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A huge marine mammal known as Steller’s sea cow once lived in the waters aroundBering Island off the coast of Siberia. It was described in 1741 by Georg W. Steller, anaturalist who was among the first Europeans to see one. In 1768 the animal became extinct.The reasons for the extinction are not clear. Here are three theories about the main cause ofthe extinction.
First, the sea cows may have been overhunted by groups of native Siberian people. Ifthis theory is correct, then the sea cow population would have originally been quite large,but hundreds of years off too much hunting by the native people diminished the number ofsea cows. Sea cows were a good source of food in a harsh environment, so overhunting bynative people could have been the main cause of extinction.
Second, the sea cow population may have become extinct because of ecosystemsdisturbances that caused a decline in their main source of food, kelp (a type of sea plant).Kelp populations respond negatively to a number of ecological changes. It is possible thatecological changes near Bering Island some time before 1768 caused a decrease of the kelpthat the sea cows depend on.
Third, the main cause of extinction of the sea cows could have been European furtraders who came to the island after 1741. It is recorded that the fur traders caught the lastsea cow in 1768. It thus seems reasonable to believe that hunting by European fur traders,who possessed weapons that allowed them to quickly kill a large number of the animals, was the main cause of the sea cow’s extinction.
Summarize the points made in the lecture, being sure to explain how they challenge the specific theories presented in the reading passage.
1. 阅读要点概括
Subpoint 1：The overhunting by native people could have been the main cause of extinction.
Subpoint 2：The extinction might result from the ecosystems disturbances that caused a decline in sea cows’ main source of food, kelp.
Subpoint 3：The main cause of extinction of the sea cows could have been European fur traders.
2. 听力要点概括
Point 1: With sea cow’s large size, Siberian people didn’t need to hunt a lot of them.
Point 2: The sea cows’main source of food was growing fine as there was no indication of broader ecosystem problems.
Point 3: The Europeans were not responsible for the extinction as the sea cow population had already been quite small before they arrived.
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