The introduction of non-native species into new environments is often viewed as a threat to biodiversity, but in certain cases, it can actually be beneficial. This is particularly true for the introduction of the Asian clam (Corbicula fluminea) into North American freshwater ecosystems. First, Asian clams improve water quality by filtering algae and suspended particles. Studies in the Potomac River have shown that dense populations of these clams can reduce phytoplankton blooms, increasing water clarity and benefiting submerged aquatic vegetation. Second, they serve as a reliable food source for native predators. For example, migratory birds such as scaups and canvasbacks have shifted their diets to include Asian clams, especially during winter when native mussels are scarce, thus supporting bird populations. Third, Asian clams can enhance nutrient cycling. Their biodeposits (feces and pseudofeces) release nitrogen and phosphorus into the sediment, which fertilizes benthic algae and invertebrates, thereby increasing overall productivity of the ecosystem. Therefore, rather than always being harmful, some introduced species can play positive ecological roles.
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The professor challenges the reading's claim that Asian clams are beneficial. While the reading cites improved water clarity, the professor notes that excessive filtration can reduce phytoplankton to levels that starve native zooplankton, which are crucial for larval fish. This disruption cascades up the food web, harming fish populations that rely on zooplankton. Second, although the clams provide food for some birds, the professor points out that they compete with native mussels for space and food, and their sharp shells are indigestible for many predators, causing internal injuries. For instance, studies have shown that bird species that consume Asian clams have lower survival rates compared to those eating native prey. Third, the claim about nutrient cycling is misleading. The clams' biodeposits may increase nutrients locally, but they also stimulate harmful algal blooms in downstream areas, leading to oxygen depletion and fish kills. Moreover, the clams' rapid reproduction can alter sediment composition, smothering native benthic communities. Thus, the apparent benefits are outweighed by significant ecological disruptions.
📝 Hướng dẫn viết Summarize the points made in the lecture, being sure to explain how they cast doubt on the specific points made in the reading passage.
📊 TOEFL Integrated Writing Rubric
Content Accuracy: Accurately capture the relationship between lecture and reading
Information Completeness: Cover all key counter-arguments from the lecture
Structure & Coherence: Logical organization and progression
Language Use: Grammar, vocabulary, expression accuracy
Scored 0-30 based on holistic scoring (original 0-5, scaled to 30).
