阅读材料：
The sea otter is a small mammal that lives in waters along the western coast of North America from California to Alaska. When some sea otter populations off the Alaskan coast started rapidly declining a few years ago, it caused much concern because sea otters play an important ecological role in the coastal ecosystem. Experts started investigating the cause of the decline and quickly realized that there were two possible explanations: environmental pollution or attacks by predators. Initially, the pollution hypothesis seemed the more likely of the two.
海獭是一种生活在北美西海岸从加州到阿拉斯加沿线的小型海洋哺乳动物。几年前，阿拉斯加海岸的海獭数量急剧减少。由于海獭对于维持海岸沿线生态平衡有非常重要的作用，所以其数量下降引起了广泛的关注。专家开始研究海獭数量下降的原因，并且很快发现了两个可能的原因，分别是环境污染和天敌捕食。最初，环境污染说看起来更为可信。
The first reason why pollution seemed the more likely cause was that there were known sources of it along the Alaskan coast, such as oil rings and other sources of industrial chemical pollution. Water samples from the area revealed increased levels of chemicals that could decrease the otters’ resistance to life-threatening infections and thus could indirectly cause their deaths.
为什么污染说更为可信呢?这是因为阿拉斯加海岸确实有污染源，比如石油带或者其他的工业污染物。该地的水样显示水中化学物的含量增加，其数量可以导致海獭对于对于一些致命感染丧失抵抗力，进而间接导致数量下降。
Second, other sea mammals such as seals and sea lions along the Alaskan coast were also declining; indicating that whatever had endangered the otters was affecting other sea mammals as well. This fact again pointed to environmental pollution, since it usually affect the entire ecosystem rather than a single species. Only widely occurring predators, such as the orca (a large predatory whale), could have the same effect, but orcas prefer to hunt much larger prey, such as other whales.
第二，在阿拉斯加海域，包括海豹和海狮在内的其他海洋哺乳动物的数量也有所下降。这表明造成海獭数量下降的原因也对其他海洋哺乳动物造成了影响。原因再次指向了环境污染，这是因为环境污染问题常常不是仅仅影响某个特定物种，而是影响整个生态系统。除此之外，只有像逆戟鲸(一种大型肉食鲸)那种食性广泛的动物才能通过捕食呈现相同的效果。但是，逆戟鲸更喜欢捕食像鲸类这种的大型猎物。
Third, scientists believed that the pollution hypothesis could also explain the uneven pattern of otter decline: at some Alaskan locations the otter populations declined greatly, while at others they remained stable. Some experts explained these observations by suggesting that ocean currents or other environmental factors may have created uneven concentrations of pollutants along the coast.
第三，科学家认为污染说同时可以解释海獭数量变化的不均衡性。所谓不均衡性是指在阿拉斯加某些区域海獭数量下降明显，而在其他地方海獭的数量保持未定。一些专家指出这样的观察结果表明洋流和其他环境因素导致了海岸线上污染物的不均衡聚集。
阅读部分需要提取的观点是：
- Main point: 海獭数量下降的原因是环境污染
- Sub point 1:阿拉斯加海域确有污染源
- Sub point 2:除海獭外，还有其他哺乳动物的数量下降
- Sub point 3:不同区域海獭数量下降情况不同，可以解释为污染物聚集情况不同导致的。
听力材料：
Well, ongoing investigations have revealed that predation is the most likely cause of sea otter decline after all.
目前的研究表明天敌捕食是最可能导致海獭数量减少的原因。
First, the pollution theory is weakened by the fact that no one can really find any Dead Sea otters washing off on Alaskan beaches. That’s not what you would expect if infections caused by pollution started killing a lot of otters. On the other hand, the fact that it’s so hard to find dead otters is consistent with the predator hypothesis. If an otter is killed by a predator, it’s eaten immediately so it can’t wash up on shore.
首先，由于没有人发现有海獭尸体被冲上阿拉斯加海滩，所以污染理论捕食那么站得住脚的。如果污染造成的疾病感染导致了海獭数量的剧减，那么情况不该如此。而且，很少发现海獭尸体也恰恰说明了天敌捕食假说。捕食者杀死了海獭后，会很快地吃掉它，那么我们也就不会再岸上发现海獭尸体了。
Second, although orcas may prefer to hunt whales, whales have essentially disappeared from the area because of human hunters. That means that orcas have had to change their diet to survive and since only smaller sea mammals are now available, orcas have probably started hunting those. So, it probably is the orcas that are causing the decline of all the smaller sea mammals mentioned in the passage - the seals, the sea lions, and the sea otters.
第二，尽管逆戟鲸喜欢捕食鲸类，但是鲸类的数量却由于人类的捕猎活动而剧烈减少。这意味着逆戟鲸不得不为了生存而更改捕食习惯。目前来看只有一些小型海洋哺乳动物还可以捕捉，逆戟鲸很可能开始以它们为食了。所以，很可能是逆戟鲸的捕食导致了文中提到的诸如海豹、海狮和海獭等小型哺乳动物的数量减少。
And third, the uneven pattern of otter decline is better explained by the orca predation theory than by the pollution theory. What happens to otters seems to depend on whether the location where they live is accessible to orcas or not. In those locations that orcas can access easily, the number of sea otters has declined greatly. However, because orcas are so large, they can’t access shallow or rocky locations. And shallow and rocky locations are precisely the types of locations where sea otter populations have not declined.
第三，海獭数量变化的不均衡性恰好表明了逆戟鲸捕食理论比污染理论更合理。海獭数量的增减取决于它们生存的地方逆戟鲸是不是可以进入。在那些逆戟鲸很容易进入的海域，海獭的数量急剧减少。然而，因为逆戟鲸个头很大，所以他们不能进入那些过浅或者充满岩石的海域。而那些过浅和充满岩石的海域，恰恰是海獭数量保持稳定的地区。
听力笔记内容：
Main point: 天敌说更为可信
Sub point one: 没有发现海獭的尸体
Sub point two: 多种小型海洋哺乳动物的减少是因为逆戟鲸捕食
Sub point three: 逆戟鲸个头很大，有的海域不能进入，解释了为何数量变化不均衡
提示：在阅读部分出现的陌生的名词(包括专有名词)要记下来，因为听力出现的时候我们要根据读音判断出现位置和大致含义。
