Viết tích hợp TOEFL

📖 Đoạn văn

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. One such example is the deliberate introduction of the tamarisk beetle (Diorhabda carinulata) to control the invasive tamarisk shrub in the southwestern United States. While tamarisk was originally introduced for erosion control, its spread has caused ecological damage. However, the beetle's introduction has had unintended positive effects. First, the beetle has successfully reduced tamarisk cover, allowing native riparian vegetation such as cottonwoods and willows to recover. This restoration of native plants has improved habitat for bird species like the southwestern willow flycatcher, whose populations have begun to rebound in areas where tamarisk is controlled. Second, the beetle's feeding activity has increased the availability of open water in arid regions. Tamarisk consumes large amounts of groundwater, and its removal has led to higher water tables and stream flows, benefiting both wildlife and local human communities. Third, the beetle itself provides a new food source for native insectivores. Birds, reptiles, and other predators have adapted to feed on the beetle, adding to the local food web. Thus, despite initial concerns, the introduction of this biocontrol agent has enhanced ecosystem health and resilience in the long run.

🎧 Bài giảng

The lecture challenges the reading's claim that the tamarisk beetle introduction has been beneficial. While the beetle does defoliate tamarisk, its effects are far from uniformly positive. First, the beetle's control of tamarisk is not selective. In many areas, the beetle has spread beyond tamarisk stands and has been observed feeding on native plants, including willows and cottonwoods, which the reading claims are recovering. This has actually set back restoration efforts, as these native trees are also damaged. Second, the alleged increase in water availability is misleading. Although tamarisk consumes water, its deep roots also help stabilize riverbanks. With tamarisk removed, erosion has increased, leading to siltation of streams and reduced water quality. Moreover, the water saved by removing tamarisk is often taken up by other invasive weeds, so the net gain in water flow is negligible. Third, the claim that the beetle provides a new food source is overstated. The beetle is not palatable to many native predators due to its chemical defenses, and its populations fluctuate dramatically, sometimes crashing after heavy defoliation. As a result, native insectivores do not reliably rely on it, and in some cases, they have shifted their feeding to more vulnerable native insects, disrupting the existing food web. Thus, the beetle's introduction has created new ecological problems rather than solving the original ones.