Reading
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 muchlarger 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.
Listening
Well, ongoing investigations have revealed that predation is the most likely cause of sea otter decline after all. Well, ongoing investigations have revealed that predation is the most likely cause of sea otter decline after all.
First, the pollution theory is weakened by the fact that no one can really find any dead sea others 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.
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.
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.
Question:Summarize the points made in the lecture.being sure to explain how they cast doubt on specific points made in the reading passage.
The reading passage discusses causes of possible declining of sea otters along Alaskan. However, the professor in the listening passage disagree and refutes each of the author’s reasons by giving others recent evidences.
First, the reading claims that the declining of sea otters are caused by the environment pollution on the Alaskan coast. Evaluation of water samples revealed the presence of oil, as a result it would cause the death of the mammas by losing the resistance and exposing to infections. The professor refutes this point by saying that recent investigations have not found proofs of sea otters cover with oil.
Second, the article posts that the oil pollution has not effected all the ecosystem of Alaskan coast. Sea otters and others mammals has declined but Orcas would not have the same risk because they prefer whales where they live far from coast. However, the professor disagree by saying that the main prey of orcas has disappeared by fleeing from hunting human.
Third, the reading says that the population of otters are different in places, in one place the numbers are stable while in others are remarkably declining caused by ocean current that carry oil. The professor opposes this point by explaining the pollution is not causing. At the declined population place is because the coasts are shallow and Orcas cannot enter while the other place is easy to access.
TOEFL listening lectures: Why does the professor mention the weather at the beginning of his lecture?