ReadingThe 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 pose

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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 author states there is no way to stop the invasion of zebra mussel. However, the speaker casts doubt upon this view by making contrary arguments and explaining how to control it.

First, the speaker points out that people in the past did not know how to stop it because they lacked of knowledge. However, he says ships can be stopped easily; ships should empty the water in the ocean rather than in fresh water, so the salty water would kill zebra population. This casts doubt upon the writer's claim who says zebra mussel cannot be stopped colonizing North America water soon.

Second, the lecturer claims that zebra mussel does not have predators, but that was just in the beginning. Moreover, he mentions some species have changed their feeding habits, so they now eat it. This casts doubt on the author's opinion that states zebra mussel does not have any predator, which makes it the dominant species in their habitat.

Finally, the orator remarks the zebra mussel positive impacts on fish population over the negative effects. Therefore, the mussel nourishes fish on the bottom of the ocean. However, the author disagrees with the lecturer by explaining that the zebra mussel competes with fish for plankton, and thus they decrease the numbers of fish in the ocean.

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