It would be difficult to exaggerate the economic and ecological impact of the rabbit prior
to myxomatosis. In good seasons there may have been one billion rabbits. As 16 rabbits
eat as much as one sheep (Short,1985), this is equivalent to approximately 60 million
sheep and the consequent loss of production. The economic impact of present day rabbit
populations is not well quantified but is estimated to be in the order of 90 million dollars
in lost production and about 20 million spent on control (Sloane et al.1988).
The rabbit impacts upon native wildlife in many ways:
1. By directly competing for food and habitat the rabbit has displaced many small to medium size marsupials such as the greater bilby, Macrotis lagotis (now an endangered species), and the burrowing bettong Bettongia lesueur, (now extinct on the mainland). The disapperance of these marsupials is reputed to have occured only after areas were invaded by rabbits. (Willson et al 10,1992)
2. Due to selective grazing the rabbit has changed ecosystem composition radically. When an ecosystem is changed, the dependant fauna are displaced by a depeletion in thier required food source and breeding grounds. This has marginalised various species into smaller populations and effected thier reproductive capability.
3. In its spread across Australia, the rabbit took advantage of pre-existing burrows and evicted various burrowing mammals such as the rufous hare-wallaby Lagorchestes hisutus, the bilby Macrotis lagotis and the burrowing bettong Bettongia lesueur, from their burrows. This factor contributed to the regional extinction of the bilby and of the burrowing bettong.
4. A colony of rabbits will support a high number of predaters such as feral cat and foxes. These predators put stress on small populations of native mammals. After a crash in the numbers of rabbits during a drought, the predator numbers initially drop much less dramatically than the rabbit and remain high putting intense pressure on the small populations of native mammals. (Myer & Parker, in Williams et al, 79). Unlike the rabbit, which can quickly recover from a population crash, native mammals are not such prolific breeders and their numbers increase slowly. They rarely repopulate areas where they have become locally extinct due to isolation.
5. Wildlife were often killed by poisions and traps set for rabbits. Rat-kangaroos, tiger-cats and magpies were some of the species inadvertently effected. Goannas, wombats and bandicoots were often directly poisioned as they were percieved as a menace to the farmer (Rolls,173-5).
6. Late last century the populations of species plumeted as shooters hired to cull rabbits, also culled wildlife for the pelt trade.
7. In South East Australia and South Australia bounties and bonuses were paid for wombats scalps up untill 1966, as the wombat was destructive to rabbit proof fences (Rolls, 162-3).
Due to selective grazing, the rabbit has changed ecosystem composition radically "Biomass and cover are reduced as perennial grasses and shrubs are replaced with annual species and then an increasing number of unpalatable and woody weeds" (Williams).
During drought, rabbits will kill trees and shrubs by ring barking and digging to eat the roots in search of moisture. Hence, rabbits not only control species germination, but the species composition of mature plants thus effecting biodiversity.
C. Soil Erosion
Due to high population numbers, the impact of rabbits prior to the introduction of myxomatosis was imense. In the semi-arid and arid zones the rabbit continues to degrade the soil by destroying the stability of the soil by the removal of vegetation cover. By denuding the landscape, as rabbits will in droughts, it is leaving the soil highly susceptible to various forms of erosion and loss of fertility. Phillip Island (above and below) has severe erosion problems due to the presence of rabbits.