英语听力文摘 English Digest 455、奇妙的海洋食物链（在线收听）

Overfishing Whales and the Environment
过度捕鲸与环境

      Until recently, when scientists studied changes in open-ocean ecosystems, they looked at changes in the bottom of the food chain that gradually make their way to the top, like food shortages and shifts in the ocean environment. A recent study, however, suggests that more attention needs to be given to top-down influences as well.

      For example, over the past few decades, there’s been a major collapse of the populations of harbor seals, fur seals, sea lions, and sea otters living on the coasts of western Alaska. Now a new study suggests that all this destruction can be traced back to humans overfishing sperm and baleen whales between 1946 and 1979.

      Killer whales, who live at the top of the ocean food chain, used to prey on sperm and baleen whales. When these were no longer available, the killer whales were forced to fish farther down the food chain. First they fed on seals, which were the easiest to catch and the most nutritionally valuable.

      Then, when seals became rare, the killer whales turned to sea lions. When the sea lion population crashed, they started fishing sea otters. The domino effect reaches even farther. Without sea otters keeping sea urchins in check, their population has exploded. And they’ve destroyed the kelp forests, and with it, the kelp forest ecosystem in southwestern Alaska.

      Animals switching food sources can have a very serious effect on the environment. And that we humans, as the ultimate predators, need to give a lot of thought to the choices we make. They may affect the food web in unintended ways.

      直到最近，当科学家们在研究远洋地区生态系统的变化时，他们观察了处于食物链最底层并且在慢慢上升的（生物）变化，比如说食物短缺、海洋环境的变化。然而，最近的一项调查显示，人们还应该更多的注意（食物链）自上而下的影响。

      比方说，在过去的几十年里，生活在阿拉斯加西海岸的斑海豹、海狗、海狮和海獭数量极具下降。现在，一项新的调查显示，所有的这些毁坏都可以归咎于人类在1946年到1979年期间过度捕捞抹香鲸和须鲸。

      虎鲸是在海洋食物链的最高层，它习惯于捕食抹香鲸和须鲸。当虎鲸不能再捕食它们的时候，它就被迫捕食处于更下一层的食物链（的生物）。她们先捕食的是最容易捕捉且最有营养价值的海豹。

      接着，当海豹变得稀少后，虎鲸便转向了海狮。而又当海狮的数量变少后，虎鲸又开始猎杀海獭了。多米诺效应又继续延伸。没了海獭约束海胆，海胆的数量变得庞大起来。并且破坏阿拉斯加啊西南部的海藻林生态系统。

      动物间交换食物来源对环境有着很严重的影响。而作为终极捕食者，人类需要好好思考一下他们所做的选择。（因为）这些选择可能会以人们所不知道的方式来影响食物网。

      Vocabularies and expressions：

      harbor seal：斑海豹
      fur seal：海狗
      sea lions：海狮
      sea otters：海獭
      sea urchin：海胆
      sperm whales：抹香鲸
      baleen whales：须鲸
      killer whale：虎鲸
      domino effect：多米诺效应（在一个相互联系的系统中，一个很小的初始能量就可能产生一连串的连锁反应，也称之为“多米诺骨牌效应”。）
      kelp forest：海藻林
      keep in check: 阻止，约束