1、Zebra mussels:

阅读材料：

Zebra mussels are spreading rapidly from East Europe, where they initially originated, to the vast areas of North America. This unique species invades and dominates almost any fresh-water ecological system, wherever they happen to travel. To make it even worse, few effective measures have been developed so far to control the explosion of zebra mussels.

First, as demonstrated in history, the invasion of zebra mussels has for all time been unstoppable. This is because zebra mussels are so smart that they have long adapted to fasten themselves to human vehicles. For example, ever since 19 century, ships traveling on fresh water routes in East Europe have been found carrying tons of zebra mussels on the bottoms. Of course, this is not the only way mussels can travel with ships. Thousands of these little creatures actually hide and multiply themselves in the “ballast water”- water intentionally filled in to balance the ships.

Second, zebra mussels always dominate new areas soon after their first successful invasion. This shellfish has been considered a hardy creature, because they can easily adapt into almost any environment and still keep their amazing reproduction rate during the procedure of adaptation. What’s more, since zebra mussels are new to the ecological systems of North America, hardly any predators of this shellfish are known.

Third, the pollution caused by zebra mussels’ waste can first reduce the quality of a water body and soon after that, the overall fish number dwelling in it. In addition, zebra mussels devour food and nutrition which would otherwise feed many other local fishes.

听力材料：

Female Professor:

Well, this is the popular belief of zebra mussels. But I won’t say that they are really such a big threat. On the contrary, most information mentioned in this passage has been exaggerated. The spread of zebra mussels is, of course, stoppable, if proper measures are taken.

First, yes, in history, people didn’t stop zebra mussels’ invasion. But that was because people didn’t have the knowledge then. Now we know how to deal with the ballast water for sure. For example, when ships travel on the sea waters, they can empty all the fresh ballast water and then refill an equal amount of salt water. Because although zebra mussels are hardly shellfish, they are fresh water species anyway and salt water kills them almost immediately.

Second, they say the mussels can dominate the whole ecological system of North America. That is pure nonsense. Of course, because the mussels are new to America, they have no predators now. But it doesn’t mean they will find North America a safe playground forever. For example, once a bird finds zebra mussels are more than tasty, sooner or later, all birds will know there comes a kind of great new food.

Third, studies have demonstrated that although the pollution caused by mussels’ waste may reduce the

number of some fish living in the upper water, other fish, especially the fish living in the bottom, actually increase in numbers.

范文：

The professor makes the point that zebra mussels are not dangerous any more now as they used to be, since people have found ways to solve the problem.

First, the professor holds that people now know how to stop zebra mussels’ invasion. When ships travel on the sea, people can simply use sea water to replace the fresh ballast water. Because the zebra mussels cannot live in sea water, this method can kill them soon. This is opposite to the idea in the passage that the invasion of zebra mussels is unstoppable.

Second, the professor does not believe that zebra mussels can dominate the whole ecological system of North America. They may not have predators at the beginning, but once the birds find out that they are delicious, they will certainly have a lot of predators soon. This is

contrary to the idea in the passage that zebra mussels have hardly any predators.

Third, the professor thinks that when the zebra mussels reduce the number of fish living in the upper water, they actually benefit those fish living in the bottom of the water. It rebuts the idea in the reading that mussels will make local fish decrease.

2、SALVAGE LOGGING

阅读材料：

Salvage logging has been routine for centuries in American forest industry. People working in this trade remove dead or diseased trees to improve the general health of a forest, especially after an outrage of wildfire. Salvage logging helps the forest ecosystem to stay protected.

First, dead trees died from wildfire have to be removed to spare spaces for new trees. This is because of left untouched, dead trunks are very likely to prevent baby

trees from obtaining sufficient sunshine and rainwater. Moreover, studies have proved that forests after salvage logging usually recover faster than if they were left unchecked, when wildfires occur.

Second, salvage logging also helps to minimize the damage caused by insects which often dwell on the dead trees. Some of these insects are deadly poisonous to people who happen to get close to them. Others would have rendered serious economic loss, if proper measures are not taken against them before they reproduce rapidly and spread over the whole forest.

Third, salvage logging brings economic benefits to the local community as well. On one side, salvage logging usually involves a great deal of manual work and thus it creates some job opportunities for the local people. On the other side, dead trunks collected in the process of logging could also be used as precious firewood.

听力材料：

Male Professor

Ah, I don’t quite agree with those points about salvage logging. Actually, logging apparently goes against the nature, if I have to say something about it. Although as a tradition, salvage logging has been lasting for centuries in North America, more people today start to realize its threats to both the forest and the local community.

First, dead trees do not have to be cleaned because rotten trunks and leaves provide valuable nutrition for growing young trees. In addition, with the help of bacteria and rainwater, it usually doesn’t take long for those dead trees to start to decompose and eventually leave enough space for the baby trees to grow.

Second, of course dead trees would give rise to some insects sooner or later. But we have to also note that those insects eat simply part of nature, since they have coexisted peacefully with other animals and plants in the forest for many, many centuries. Furthermore, the quick reproduction of insects also provides birds an opportunity to feed themselves as well.

Third, salvage logging is very expensive in economy if we look at it in a long run. This is because renting the helicopters, keeping the forest vehicles, and the forest management all make salvage logging a very costly process to take. Besides, the jobs created in the logging process are mostly temporary and they only benefit those professional workers.

范文：

The reading material says that salvage logging helps the forest ecosystem to stay protected, but the lecture refutes the idea by stating that salvage logging goes against nature and threats the forest and the local community.

First, the professor doubts the idea in the passage that the salvage logging can free up space for young trees. He claims that the dead bodies of trees provide much nutrition for new trees to grow up, instead of blocking the space. In addition, dead trees decompose quickly and leave enough space for the baby trees to grow.

Second, while the passage says that salvage logging

helps to minimize the damage caused by insects which often dwell on the dead trees, the professor raises a contradictory idea that these insects have lived in the forests for many years and do not actually cause damages to the forest. He also argues that the insects living on the dead trees become food for birds.

Third, the passage says that salvage logging provides many job opportunities. But the lecture rebuts the idea and says that the process can be very expensive. What’s more, the task only benefits professional workers, not the local community.