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阅读材料： 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 pest. Several measures have been proposed to stop the spread of the cane toad in Australia. 蔗蜍是一种原产于中南美洲的两栖动物，个头很大，重大1.8公斤。蔗蜍是在1935年由人工引进至澳大利亚的。因为它们可以吃害虫，所以希望借由它们保护农作物。不幸的是，蔗蜍繁殖迅速。而且，数量巨大的蔗蜍群体已经开始威胁一些对农业无害的小型本土生物。现在提出了几个解决蔗蜍在澳大利亚蔓延的方法。 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. 第一个阻止蔗蜍蔓延的方法是建立一个全国范围的围栏。用围栏控制蔗蜍增长的方式是通过阻止它们迁徙实现的，这样它们就不会聚集到那些尚未被它们控制的地区了。这种方法以前就用过。在二十世界初期，澳大利亚就从用这种围栏阻止兔子的过度增长。兔子由澳大利亚从他国引进后，给当地的生态平衡造成了巨大的灾难。 | 
