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第1段
1 .<-NARRATOR:->Now listen to part of a lecture on the topic you just read about.
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第2段
1 .<-FEMALE PROFESSOR:->The truth is, we don’t know what the main cause of extinction of Steller’s sea cow was.
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2 .There are problems with each of the theories that you read about.
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第3段
1 .First, these sea cows were massive creatures—they were up to nine meters long and could weigh over ten tons—just enormous.
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2 .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.
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3 .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.
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第4段
1 .Second, about a hypothetical decrease in kelp caused by ecological disturbances.
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2 .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.
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3 .For example, it would have caused a decline in other marine animals, like whales.
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4 .But fishing ships in the area did not report a whale decline.
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5 .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.
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第5段
1 .Third, it might seem like the European traders were responsible because the sea cows became extinct soon after the Europeans arrived.
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2 .But actually, by the time the Europeans arrived, the sea cow population was already quite small.
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3 .We have evidence that the sea cow population was at its largest hundreds of years before the 1700s.
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4 .So something was causing a serious and ongoing decrease in the sea cow population long before Europeans arrived in the Bering Island area.
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5 .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.
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