In this set of material, the reading passage suggests three theories of the main cause of Steller's sea cow extinct. However, the lecture finds all the idea dubious and skeptical and unlikely to help them and provides some evidence to refute them all.
First of all, the reading passage states the extinction might be occurred because of the over hunted caused by the native Siberian people. As the sea cows were a good source of food in a harsh environment where the Siberian people live. On the contrary, the professor opposites this theory by explaining that the sea cows were very massive animals and weight above two tons, at the same time the Siberian population was very small. In order to that, it is unbelievable that such small population was fed on the large quantity of massive animals.
Furthermore, the reading passage poses that ecosystems disturbances might be another cause of sea cow extinction as they were made decline in the main source of food of the sea cows. Conversely, the professor refutes this reason by saying if that really was happened, it would affect other marine animals as well, because they also used to eat the kelp plant. But there were not any decline in other marine animals habitat.
Finally, the reading passage mentioned that the European people traders might caught the sea cows during their visit. In contrast, the professor refutes this reason by claims that the extinction was already done before the European trader came to the island, and they were the last people went to the island.