TOEFL統合ライティング

📖 読解文章

The extinction of the passenger pigeon (Ectopistes migratorius) in 1914, once the most abundant bird in North America, is often attributed to overhunting by humans. However, recent research suggests that natural factors may have played a more significant role than previously assumed. Three key arguments support this view. First, the passenger pigeon's population experienced dramatic fluctuations due to climatic events, such as the Little Ice Age, which reduced its primary food sources—acorns and chestnuts—leading to periodic massive die-offs. Second, the species had extremely low genetic diversity, as evidenced by DNA analysis of museum specimens, making it vulnerable to diseases and environmental changes. This genetic bottleneck, not human hunting, could have driven the population to a critical threshold. Third, the passenger pigeon relied on a highly social, colonial breeding strategy, requiring huge flocks to stimulate reproduction. When natural factors, such as storms or predators, disrupted these flocks, breeding success plummeted, and the population could not recover. Thus, these natural vulnerabilities, not human actions, were the primary cause of the pigeon's demise.

🎧 リスニング講義

The reading's claims are not convincing. While it is true that climatic events like the Little Ice Age affected food supplies, these fluctuations were not unique to the passenger pigeon; many other bird species survived similar stresses. The key difference was human hunting. Historical records show that commercial hunters systematically slaughtered millions of pigeons each year, using nets, guns, and even sulfur fumes to destroy nesting colonies. This relentless pressure, not climate, reduced populations to unsustainable levels. Regarding genetic diversity, the reading cites low diversity as a cause, but this is misleading. Low genetic diversity is common in many species that thrive, and the passenger pigeon's large population size before human contact suggests that its genetic makeup was sufficient for survival. The real problem was that hunting removed individuals faster than reproduction could replace them. Finally, the colonial breeding strategy was a double-edged sword. While it required large flocks, these flocks were actually an advantage against natural predators. However, they made the pigeons easy targets for humans, who could wipe out entire breeding colonies in a single season. Without such intense human exploitation, the passenger pigeon could have persisted, as it had for millennia.