The possible causes of extinction of Steller's sea cow are explained by the passage. However, the lecture completely disagrees with the reasonings given by the passage providing some explanations.
Firstly, the passage claims that overhunting of sea cows by the groups of native Siberian people had cause the extinction. In contrast, the lecture affirms that the animal was so massive in size weighing about tens tons which would be more than enough to feed the entire group of people for a month. Therefore, overhunting could not be the cause.
Secondly, the passage talks about the ecosystem disturbance which led to the decline in seafoods (kelps) for the cows causing decline in number. But the lecture says the cause is inconsequential and if there was imbalance in the ecosystem of Bering island, other marine animals would also be affected. There was no any decline in other animals and even kelps were growing well. Thus, the second cause is ruled out by the passage.
Thirdly, the passage contends hunting of sea cows for fur trading by the Europeans had cause the decline of sea cows. The lecture, in other hand, explains that their numbers were relatively low before the arrival of Europeans. So, they could not be taken as the cause being last to arrive in the island.