Populations of the yellow cedar, a species of tree that is common in northwestern North America, have been steadily declining for more than a century now, since about 1880. Scientists have advanced several hypotheses to explain this decline.One hypothesis

both the reading passage and the professor are discussing the hypothesis about the causes of yellow cedar's declining during the recent century. the reading passage claims that the cedar bark beetle, the brown bears and the gradual changes of climate are the feasible reasons for yellow cedar's decreasing. however, the lecturer finds all the ideas implausible and presents some evidence to refute them all and support the idea that none of the reasons mentioned in the passage can be considered possible explanation for this decline.

the reading passage holds the view that maybe the insect parasites, particularly the cedar bark beetle, are responsible for the cedar's decreasing since there have been some evidence which confirm the fact that beetle attacks adversely affect the yellow sedar and kills them. conversely the lecturer rejects the point in the reading and brings up the idea that healthy yellow cedars are much more resistant to insects than other species. they have powerful chemical which are poisonous to insects, so if the beetle attack the tree, it will damage itself and die. as a result this hypothesis seems to be really dubious.

furthermore, the other hypothesis on which the passage relies is that the brown bears maybe account for the decline because they feed from the tree bark which is superseded the wild berries,the bears main food and their improper and agressive feeding habits may cause the decline. on the contrary the professor takes the opposite view and underlines the fact that there is no bear on islands, while the cedar population have decreased as well in this place. therefore, regarding the fact that the decline occur in the places with and without bears, he assert that this hypothesis can't be the reason for the decline.

finally the last reason from the author's point of view is that the gradual changes of climate is a factor that can be considered as the possible reason for the decline. since these changes can adversely influence the roots of the cedar trees by means of causing them to grow in the late winter instead of in the early spring. in contrast, the speaker dismisses this issue due to the fact that the cedar trees primarily die at lower elevation where it is warmer rather than higher elevation where it is colder. in other words, more trees die in the relatively warm places where it is at the low elevation. consequently this claim doesn't possess a strong support in order to be acceptable.