The sea otter is a small mammal that livesin waters along the western coast of North America from California to Alaska.When some sea otter populations off the Alaskan coast started rapidly declininga few years ago, it caused much concern because sea otters play an importantecological role in the coastal ecosystem. Experts started investigating thecause of the decline and quickly realized that there were two possibleexplanations: environmental pollution or attacks by predators. Initially, thepollution hypothesis seemed the more likely of the two.
The first reason why pollution seemed themore likely cause was that there were known sources of it along the Alaskancoast, such as oil rings and other sources of industrial chemical pollution. Watersamples from the area revealed increased levels of chemicals that coulddecrease the otters’ resistance to life-threatening infections and thus couldindirectly cause their deaths.
Second, other sea mammals such as seals andsea lions along the Alaskan coast were also declining; indicating that whateverhad endangered the otters was affecting other sea mammals as well. This factagain pointed to environmental pollution, since it usually affect the entire ecosystemrather than a single species. Only widely occurring predators, such as the orca(a large predatory whale), could have the same effect, but orcas prefer to huntmuch larger prey, such as other whales.
Third, scientists believed that thepollution hypothesis could also explain the uneven pattern of otter decline: atsome Alaskan locations the otter populations declined greatly, while at othersthey remained stable. Some experts explained these observations by suggestingthat ocean currents or other environmental factors may have created unevenconcentrations of pollutants along the coast.
- Main point: 海獭数量下降的原因是环境污染
- Sub point 1:阿拉斯加海域确有污染源
- Sub point 2:除海獭外，还有其他哺乳动物的数量下降
- Sub point 3:不同区域海獭数量下降情况不同，可以解释为污染物聚集情况不同导致的。
Well, ongoing investigations have revealedthat predation is the most likely cause of sea otter decline after all.
First, the pollution theory is weakened bythe fact that no one can really find any Dead Sea otters washing off on Alaskanbeaches. That’s not what you would expect if infections caused by pollutionstarted killing a lot of otters. On the other hand, the fact that it’s so hardto find dead otters is consistent with the predator hypothesis. If an otter iskilled by a predator, it’s eaten immediately so it can’t wash up on shore.
Second, although orcas may prefer to huntwhales, whales have essentially disappeared from the area because of humanhunters. That means that orcas have had to change their diet to survive and sinceonly smaller sea mammals are now available, orcas have probably started huntingthose. So, it probably is the orcas that are causing the decline of all thesmaller sea mammals mentioned in the passage - the seals, the sea lions, andthe sea otters.
And third, the uneven pattern of otterdecline is better explained by the orca predation theory than by the pollutiontheory. What happens to otters seems to depend on whether the location where theylive is accessible to orcas or not. In those locations that orcas can accesseasily, the number of sea otters has declined greatly. However, because orcasare so large, they can’t access shallow or rocky locations. And shallow androcky locations are precisely the types of locations where sea otterpopulations have not declined.
Main point: 天敌说更为可信
Sub point one: 没有发现海獭的尸体
Sub point two: 多种小型海洋哺乳动物的减少是因为逆戟鲸捕食
Sub point three: 逆戟鲸个头很大，有的海域不能进入，解释了为何数量变化不均衡
The lecturer and the reading passagesuggest two competing theories, _______V.S.__________, to explain______________________.
The professor reasons that _____________ isnot caused by _______but rather by ______. In contrast, the reading passageattributes _______ to _____, citing evidence of ______________.
Furthermore, the lecturer argues that________ are likely factors in ________, because ________.
The reading passage, on the other hand,rules out this theory based on __________, and instead approves of the______theory as the only explanation for ______________.
Finally, according to the lecturer, ________________.However, the reading passage argues that _________ factors betterexplains_________.
The lecturer and the reading passagesuggest two competing theories, the predation theory vs. the pollution theory,to explain why the sea otter population is in rapid decline.
The professor reasons that the absence of DeadSea otters washed up the coast suggests that their decline is not caused by seapollution but rather by sea predators who consume their bodies after killingthem. In contrast, the reading passage attributes the death of sea otters topollution, citing evidence of increased sources of ocean contaminants whichlead to greater vulnerability to infections.
Furthermore, the lecturer argues that orcasare likely factors in the disappearance of sea otters, because the scarcity ofwhales, their usual prey, has left them with no other choice but to start huntingsmaller mammals like the otters for food. The reading passage, on the otherhand, rules out this theory based on the orca’s prepaying habit, and insteadapproves of the pollution theory as the only explanation for the decline ofboth large and small sea mammals across the entire ecosystem.
Finally, according to the lecturer, theuneven pattern of sea otter decline corresponds to the distribution of theorcas; she argues that the fact that their population has declined most rapidlywhere orcas are most prevalent further validates the predation theory. However,the reading passage argues that chaneable environmental factors, which lead todifferent concentrations of pollutants, better explains the varying pattern ofsea otter decline.