阿拉斯加州的海獭数目在削减?是情形污染?仍是天敌捕食?
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.
海獭是一种糊口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目急剧削减。因为海獭对于维持海岸沿线生态平衡有很是主要的浸染，所以其数目下降引起了普遍的关注。专家起头研究海獭数目下降的原因，而且很快发现了两个可能的原因，分袂是情形污染和天敌捕食。最初，情形污染说看起来更为可托。
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.
为侍趵碹染说更为可托呢?这是因为阿拉斯加海岸确实有污染源，好比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添，其数目可以导致海獭对于对于一些致命传染损失踪抵当力，进而借居导致数目下降。
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，搜罗海豹和海狮在内的其他海洋哺乳动物的数目也有所下降。这剖明造成海獭数目下降的原因也对其他海洋哺乳动物造成了影响。原因再次指向了情形污染，这是因为情形污染问题经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)那种食性普遍的动物才能经由过程捕食呈现不异的效不美观。可是，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诠释海獭数目转变的不平衡性。所谓不平衡性是指在阿拉斯加某些区域海獭数目下交窬淘，而在其他处所海獭的数目连结不决。一些专家指出这样的不雅察看结不美观剖明洋流和其他情形身分导致了海岸线上污染物的不平衡聚积。
阅读部门需要提取的不雅概念是：
- Main point: 海獭数目下降的原因是情形污染
- 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目下降
- Sub point 3:分歧区域海獭数目下降情形分歧，可以诠释为污染物聚积情形分歧导致的。
听力材料：
Well, ongoing investigations have revealed that predation is the most likely cause of sea otter decline after all.
今朝的研究剖明天敌捕食是最可能导致海獭数目削减的原因。
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，因为没有人发现有海獭尸身被冲上阿拉斯加海滩，所以污染理论捕食那么站得住脚的。如不美观污染造成的疾病传染导致了海獭数目的剧减，那么情形不应如斯。而且，很少发现海獭尸身也恰恰说了然天敌捕食假说。捕食者杀死了海獭后，会很快地吃失踪它，那么我们也就不会再岸上发现海獭尸身了
