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The geographical isolation of Australia has resulted in the development of many delicate ecosystems that are very sensitive to exotic invaders and in many cases cannot provide natural predators for many of the species introduced. The introduction and spread of invasive animals can disrupt the balances between populations, and possibly develop into environmental problems. Given the drastic destruction that some introduced species have caused, no more exotic species should be introduced to Australia. Feral cats and red foxes are two of the major invasive alien species in Australia and have been held responsible for the decline and extinction of many native animal species. Besides, rabbits - once causing the infamous rabbit plague in Australia - are a well-known invasive species devastating to vagetations. Although some species, cane toads for example, were introduced for good purposes, they often ended up being uncontrollable and very destructive.
Feral cats are a serious threat to many local animals in Australia. They are descendants of domestic cats that have returned to the wild (Holton, 2007). Cats were first introduced to Australia as pets in 1804 and became feral over 90% of the continent by 1890 (Abbott, 2008). Feral cats can spread very fast and are highly skilled predators. They can colonise a wide range of habitats. As carnivores, they eat a wide range of prey and can survive with limited access to drinking water. The survival rate of kittens is not high, but cats can breed in any season, allowing rapid increases in numbers. Cats have direct impacts on native fauna through predation. They can kill vertebrates weighing as much as 3 kg (Dickman 1996), but more likely kill mammals weighing less than 220 g and birds less than 200 g. They also kill and eat reptiles, amphibians and invertebrates (Dickman 1996). Cats can also have indirect effects on native fauna by carrying and transmitting infectious diseases (DEH 2004). They are thought to have contributed to the extinction of a number of small to medium-sized mammals species and ground-nesting birds species in the arid zone, and to have seriously affected populations of bilby, mala and numbat (DEH 2004). For example, Cats are believed to be a factor in the extinction of the only mainland bird species lost since European settlement, the Paradise Parrot ("Psephotus pulcherrimus"). Such devastation was never expected. Therefore, it is not wise to take the risk to introduce any other species to Australia.
In addition to feral cats, red foxes are also one of the most destructive invasive species in Australia. Red foxes were first introduced to Australia for the purpose of the English traditional fox hunting sport. Current estimates show that there are more than 7.2 million red fox with a range spreading throughout most of the continental mainland (Macdonald, 1987). Large populations of red fox can be found in all states in Australia except Tasmania and are widespread except the Kimberley, northern Queensland, and the Top End of the Northern Territory. Red foxes are responsible for the extinction of at least ten native species in Australia. For example, the increase in populations of red foxes corresponds with the decline and extinction of population of the Potoroidae family, including the extinction of the Desert rat-kangaroo (Short, 1999). Additionally, The spread of the red fox population is directly linked to the decreases in the distributions of some medium-sized ground-dwelling mammals species including burrowing bettongs, brush-tailed bettongs, bridled nailtail wallabys, bilbys, numbats, rufous bettongs and quokkas ("Threat Abatement Plan", 2001). Most of these species are now limited in areas where red foxes are rare or not present, such as islands. Although some management has been conducted, there are still huge populations of red foxes killing millions of animals. Government must learn from this and allow no more introduced species.
Besides cats and foxes killing animals, rabbits cause great damage to vegetations in Australia. Rabbits were first introduced to Australia in 1788 as food animals. Once they were released, rabbits' population has grown very fast due to their lack of natural enemies and their vast range of diet. They are considered as the most serious known factor in species loss in Australia. It is stated that rabbits often destroy young trees in forests, orchards, and on properties by ring barking them (Australian Encyclopaedia). Consequently, rabbits are also responsible for serious land erosion problems, because they eat native plants, leaving the top soil exposed and vulnerable to sheet, gully, and wind erosion. Removing this top soil is devastating to the land because it takes hundreds of years to regenerate. Additionally, rabbits cause millions of dollars of agricultural damage. It has taken many years to control the rabbit plague until the introduction of rabbit haemorrhagic disease (RHD) successfully reduced the population of rabbits in Australia in 1991 ("Rabbit Calicivirus Disease"). Once a bio-invader spreads out, it is nearly impossible or very hard to control. To prevent this from happening, it is better to prevent any exotic species from entering Australia.
Some experts claim that it is beneficial to introduce some exotic species for good purposes such as controlling pests, but according to some examples of failure, it is not safe to do so. For instance, the cane toads, native to Central and South America, were introduced from Hawaii to Australia in June 1935 by the Bureau of Sugar Experiment Stations and were used to control the native cane beetles which are detrimental to sugar cane crops (Australian Museum). Since they were released, toads have multiplied in population rapidly and now have a total population of over 200 million and are known to spread diseases affecting local species. Unfortunately, not only has the introduction of the toads caused large environmental detriment, but also there is no evidence that they have impacted the population of the cane beetles which they were introduced to predate ("Killing off the cane toad"). The main effects that cane toads have caused are the decrease in number of native species which die eating cane toads, the poisoning to pets and humans, the decrease of native fauna that are preyed on by cane toads, and depleted prey populations for native species that prey on insects, such as skinks (National Geographic). Even if the intention is good, accepting introduced species is a risk that government cannot afford to take.
In general, no more introduced species should ever be allowed in Australia. Feral cats and red foxes cause extinction; rabbits destroy vegetations and lands; toxic cane toads poison local predators. These are real-life example warning the government of Australia not to accept alien species any more.