland within 30 years and reaching Western Australia within 40 years of its release. Stodart and Parer suggest that it has the fastest dispersal rate of any colonising mammal in the world.
The introduction of the rabbit had an enormous impact on the native wildlife, displacing many small-medium sized native mammals; the greater Bilby, Bettongia Leseur are but a few. The displaced was largely due to competition for food and the altering of their ecosystems. With the enormity of their population, widespread impacts were inevitable: depletion of native vegetation, competition for space, resulting warrens and burrows of native animals being overtaken by the rabbit. Predators such as foxes increased in numbers as a direct result of the high number of rabbits to prey on. Unfortunately when the rabbit numbers declined due to drought, etc the foxes would turn on the small native mammals. The economic implications were also enormous; Sloane etal (1988) puts the impact at approximately $90 million in lost production and a further $20 million on the control.
The European rabbit has an extremely high fertility rate together with a relatively short gestation period of approximately 30 days. They are able to fall pregnant immediately after giving birth. Their litter size fluctuates between four to seven kittens. Although small at birth, weighing about thirty-five grams they are able to increase their birth weight by a staggering 600% by the time they are ready to leave the warren, generally at about 21 days of age. They will be capable of breeding when they reach an age of 3-4 months (Parer 1977). The prolificacy of their breeding season is regulated by rainfall and hence the availability of food. When the rainfall is in short supply or during the occurrence of a drought, the breeding season will be short, litter sizes will be smaller and fewer females will breed (Twigg et al 1998).
The warren provides the newly born and the young kittens with shelter from the harsh elements and protection from predators. This is especially so in the open, cleared grazing land where there is little, if any, shelter or protection provided by natural vegetation (Parer 1997). Linton (2001) supports this by stating that the rabbit lowers it chances of survival outside the warren and that the warren is the centre of the rabbit’s life. This therefore seems to suggest that the destruction of the warren would facilitate in the long term eradication of the rabbit.
“Rabbit control is the (artificial) imposition of mortality. It is generally assumed that, under normal circumstances, 100% mortality as a result of control is unlikely” (Wheeler and King 1985:224). They continue by suggesting that since complete eradication is not possible, one needs to ascertain when the best time of impact would be. The best time would seem to be when the rabbit is in its’ most vulnerable state, i.e. when it is a kitten. Wheeler and King (1985) argue that by targeting the young kittens, resources are increased for those that survive and for the litters born later in that year. They suggest that targeting the adults just at the commencement of their breeding season would impact more on the actual population size.
That is, there would be a reduction in the total number of kittens or litters born during that breeding season. Linton (2001) argues that the greatest influence on the control of the rabbit is the actual rabbit habitat itself. That is a habitat which has a high rabbit population will always be highly susceptible to re-establishment. Linton (2001) continues by suggesting that a control program which concentrates on those components of the habitat that make it susceptible to rabbit infestation, would then greatly reduce re-establishment of the rabbit. Twigg etal (1998) suggest that the rabbit problem needs to be addressed on a regional basis, not as an Australian wide problem, even though it is. Parer (1982) supports this but, suggests that in it infancy a control program aimed at isolated populations would be more beneficial. Parer puts this down to the reduced invasion by immigrants due to its location and therefore the reduced population would be kept at low densities by resident facultative predators.
It would seem that the complete eradication of the rabbit in Australia is insurmountable due to the enormity of our country and its’ varied landscape. However with a management plan the targets individual regions, a reduction in the population density of the rabbit is achievable. This would involve a combination of control methods which would incorporate environmental, biological, economic factors (Linton 2001, Twigg etal 1998.Parer 1982). The biological controls still have an effect on the rabbit, so this together with mechanical controls and timing would beneficial to the overall problem. Mechanical controls such as warren ripping is an important element in the long term plan and the commencement of this should be when the rabbit population is low (Linton 2001).
Therefore the likelihood of re-colonisation is reduced. Follow up procedures of fumigation; poisoning might be necessary to ensure that the population has been eradicated from within that area. The timing of these methods seems to play an intricate role in the rabbits’ demise. Perhaps with time and proper management the eradication of the rabbit is feasible, but not without the financial support and backing of both regional and commonwealth governments. The key seems to be to start on a small scale and work up to larger regional control or eradication.
