TPO 23综合
The reading passage indicates that scientists provide three hypotheses to explain the decline of the yellow cedar trees. However, the professor says none of the explanation is convincing.
The first hypothesis is that insect parasites, especially the cedar cark beetle cause the decline of the yellow cedar. However, the professor refutes that the beetle is not the fundamental reason. He says healthy trees are more resistant to insect infestation than unhealthy ones. The arms and leaves of the cedar trees are full of chemical which are toxic to insects. So healthy cedar trees are unlikely to suffer from insect damage. For the dead trees, they are not attacked by beetles and they are died for other reasons.
Second, the hypothesis indicates that bears’ aggressive feeding habits attribute to the decline of cedar trees. However, the professor argues that bears do not cause the overall decline of the trees. Cedar trees mainly grow in the mainland and island just off the coasts where no bears exist. The decline of island may lead to the decline of the trees and that has nothing to do with bears.
The third hypothesis implies that climate changes is the cause of the decline. However, the professor indicates that the reading neglects one important factor that majority of the trees are dying in low elevation, where is warm. As a result, the frozen effect, which appears in high elevation, has nothing to do with the decline.
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发表于 2014-9-14 10:56:15
