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Goannas, Rabbits and Foxes

Rabbits were first brought to Australia in 1788 with the First Fleet and were bred for food, mostly kept in cages or small enclosures. Some got away and others were deliberately released over the next 80 years but they failed to colonise the Australian continent to any noticeable extent. The native predators killed them off.

In 1845, Foxes were introduced to Victoria so the rich people could hunt them in the traditional English way. By this time many of the predators that might have controlled Foxes had been either exterminated completely in the area or greatly reduced in number. Foxes steadily spread across the state and eventually over most of the continent.

The Foxes were mostly eating native animals including young Goannas and other potential Rabbit predators. One predator of both Rabbits and Foxes is the Wedge Tailed Eagle. These had been killed off in the millions and their population level had dropped quite low.

In Tasmania the native predators were different and Rabbits became a problem much earlier than on the continent of Australia. They were released in Tasmania in the 1820's and by 1828 were a serious problem.

In 1869 there was yet another release of Rabbits. This release was in Victoria where Foxes were already established. This time the Rabbits not only flourished round their release site, but spread across two thirds of the continent, invading it at a speed unmatched by any other introduced land animal without the power of flight.

Tasmania with an area of over sixty eight thousand square kilometres is by far the biggest island off the coast of Australia. The second biggest island is Melville Island which is in the tropics and is too hot for Rabbits. Kangaroo Island off the coast of South Australia with about four thousand four hundred square kilometres is the third biggest island.

Rabbits were introduced to Kangaroo Island but the native predators killed them off. In particular, the Rosenberg’s Goannas, Varanus rosenbergi, discovered that baby Rabbits are tasty and easy to catch for an animal like the Goanna which can go down the Rabbit burrows easily. The babies of similar sized native animals were not so easy to catch because in most cases their mothers carried their babies round in pouches.

A contributing factor to the total extermination of Rabbits on the island may have been the greater number of Wedge Tailed Eagles on Kangaroo Island. The island has a number of serious trace element deficiencies in many of its soils and these were not sorted out until the pioneering scientific work of Edward D.

Carter and others in the 1950’s. This meant that until the 1950’s there were not many sheep on the island and a relatively small population of people. Wedge Tailed Eagles had not been killed off to the same extent as on the mainland.
One major difference between Tasmania and Kangaroo Island is that Tasmania has no Goannas while Kangaroo Island has about 20 Goannas per square kilometre.
Predator/Prey Mismatch
Foxes are well known for eating Rabbits. They have been part of the same ecosystems for a very long time, so although many rabbits are killed by Foxes, Foxes do not exterminate Rabbits. A prey animal either develops defenses against their predator or becomes extinct in that ecosystem.  Rabbits can escape down their borrows to escape Foxes, but against a thinner predator like a Goanna this defense is ineffective. In a balanced ecosystem on a continent or large island Foxes would not be the top land predator.It is likely that the success of at least some of the exotic invaders of the Australian continent and islands was because the ecosystems were already very badly degraded.

Picture Sources

The Pictures of Rosenbergs Goannas used in this article 
are by Paul Asman and Jill Lenoble, and are used under a Creative Commons license 2.0.