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。