Reading
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 means that they compete for food with many freshwater fish species.
Question
Summarize the point made in the lecture, being sure to explain how the cast doubt on specific points made in the reading passage.
The lecturer claims that there are ways to control the zebra mussel so it is not clear that the fresh water shellfish is a treat to freshwater fish populations in all of North America. This contradicts the reading's assertion that its invasion cannot be stopped and poses a serious threat to it.
The first point made in the passage is that according to the history of the zebra mussel, the invasion has not been stoppable. The lecturer, however, challenges this particular viewpoint by arguing that it is suggested because we did not have enough knowledge about it at that time. He adds that it is stoppable to carry out the zebra mussel. When ships travel from Europe to North America, they could empty out their ballast water and refill with ocean water which kills the zebra mussel.
Another point the article puts an emphasis on is that the zebra mussel can easily dominate new habitats because they would not have any predators. In contrast, the lecturer contends that it might be often true but it is only in the beginning. That is, the local birds in the new habitat would soon figure out there is new prey for them and begin to eat the zebra mussel a lot. So, the zebra mussel is unlikely to dominate after all.
Finally, it is stated in the reading that a decline in the overall fish population is expected by the zebra mussel because it eats the food of many freshwater fish species, plankton. On the contrary, it is accentuated in the lecture that it is not true that overall fish population in habitats would decline. Even though plankton eating fish population will decline, as the zebra mussel provides nutrients for fish that live at the bottom of lake, the number of bottom eating fish would increase.
Attribute Value Ideal
Score: 26 in 30
Category: Very Good Excellent
No. of Grammatical Errors: 0 2
No. of Spelling Errors: 0 2
No. of Sentences: 13 12
No. of Words: 303 250
No. of Characters: 1388 1200
No. of Different Words: 154 150
Fourth Root of Number of Words: 4.172 4.2
Average Word Length: 4.581 4.6
Word Length SD: 2.301 2.4
No. of Words greater than 5 chars: 91 80
No. of Words greater than 6 chars: 66 60
No. of Words greater than 7 chars: 41 40
No. of Words greater than 8 chars: 22 20
Use of Passive Voice (%): 0 0
Avg. Sentence Length: 23.308 21.0
Sentence Length SD: 7.032 7.5
Use of Discourse Markers (%): 0.538 0.12
Sentence-Text Coherence: 0.397 0.35
Sentence-Para Coherence: 0.609 0.50
Sentence-Sentence Coherence: 0.143 0.07
Number of Paragraphs: 4 4