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 overhunting, a closer examination of the evidence suggests that climate change was the primary driver. First, the timing of the extinctions correlates strongly with major climatic shifts. The most significant die-offs occurred during the transition from the last glacial maximum to the Holocene, a period of rapid warming and dramatic changes in vegetation. For example, in North America, the extinction of the mammoth steppe ecosystem coincided with the expansion of forests and the loss of open grasslands, which were the preferred habitats of these megafauna. Second, many megafauna species had survived previous human hunting pressures for thousands of years, indicating that humans were not a decisive factor. In Africa, where hominids had coexisted with large mammals for over two million years, only a small percentage of megafauna went extinct, even though hunting technology was advanced. Third, the extinction pattern was selective: species with large body sizes and specialized diets, such as grazers dependent on grassy plains, were disproportionately affected. In contrast, generalist herbivores and smaller animals, which could adapt to changing food sources, survived. This selectivity is more consistent with a habitat-based explanation than with human hunting, which would not discriminate based on dietary specialization.
🎧 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 a lecture on the same topic from a professor of paleontology.
"The reading argues that climate change was the main cause of the late Pleistocene megafauna extinctions, but this view is problematic. Let’s look at the evidence more carefully. First, the correlation between climate change and extinction is not as clean as the reading suggests. Many species went extinct well after the major warming began, and some extinctions occurred during stable climatic periods. For instance, in Australia, the giant marsupials disappeared about 46,000 years ago, during a period of relatively stable climate, yet shortly after the arrival of humans. This timing strongly implicates human activity, not climate. Second, the claim that megafauna survived earlier human pressures in Africa ignores the fact that African mammals co-evolved with hominids for millions of years, developing behavioral adaptations to avoid hunters. In contrast, megafauna in the Americas, Australia, and many islands had no prior exposure to humans, making them extremely vulnerable to even low-level hunting. When humans arrived, these naive animals were easily killed, leading to rapid declines. Third, the selective extinction of large, specialized grazers can also be explained by human hunting. Hunters preferentially targeted large game because they provided more meat and resources per kill. Large animals also have slower reproductive rates, so their populations were slower to recover from hunting losses. The reading’s assumption that hunting would not be selective is simply incorrect. Therefore, when we consider the full evidence, human overhunting remains a far more compelling explanation."
📝 写作提示 Summarize the points made in the lecture, being sure to explain how they cast doubt on the specific points made in the reading passage.
