TPO-23

The lecturer revises the points made in the passage about the assumptions of decline in populations of the yellow cedar species of trees common in northwestern North America during 17th century.

There are three hypothesis mentioned in the passage, which claim the causes of decline in population of yellow cedar trees over the years; While the lecturer casts doubts on each point with some scientific evidences.

Firstly, the passage states insect parasite known as the cedar bark beetle responsible for decline. However, the lecturer asserts that, healthy yellow cedar were resistant to beetle barks. Moreover, the trees released chemicals in barks, which could be poisonous to the insects beetles. Also, the trees were already weaken when beetles attacked them. Therefore, beetles are not the fundamental cause of it.

According to the second hypothesis in the passage the brown bears clawed to eat tree bark. The barks of cider trees contains high sugar and brown bears eat the food with high sugar contents. Therefore, beers could make them weak by eating them. On the other hand, according to the evidences provided by the lecturer, the decline seen in the whole northern coast from mainland to island. Matter of the fact is that, decline in the population was seen in the island where there were no bears. That indicates that bears were not the main cause of the population decline.

According to the third hypothesis provided in the passage, decline could be caused by gradual change in climate over the hundred years. It also asserts that, there was damage to growing roots during cold season because they are highly sensitive to winter . However, according to the lecturer the decline in population was also seen in the lower region with warm climate. Therefore, cold weather couldn't be the primary reason.