Both the reading part and the lecture talk about the reason why Steller's sea cow is extinct. The author of the article points out three theories that may contribute to the extinction of Steller's sea cow. However, the lecturer disagrees with him, thinking all of the hypothesis is not convincing.

To begin with, the author suggests that overhunting by native people may be the main reason. It is said that because sea cows were an important food source at that time, people tended to hunt them to maintain life. The lecturer admits the value of sea cows as food. But he posits that the size of sea cows was massive, and one single sea cow could feed a village of people to survive against the background that the population of humans at that time was small. So native people would not overhunt it.

Secondly, the author contends that ecosystem disturbances are another potential contribution, causing a decline of kelp which was the main source of food for Steller's sea cow. The lecturer argues that if the degradation of the ecosystem near Bering Island indeed occurred around 1768, other species would be affected too. However, the fish ships did not report the number of wheals declined at that time. It indicates that the kelp probably grew well, and had no impact on sea cows.

Finally, the author asserts that the fur traders from Europe are responsible for the extinction of sea cows. He backs up it with the recording that the fur traders caught the last sea cow in 1768. The lecturer casts her doubt on it by bringing up the fact that the amount of sea cows had declined to a small number long before the fur traders arrived on the island, about 100 years before. So such a conclusion suffers a factual problem.