The zebra mussel, a freshwater shellfish native to Eastern Europe, has long been spreading out from its original habitats and has now reached parts of North America. There are reasons to believe that this invasion cannot be stopped and that it poses a serious threat to freshwater fish populations in all of North America.
First, the history of the zebra mussels’ spread suggests that the invasion might be unstoppable. It is a prime example of an invasion made possible by human transportation. From the zebra mussel’s original habitats in Eastern Europe, ships helped spread it out along new canals built to connect Europe’s waterways. The mussel can attach itself to a ship’s bottom or can survive in the water—called “ballast water"—that the ship needs to take on to properly balance its cargo. By the early nineteenth century, the mussel had spread to the whole of Europe. It was later carried to the east coast of North America in the ballast water of ships traveling from Europe. The way ships have spread the zebra mussel in the past strongly suggests that the species will soon colonize all of North America.
Moreover, once zebra mussels are carried to a new habitat, they can dominate it. They are a hardy species that does well under a variety of conditions, and they have a high rate of reproduction. Most important, however, zebra mussels often have no predators in their new habitats, and species without natural predators are likely to dominate their habitats.
Finally, zebra mussels are likely to cause a decline in the overall fish population in habitats where they become dominant. The mussels are plankton eaters, which mean that they compete for food with many freshwater fish species.
Summarize the points made in the lecture, being sure to explain how they support/contradict specific points made in the reading passage.
Now listen to part of a lecture on the topic you just read about.
Contrary to what you just read, there are ways to control the zebra mussels’ spread. What's more, it is not so clear that the mussel is a serious threat to fish populations. True, the spread of zebra mussels couldn't be controlled in the past, but that's because people didn't have enough knowledge. In fact, there are effective ways to stop ships from carrying the mussels to new locations. Here's an
example. The way zebra mussels usually travel across the ocean is that a ship takes on some fresh "ballast water" in Europe and then empties that water into American waterways when it arrives. Full of zebra mussels, but the ship can be required to empty out the freshwater and refill with ocean water while still out in the ocean. Saltwater will kill the mussels.
Second, it's true that zebra mussels often don't have predators in their new habitats, but that's only in the beginning. What's been happening in Europe is that local aquatic birds sooner or later notice there's a new food source around and change their habits to exploit it. They switch from whatever they were eating before to eating zebra mussels. And birds can eat a lot of mussels. So zebra mussels aren't so likely to dominate their new habitats after all.
Finally, even in habitats where zebra mussels become dominant, is the overall fish population likely to decrease. It's true that zebra mussels may have a negative impact on fish that eat plankton. But on other fish, they can have a positive impact. For example, the mussels generate nutrients that are eaten by fish that feed near the bottom of the lake or river. So bottom-feeding fish populations may increase, even if plankton-eating fish population decrease.
Main points: The invasion of zebra mussel to North America is unstoppable and puts a threat to freshwater fish.
Sub point 1: History suggests invasion might be unstoppable.
Sub point 2: Mussels can dominate their new habitat.
Sub point 3: Mussel threats fish population after they dominate the area.
Attitude: Contrary. Unsure mussels’ threat to fish population.
Sub Point 1: There are effective ways to stop mussels in today’s knowledge. (Salt water)
Sub Point 2: Birds can be predators and eat mussels.
Sub Point 3: Bottom-feeding fish may increase while plankton-eating fish decrease but overall fish population only likely to decrease.
Contrary to what is argued in the passage, the lecture demonstrates how zebra mussels are not likely to become an imperative threat to freshwater fish populations in North America.
First of all, recently uncovered knowledge of zebra mussel has shed light on new ways to prevent their invasion. Even though people in the past have not been able to stop the spread of zebra mussels, knowledge and technology may help abate their spread in the future. For example, although a large amount of zebra mussels spread to America by staying in the ballast water that is emptied halfway on the journey across the Atlantic and replenished with sea water, the zebra mussels can be exterminated as soon as they are directly exposed to salt water.
以L的观点起头，谈sub topic 1，说明现代科技相比旧时代科技有机会阻止mussels spreading. 随后举听力中的具体例子为证明。
Furthermore, zebra mussels do not often dominate a new habitat for a long period of time. The lecture agrees that although zebra mussels may have no predators and reproduce rapidly in the beginning, it would not be long before predators take notice of this new source of food, and develop a taste for it, thus preventing its domination.
还是以L观点出发，谈sub topic 2, 说明mussels不会经常统治它的栖息地。L中同意短时期内mussels没有天敌，但没过多久掠食者就会发现这些新的美味从而结束mussels的统治。
Finally, zebra mussels alone would not cause the decline of overall fish populations. Although zebra mussels may perhaps cause the decline of plankton eaters, as the passage suggests, they would also provide nutrients for bottom-feeding fish and ultimately cause the population of those fish to increase.
讨论sub topic 3, 说明mussels不会造成总体鱼群数量的下降。L说虽然他同意R的观点(mussels造成plankton eaters下降)，但是伴随而来的bottom-feeding fish的数量上升从而导致总体鱼群数量上身。
1. Recently uncovered knowledge of zebra mussel has shed light on new ways to prevent their invasion
/Newly discovered/ It is recently found out/
Shed light on: 给与了希望
/Shown/ Given/ Illuminated
2. The lecture agrees although………….(R的观点)………..，….…(L的观点会抵消R的观点)….。
/Finally/ At last/ In the end/ After all/
Ultimately, the success of his company depends on good marketing.