TPO15

Reading

The cane toad is a large (1.8 kg) amphibian species native to Central and South America. It was deliberately introduced to Australia in 1935 with the expectation that it would protect farmers’ crops by eating harmful insects. Unfortunately, the toad multiplied rapidly, and a large cane toad population now threatens small native animals that are not pests. Several measures have been proposed to stop the spread of the cane toad in Australia.

One way to prevent the spread of the toad would be to build a national fence. A fence that blocks the advance of the toads will prevent them from moving into those parts of Australia that they have not yet colonized. This approach has been used before: a national fence was erected in the early part of the twentieth century to prevent the spread of rabbits, another animal species that was introduced in Australia from abroad and had a harmful impact on its native ecosystems.

Second, the toads could be captured and destroyed by volunteers. Cane toads can easily be caught in simple traps and can even be captured by hand. Young toads and cane toad eggs are even easier to gather and destroy, since they are restricted to the water. If the Australian government were to organize a campaign among Australian citizens to join forces to destroy the toads, the collective effort might stop the toad from spreading.

Third, researchers are developing a disease-causing virus to control the cane toad populations. This virus will be specially designed: although it will be able to infect a number of reptile and amphibian species, it will not harm most of the infected species; it will specifically harm only the cane toads. The virus will control the population of cane toads by preventing them from maturing and reproducing.

Listening

听力原文

The cane toad won’t be as easy to get rid of as the reading suggests. The measures proposed by the reading are likely either to be unsuccessful or to cause unwanted environmental damage.

First of all, a national fence probably won’t stop the spread of the toad. That’s because young toads and toad eggs are found in rivers and streams. No matter where the fence is located, at some point there will be rivers or streams flowing from one side to the other. These waterways will be able to carry the young toads and their eggs to the other side. Since it’s only necessary for a few young toads or eggs to get through the fence in order to establish population on the other side, the fence is unlikely to be effective.

Secondly, a massive group of volunteers could have success trapping and destroying toads. But it’s likely that these untrained volunteers would inadvertently destroy many of Australia’s native frogs—some of which are endangered. It’s not always easy to tell the cane toad apart from native frogs, especially when it’s young.

Third, using the virus is a bad idea because it could have terrible consequences for cane toads in their original habitat in Central and South America. You might be wondering, how can a virus released in Australia cause harm in the Americas? Well, Australian reptiles and amphibians are often transported to other continents, by researchers or pet collectors, for example. Once the animals infected by the virus reach Central and South America, the virus will attack the native cane toads and devastate their populations. That would be an ecological disaster because in the Americas, cane toads are a native species and a vital part of the ecosystem. So if they’re eliminated, the whole ecosystem will suffer.

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