The Extinction of the Megafauna: A Climate-Driven Event
During the late Pleistocene, roughly 50,000 to 10,000 years ago, many large terrestrial mammals—such as woolly mammoths, saber-toothed cats, and giant ground sloths—went extinct. While some scientists attribute these extinctions to human hunting, a closer examination of the evidence suggests that climate change was the primary driver. First, the timing of these extinctions correlates strongly with major climatic shifts, particularly the warming at the end of the last Ice Age. As temperatures rose, habitats such as grasslands and tundra shrank, reducing the available range for megafauna. For example, woolly mammoths, which depended on cold, dry steppe environments, saw their habitat contract drastically as forests expanded. Second, many megafauna species had survived earlier human hunting pressure during interglacial periods, indicating that they were not inherently vulnerable to human predation. Fossil records show that species like the giant deer coexisted with early humans for thousands of years before their final disappearance, suggesting that hunting alone could not have caused their extinction. Third, the extinction event affected not only large herbivores but also large carnivores, which would not have been directly targeted by human hunters. If overhunting were the cause, we would expect a selective impact on prey species, yet the pattern shows a broad die-off across trophic levels, consistent with a collapse of entire ecosystems due to climate-induced habitat loss.
🎧 听力讲座
🎯 模拟考试环境：先点「播放讲座」听完内容，再作答。听力原文默认隐藏。
Now listen to part of a lecture on the same topic from a biology professor.
While the reading makes a case for climate change, I argue that human hunting was the decisive factor in the megafauna extinctions. Let's examine the evidence. First, the timing argument is misleading. The extinctions occurred not during the initial warming but at the end of the Younger Dryas, a cold snap that followed a brief warming trend. This cold period saw a return of glacial conditions, yet extinctions continued. In fact, many species, such as the woolly mammoth, survived on isolated islands like Wrangel Island for thousands of years after the main climate shift—until humans arrived there. This pattern strongly implicates human activity, not climate. Second, the claim that megafauna coexisted with humans for long periods is flawed. In many regions, such as North America, megafauna disappeared within a few hundred years of human arrival, not after thousands of years. The fossil record shows that species like the mammoth vanished rapidly after the Clovis people appeared, which is consistent with overhunting. Moreover, the idea that they were resilient to hunting is contradicted by the fact that large, slow-reproducing animals are highly susceptible to even modest hunting pressure. Finally, the extinction of carnivores does not undermine the hunting hypothesis. When humans eliminated large herbivores, carnivores lost their prey base and subsequently starved. This cascade effect explains why carnivores went extinct without being directly hunted. Thus, the pattern of extinction aligns far better with human overexploitation than with climate change.
📝 Инструкция по письму Summarize the points made in the lecture, being sure to explain how they cast doubt on the specific points made in the reading passage.
📊 TOEFL Integrated Writing Rubric
Content Accuracy: Accurately capture the relationship between lecture and reading
Information Completeness: Cover all key counter-arguments from the lecture
Structure & Coherence: Logical organization and progression
Language Use: Grammar, vocabulary, expression accuracy
Scored 0-30 based on holistic scoring (original 0-5, scaled to 30).
