托福综合写作

📖 阅读文章

The extinction of the woolly mammoth has long been attributed to human overhunting, but recent evidence suggests that climate change was the primary driver. First, the timing of mammoth decline correlates more closely with warming temperatures than with human expansion. Radiocarbon dating shows that mammoth populations began to shrink significantly around 15,000 years ago, as the last Ice Age ended, while human hunters had already been present in Siberia for thousands of years without causing major declines. Second, the habitat of woolly mammoths—the mammoth steppe—was highly specialized, consisting of dry, cold grasslands. As the climate warmed, this steppe was replaced by wetter, forested landscapes, which would have drastically reduced the food supply and open terrain that mammoths required. Fossil pollen records from the period confirm a rapid shift to birch and spruce forests, leaving only isolated pockets of suitable habitat. Third, genetic studies of mammoth remains reveal a loss of genetic diversity in the final millennia, indicating small, fragmented populations that were more vulnerable to environmental stress and disease. This genetic bottleneck is inconsistent with a rapid overkill by humans, which would have left a sudden, catastrophic demise rather than a prolonged decline.

🎧 听力讲座

The lecture challenges the reading's claim that climate change was the main cause of woolly mammoth extinction. While the timing of their decline does coincide with warming, this does not rule out human hunting. In fact, human populations were also expanding during this period, and archaeological sites show increasing evidence of mammoth hunting, including spear points and butchery marks. Moreover, the habitat change argument is overstated. Mammoths were adaptable animals, as evidenced by their ability to survive in diverse environments from Siberia to the Iberian Peninsula. Pollen records indicate that some mammoth populations persisted in forested regions for thousands of years, suggesting they could exploit alternative food sources like twigs and bark. Finally, the genetic diversity loss does not necessarily point to climate fragmentation. Small populations could have been the result of hunting pressure, as humans selectively targeted the largest individuals, reducing effective population size. Indeed, similar genetic bottlenecks are seen in historically overhunted species like the great auk. Thus, the reading's evidence is ambiguous and can be explained more parsimoniously by human overhunting.