TOEFL Integrated Writing

📖 Reading Passage

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 the extinctions correlates strongly with rapid climatic shifts, not with human arrival. For example, in North America, the last glacial maximum ended around 15,000 years ago, and many megafauna species disappeared during the subsequent warming period, which caused dramatic alterations in vegetation and habitat. In contrast, human populations were present for thousands of years before these extinctions, yet the die-off was sudden and synchronous with climate change. Second, climate change directly affected the food sources of megafauna. Many large herbivores relied on specific open grassland ecosystems that flourished during cold, dry conditions. As the climate warmed and became more humid, these grasslands were replaced by forests and tundra, reducing the availability of nutritious forage. This vegetation shift would have caused malnutrition and population declines, making the animals vulnerable to extinction. Third, the pattern of extinction supports a climate cause: species that were highly specialized to cold environments, such as the woolly mammoth, suffered the most, while more adaptable generalist species survived. This selective pattern is consistent with a habitat-loss scenario rather than human overhunting, which would have targeted all large prey equally.

🎧 Listening Lecture

Now listen to a lecture on the same topic. Professor: The reading argues that climate change was responsible for the extinction of megafauna, but this view is seriously flawed. Let me address each point. First, the timing argument is misleading. While it is true that some extinctions occurred during warming periods, we now have high-resolution data showing that many extinctions happened after humans arrived, even when climate was stable. For instance, in Australia, megafauna went extinct around 45,000 years ago, shortly after human colonization, and there is no major climate shift at that time. Similarly, in New Zealand and Madagascar, extinctions occurred just a few hundred years ago, well after human settlement and during a period of minimal climate change. Second, the claim that climate change destroyed food sources is not supported by fossil evidence. Pollen and plant fossil records show that many megafauna species were generalist feeders and could consume a wide variety of plants. For example, mammoth dung analysis reveals they ate grasses, sedges, and even woody plants, so the shift to forests would not have caused starvation. In fact, some megafauna thrived in warmer interglacial periods in earlier epochs, indicating they were not limited to cold habitats. Third, the selective extinction pattern actually points to human hunting. Species that went extinct were those that were large and slow-reproducing—like mammoths and giant sloths—making them easy targets and slow to recover from hunting pressure. Smaller, faster-reproducing animals survived, which is exactly what we would expect from overhunting, not from climate change. So, the evidence strongly favors human activities as the primary cause.