▶ Your Answer :
In this set of materials, both the lecturer and the writer deal with whether zebra mussels devastate native ecosystems in the United States and Canada. Regarding this issue, the lecturer refutes the writer’s claims because the lecturer claims that zebra mussels are not a major threat.
To begin with, the lecturer contradicts the writer’s first idea that zebra mussels completely change the ecosystems that they invade. The lecturer mentions that since aquatic birds changed their diets, these birds catch and eat zebra mussels. Furthermore, an increase in the number of zebra mussels is kept in check by these birds in long terms. Thus, the lecturer asserts that the writer’s first idea cannot be validated.
On top of that, the lecturer runs counter to the second idea of the reading passage that the existence of zebra mussels gives native fish negative effects. The lecturer points out that unlike the writer’s assertion, zebra mussels can help remove pollutants. The presence of zebra mussels is beneficial in order to keep water clean. Also, zebra mussels enable algae to grow in deeper water due to a decrease in the number of planktons and more light. Therefore, the lecturer goes on to say that the second idea of the reading passage is also disputable.
Finally, the lecturer casts doubt on the writer’s last idea that zebra mussels’ movement from one location to other locations is not controlled. The lecturer explains that zebra mussels do not survive in salt water but only live in clean water. To be specific, if ships fill their ballasts with seawater, zebra mussels cannot survive in this environment. Hence, the lecturer claims that the writer’s last idea does not sound convincing. |