A huge marine mammal known as Steller s sea cow once lived in the waters around Bering Island off the coast of Siberia It was described in 1741 by Georg W Steller a naturalist who was among the first Europeans to see one In 1768 the animal became extinct

A huge marine mammal known as Steller's sea cow once lived in the waters around Bering Island off the coast of Siberia. It was described in 1741 by Georg W. Steller, a naturalist 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 of the extinction.
First, the sea cows may have been overhunted by groups of native Siberian people. If this theory is correct, then the sea cow population would have originally been quite large, but hundreds of years of too much hunting by the native people diminished the number of sea cows. Sea cows were a good source of food in a harsh environment, so overhunting by native people could have been the main cause of extinction.
Second, the sea cow population may have become extinct because of ecosystem disturbances 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 that ecological changes near Bering Island some time before 1768 caused a decrease of the kelp that the sea cows depended on.
Third, the main cause of extinction of the sea cows could have been European fur traders who came to the island after 1741. It is recorded that the fur traders caught the last sea 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.

The truth is, we don't know what the main cause of extinction of Steller's sea cow was. There are problems with each of the theories that you read about.

First, these sea cows were massive creatures—they were up to nine meters long and could weigh over ten tons—just enormous. A couple of sea cows could feed a small Siberian village for months and the population of the native Siberian people wasn't very large. So while the Siberians certainly did hunt the sea cows, they didn't need to hunt a lot of them, so it's unlikely they were the ones who brought the sea cows to the point of extinction. Second, about a hypothetical decrease in kelp caused by ecological disturbances. Well, if something severe really happened in the ecosystem near Bering Island some time before 1768, it would have affected not just the kelp but also other parts of the ecosystem. For example, it would have caused a decline in other marine animals, like whales. But fishing ships in the area did not report a whale decline. Since there's no indication of broader ecosystem problems, the kelp was probably growing just fine, and the sea cows did not experience food shortage.

Third, it might seem like the European traders were responsible because the sea cows became extinct soon after the Europeans arrived. But actually, by the time the Europeans arrived, the sea cow population was already quite small. We have evidence that the sea cow population was at its largest hundreds of years before the 1700s. So something was causing a serious and ongoing decrease in the sea cow population long before Europeans arrived in the Bering Island area. Whatever this “something” was, it should be considered the main cause of the extinction, not the European traders who were just the last to arrive.

Summarize the points made in the lecture, being sure to explain how they challenge the specific theories presented in the reading passage.