托福综合写作

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The introduction of the Asian shore crab (Hemigrapsus sanguineus) to the Atlantic coast of North America is often viewed as an ecological threat. However, recent studies suggest that this invasive species may actually have several beneficial effects on the local ecosystem. First, the Asian shore crab is a voracious predator of the blue mussel (Mytilus edulis), which has become overabundant in many intertidal zones due to the decline of its natural predators, such as the sea star. By controlling mussel populations, the crab helps maintain biodiversity, allowing other sessile organisms like barnacles and algae to thrive. Second, the crab serves as a new food source for native predators, including gulls, raccoons, and larger fish. This additional prey item may bolster the diets of these animals, especially in areas where traditional food sources are scarce. Third, the crab's burrowing behavior aerates the sediment, which enhances nutrient cycling and promotes the growth of beneficial bacteria. This process can improve water quality and support the overall health of the coastal environment. Therefore, rather than being a purely harmful invader, the Asian shore crab might play a constructive role in the Atlantic intertidal community.

🎧 听力讲座

Professor: The reading presents a surprisingly optimistic view of the Asian shore crab, but I’m afraid its arguments are flawed. Let’s examine each point. First, while it’s true the crab eats blue mussels, this does not necessarily increase biodiversity. In fact, the Asian shore crab is a generalist predator that also consumes barnacles, periwinkles, and even juvenile crabs of native species. Its feeding can drastically reduce the abundance of these organisms, leading to a less diverse community, not a more diverse one. The reading’s claim is based on a narrow focus on mussels alone. Second, the idea that the crab is a beneficial food source for native predators is questionable. Observations show that most native predators, such as gulls and fish, rarely consume the Asian shore crab because of its hard shell and aggressive behavior. Instead, the crab competes with native predators for the same prey, like small mollusks, thereby reducing the food available to them. So it’s not a helpful addition to the food web. Third, the claim about burrowing improving sediment quality is overstated. The Asian shore crab typically shelters under rocks and in crevices rather than digging extensive burrows. Its activity does not significantly aerate the sediment. On the contrary, its foraging disturbs the substrate, dislodging algae and invertebrates, which can harm the local community. Thus, the supposed benefits are not supported by evidence.