托福综合写作

📖 阅读文章

The extinction of the passenger pigeon (Ectopistes migratorius) in the early 20th century is often attributed solely to overhunting. However, a closer examination of the ecological and genetic evidence suggests that the species' demise was a complex event driven by several interacting factors. First, the passenger pigeon's reproductive strategy was highly specialized. They nested in dense colonies, sometimes covering hundreds of square kilometers, and bred only when population densities were extremely high. This 'boom-or-bust' breeding behavior made them vulnerable to even slight reductions in population size, as smaller colonies failed to trigger the necessary breeding stimulus. Second, the species had a very low genetic diversity, as indicated by studies of museum specimens. This lack of genetic variation likely reduced their ability to adapt to environmental changes, such as new diseases or shifts in forest composition. Third, deforestation in eastern North America during the 19th century destroyed vast tracts of the mature hardwood forests that provided their primary food source—acorns and beechnuts. While hunting removed large numbers of birds, it was this combination of reproductive fragility, genetic uniformity, and habitat loss that ultimately pushed the species past a critical threshold from which it could not recover.

🎧 听力讲座

The lecture challenges the reading's claim that factors other than hunting were primarily responsible for the passenger pigeon's extinction. While the reading mentions reproductive fragility, genetic uniformity, and habitat loss, the professor argues that these were not independent causes but rather consequences of the dramatic population decline caused by overhunting. First, regarding reproductive behavior, the professor explains that passenger pigeons did not always require huge colonies to breed. Historical records show that smaller colonies did reproduce successfully, especially in the early 19th century. The reliance on dense colonies only became a problem after hunting had already decimated the population, creating a feedback loop. Second, the genetic evidence is misleading. The low genetic diversity observed in museum specimens may reflect the species' long-term evolutionary history, not a recent bottleneck. Many bird species with low genetic diversity, such as the whooping crane, have survived with small populations. Thus, genetic uniformity alone cannot explain the extinction. Third, while deforestation did occur, it was not severe enough to eliminate the pigeons' food supply. Large tracts of forest remained in the late 19th century, and the pigeons were known to feed on a variety of nuts and seeds, not just acorns and beechnuts. The professor concludes that the timing of the extinction—immediately following intense commercial hunting—strongly indicates that overhunting was the decisive factor, and the other factors merely exacerbated the unavoidable decline.