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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.
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 rigs 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.
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 affects 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.
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.
Summarize the points made in the lecture, being sure to explain how they support/contradict specific points made in the reading passage.
网老师提醒：用TPO作为托福备考教材和练习工具，已经成为全球考生必选。由于TPO试题的权威性和实用性，SmartStudy(网)组织了权威师资对TPO进行了逐套逐项逐题的详细讲读，包括题目理解，答题思路，解题步骤，答案分析和难点重点针对解析，覆盖最全面的做题逻辑，输出细致贴心的答题辅导，并以精美的课件清晰呈现。相信SmartStudy(网)托福版块的“TPO逐题精讲”能有效帮助您查漏补缺，完美备考。
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