decline in fish species

The passage asserts that the zebra mussel, a kind of fish, is a serious threat to other species, since they will be dominat in the area and provides three reasons of support. The professor, on the contrary, debunks this point by rejecting each reason provided by the passage. First, the article claims that zebra mussels were carring to the North America through ships and they will be dominat species in the region. The lecturer rebuts this view by saying that in the past people were not knowledable about this issue. While the ships are traveling to the North America, they can empty water in the ocean and there is no need to refilling water in the Europe, since the salt in the water will kill all zebra mussels. Second, the zebra mussel is a resistant type of fish and it has no predator in their habitats, according to the reading. The professor disavows this point by explaining that in the past, the zebra mussel had no predators, but now some types of birds have adapted to eat them, and they no longer are dominant species. Third, the author of the passage imparts that zebra mussels are eating plankton, since they compete for food with other fish species. The lecture casts doubt on this argument by saying that it is true that mussels are plankton eater, but they provide nutrient food for other species in the river which leads to increase in their population.