Now listen to part of a lecture on the topic you just read about.
现在来听一段有关你刚刚听到的话题的演讲。
The truth is we don't know what the main cause of extinction of Steller's sea cow was.
真实的情况是，我们并不知道虎头海牛灭绝的主要原因是什么。
There are problems with each of the theories that you read about.
阅读中的每一个理论都是存在一些问题的。
First, the sea cows were massive creatures.
首先，海牛是非常庞大的生物。
They were up to nine meters long and could weigh over ten tons, just enormous.
它们通常都会达到9米长，而且重达10吨，体型巨大。
A couple of sea cows could feed a small Siberian village for months.
几个海牛就够一个西伯利亚小村庄吃几个月了。
And the population of the native Siberian people wasn't very large.
况且西伯利亚本地人的人口也不是很大。
So while the Siberians certainly did hunt the sea cows, they didn't need to hunt a lot of them.
所以就算是西伯利亚人当时的确捕杀了海牛，那他们也不会捕很多。
So it's unlikely they were the ones who brought the sea cows to the point of extinction.
由此看来他们并不是造成海牛灭绝的原因。
Second, about a hypothetical decrease in kelp caused by ecological disturbances.
其次是关于生态干扰造成的海藻减少的这个假设。
Well, if something severe really happened in the ecosystem near Bering Island sometime before 1768,
如果说1768年之前，白令岛附近真的发生了严重的生态问题，
it would have affected not just the kelp, but also other parts of the ecosystem.
那么这个问题所影响的就不仅仅是海藻了，生态系统里的其他生物一定也会受到影响。
For example, it would have caused the decline in other marine animals, like whales.
比如说它就会造成其他海洋动物，像鲸鱼数量的下降。
But fishing ships in the area did not report a whale decline.
但是该地区的捕鱼船只并没有给出鲸鱼数量下降的报告。
Since there is no indication of broader ecosystem problems, the kelp was probably growing just fine, and the sea cows did not experience food shortage.
由于并没有更广泛的生态问题，海藻很可能就生长的很好并且海牛也没有经历食物短缺。
Third, it might seem like the European traders were responsible because the sea cows became extinct soon after the Europeans arrived.
第三，因为海牛在欧洲人到来不久就灭绝了所以欧洲商人的这个理由看起来似乎看起来挺合理的。
But, actually, by the time that the Europeans arrived, the sea cow population was already quite small.
但是实际上，在欧洲人到来之前海牛的数量已经非常的小了。
We have evidence that the sea cow population was at its largest hundreds of years before the 1700s.
现在有证据表明海牛在16世纪之前的数量达到了它的顶峰。
So something was causing a serious and on-going decrease in the sea cow population long before Europeans arrived in the Bering Island area.
所以在欧洲人到达白令岛之前，一定有一个东西引起了海牛数量严重、持续的下降。
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.
不管这个东西是什么，它都应该是海牛灭绝的主要原因，而不是欧洲商人。