TOEFL統合ライティング

📖 読解文章

The extinction of the passenger pigeon, once the most abundant bird in North America, is often attributed solely to overhunting. However, a closer examination of the ecological and genetic evidence suggests that habitat fragmentation and loss of genetic diversity played equally critical roles. First, the passenger pigeon depended on vast, contiguous forests of oak and beech, which provided mast (nuts) in cyclical abundance. As European settlers cleared land for agriculture and timber, these forests were broken into smaller patches. This fragmentation disrupted the pigeons' ability to locate sufficient food across their massive foraging ranges, leading to regional breeding failures. Second, the species exhibited a unique reproductive strategy called 'predator satiation': they bred in enormous colonies, sometimes containing millions of birds, to overwhelm predators. When habitat fragmentation reduced colony sizes, this strategy failed, as smaller colonies became vulnerable to predation and disease. Third, recent genetic studies of museum specimens reveal that passenger pigeons had surprisingly low genetic diversity, making them susceptible to sudden environmental changes. Even without hunting, this genetic bottleneck could have led to population crashes. Thus, while hunting was a proximate cause, the ultimate drivers were habitat loss and inherent genetic fragility.

🎧 リスニング講義

The reading presents a plausible but incomplete picture. In reality, overhunting was the dominant cause, and the other factors were either secondary or even consequences of hunting pressure. First, habitat fragmentation did occur, but it was not severe enough to eliminate food sources. Historical records show that passenger pigeons still found abundant mast in remaining forests until the final decades. The real issue was that commercial hunters, using nets and guns, killed millions of birds annually, driving populations below a critical threshold where recovery was impossible. Second, the 'predator satiation' strategy was indeed effective, but only when colonies were large. However, hunting directly reduced colony sizes, not habitat fragmentation. Hunters deliberately targeted breeding colonies, destroying nests and killing adults, which caused colonies to shrink and then collapse. This was a direct human impact, not an indirect ecological one. Third, the claim of low genetic diversity is misleading. Many species with low diversity, such as the cheetah, thrive today. The passenger pigeon’s genetic makeup was adequate for its environment. The sudden extinction was too rapid for genetic factors to play a role; a population crash from hunting happened in just decades, not centuries. In conclusion, while habitat and genetics may have been minor stressors, the overwhelming evidence points to commercial overhunting as the primary and sufficient cause.