Viết tích hợp TOEFL

📖 Đoạn văn

The introduction of non-native species into new environments is often viewed as a threat to biodiversity. However, a growing number of ecologists argue that in certain circumstances, such introductions can have net positive effects. First, non-native species can increase local biodiversity by adding new functional roles, such as pollinators or seed dispersers that were previously absent. For example, the introduction of honeybees to North America enhanced pollination for many native plants, leading to higher fruit and seed production. Second, non-native species can serve as a food source for native predators, thereby supporting higher trophic levels. In the Great Lakes, for instance, the invasive alewife provides a crucial prey base for native salmon and trout, whose populations have thrived as a result. Third, non-native species often exhibit rapid adaptation and resilience, which can help stabilize ecosystems disturbed by human activity. In urban areas, hardy non-native plants like dandelions colonize degraded soils, preventing erosion and providing habitat for insects. These examples suggest that rather than always harming ecosystems, non-native species may sometimes be beneficial, and conservation policies should consider potential advantages rather than automatically eradicating them.

🎧 Bài giảng

The reading claims that non-native species can be beneficial, but this view is overly optimistic and ignores serious long-term costs. First, while non-native species may temporarily increase species counts, this often comes at the expense of native species. For example, honeybees compete with native bees for nectar and pollen, and in many regions, native bee populations have declined sharply since honeybee introduction. Thus, the net effect on biodiversity is negative when considering native species loss. Second, the supposed benefit of non-native prey like alewives is misleading. Alewives contain high levels of thiaminase, an enzyme that breaks down vitamin B1 in predators. As a result, native salmon and trout that feed heavily on alewives suffer from vitamin deficiency, leading to reduced survival and reproductive failures. So, rather than supporting predators, non-native prey can harm them. Third, the idea that non-native plants stabilize disturbed soils ignores the fact that they often alter nutrient cycles and outcompete native vegetation. Dandelions, for example, change soil chemistry and reduce the diversity of native wildflowers, which in turn diminishes habitat quality for specialized insects. In the long run, these negative impacts outweigh any short-term benefits, so conservation should focus on preventing and removing non-native species.