TOEFL統合ライティング

📖 読解文章

The extinction of the passenger pigeon (Ectopistes migratorius) in the early 20th century is often attributed solely to overhunting. However, recent research suggests that the bird’s demise was also driven by ecological factors that made the species inherently vulnerable. First, passenger pigeons bred in massive, dense colonies, sometimes containing millions of birds. Such extreme social nesting meant that reproduction was highly synchronized: if a colony was disturbed during the breeding season, entire generations of chicks could be lost in a single event. This reproductive strategy was poorly suited to withstand repeated human intrusions. Second, the species depended on a narrow diet of mast—specifically acorns, beechnuts, and chestnuts—which are produced in irregular, boom-and-bust cycles. When mast crops failed, pigeons were forced to range over vast areas to find food, expending energy and facing starvation. This dietary specialization made them sensitive to habitat fragmentation, as suitable feeding forests were increasingly cleared by settlers. Third, genetic evidence from museum specimens indicates that passenger pigeons had extremely low genetic diversity, likely due to their large but fluctuating population sizes. This lack of variation reduced their ability to adapt to new diseases or environmental changes, such as the introduction of West Nile virus or shifts in climate. Thus, while hunting was a direct cause, these intrinsic biological weaknesses—colonial breeding, dietary inflexibility, and low genetic diversity—were equally important in driving the species to extinction.

🎧 リスニング講義

The reading’s argument that ecological factors were as important as hunting overlooks the overwhelming evidence that human activity was the decisive cause. First, the breeding colonies, though dense, were not inherently fragile. Historical records show that passenger pigeons successfully reproduced in colonies for thousands of years, despite occasional natural disturbances like storms or predators. The real problem was that hunters deliberately targeted these colonies, using nets, guns, and even burning trees to kill thousands at once. A single colony in Michigan in 1878 was reportedly killed at a rate of 50,000 birds per day over five months—this was not a natural failure but systematic slaughter. Second, while the pigeons did depend on mast, they were highly mobile and could travel hundreds of miles to find food, as documented by ornithologists. The irregularity of mast crops was a normal condition they had evolved to handle. What disrupted their food supply was the large-scale deforestation by European settlers, which removed vast tracts of hardwood forest. This habitat loss was not an intrinsic weakness but an anthropogenic change that left pigeons without adequate resources. Third, the claim of low genetic diversity is misleading. Recent DNA studies actually show that passenger pigeons had a relatively large effective population size and moderate genetic variation, similar to other thriving bird species. The low diversity seen in some museum samples is likely an artifact of post-mortem DNA degradation. Therefore, the extinction was not due to internal flaws but to overhunting and habitat destruction—both caused by humans.