There have been many types of Ecological disaster. In this article, I am going to confine myself to disasters caused to Australian Ecosystems by introducing other animals.
The most famous one was the introduction of the Rabbit. The Rabbits in Australia are mostly descended from a deliberate introduction of a group of wild Rabbits in Victoria in south eastern Australia. These were released in the hope of starting a business selling rabbit skins.
This did actually happen, but there was nothing to stop the rabbits spreading to the rest of mainland Australia. Now we have billions. They compete with the native animals, denude the landscape of vegetation and severely reduce the potential carrying capacity of the land for grazing animals like sheep and cattle.
After doing this to the mainland, some early settlers actually introduced rabbits to Kangaroo Island. On Kangaroo Island they were killed off by the goannas which went down the rabbit burrows and ate the babies. On the mainland rabbits also had predators, but one of the main ones, the Wedge-tailed Eagle, was killed off in huge numbers in the mistaken belief that they kill lambs.
As well as the disasters caused to land ecosystems there have been ones in the water. The most obvious one was the deliberate introduction by the government of European Carp to the River Murray. The Murray-Darling River system is by far the most extensive in Australia. The carp were introduced to eat the weeds growing in the river. Now they have taken over much of the river’s ecosystem and severely reduced the numbers of native fish.
There are many other examples of introducing other fish and totally disrupting aquatic ecosystems. One I have personally observed was the extinction of the Lake Eacham Rainbow fish.
In almost every little waterway in my area there are to be found numbers of Mosquito Fish (mostly Gambusia affinis). These were introduced by the government to control Mosquitoes. Recent research suggests that they are not even particularly good at this, and the large numbers of native species were already controlling Mosquitoes very well. Now it is illegal to even possess Mosquito fish.
Of course, not all the introductions were done by the government. Guppies got into a system of underground waterways in Western Australia. These have decimated the native fish adapted to this environment. Aquarists need to be very careful to not release any of their fish into waterways. They also need to be aware that unexpected things can happen. Ponds can overflow. Normally we would not expect fish to be able to get out of an aquarium into natural rivers, but a friend of mine had a very nice Oscar. A sudden flood came. My friend, together with his wife and baby spent the night on top of their house while the water flowed through their house. Their Oscar went out with the water. In this case, the fish would not have survived. It would have been swept out to sea and died, but there must be cases where this type of unexpected event does put fish into waterways they can survive in.
A ridiculous introduction was the fox. They were brought to Australia so the upper class English who were coming to Australia could enjoy their fox hunting.
Not all introductions have been disastrous. The Prickly Pear Cactus got into the state of Queensland where it was spreading at an enormous rate. (The prickly Pear was introduced by the British Government with the first fleet.) A moth was introduced, with the very descriptive name; Cactoblastis cactorum. The caterpillars of this moth totally devastated patches of the invasive weed in tropical areas. It was not effective in cooler areas, but remains one of the most spectacular examples of biological weed control.
Another government attempt at biological control was the introduction of the Cane Toad, Bufo marinus, to control beetles in sugar cane. They failed to control the beetles and are now spreading steadily through Australia, disrupting ecosystems as they go.
Not all destructive introductions in Australia were of foreign species. There are several cases of native Australian Animals being taken to different places in Australia and causing destruction. Koalas were introduced to Kangaroo Island and increased to such an extent that they are killing off the Manna Gum trees.
There are large numbers of other examples of deliberate introductions that have caused havoc.