托福综合写作

📖 阅读文章

The extinction of the woolly mammoth has long been attributed to human hunting, but recent evidence suggests that climate change was the primary driver. First, the warming period at the end of the Pleistocene (about 10,000 years ago) drastically reduced the mammoth's habitat. As glaciers retreated, the vast steppe-tundra—their primary grazing ground—was replaced by dense forests and wetlands, which offered unsuitable forage. Fossil pollen records show this vegetation shift coincided with mammoth population declines. Second, climate change led to increased precipitation and warmer winters, creating ice layers on snow that prevented mammoths from accessing the grasses beneath. Unlike other megafauna, mammoths had specialized teeth for grazing tough, dry grasses, so they could not adapt to the new vegetation. Third, isolation in small refugia, such as islands, accelerated their extinction. For example, the last known mammoths on Wrangel Island survived until 4,000 years ago, but genetic studies reveal that this small population accumulated harmful mutations, leading to reduced fertility and eventual demise—a consequence of climate-driven habitat fragmentation, not human overhunting.

🎧 听力讲座

The lecture challenges the reading's argument that climate change was the primary cause of mammoth extinction. While the reading cites habitat loss, the lecture notes that mammoths were not confined to a single habitat type. Fossil evidence shows that mammoths thrived in diverse environments, including grasslands and even some forested areas, during earlier warm periods. Thus, vegetation shifts alone cannot explain their extinction. Second, the ice-layer theory is problematic. Ice layers would have also affected other herbivores, yet species like caribou and musk oxen survived. Moreover, mammoths had long, curved tusks and flexible trunks that could have been used to break through thin ice to reach vegetation, a behavior observed in modern elephants. Finally, the island extinction example is misleading. Radiocarbon dating shows that human hunters arrived on Wrangel Island at least 500 years before the mammoths disappeared. The small population may have been killed off by hunting, not just genetic defects. In fact, simulated models demonstrate that a human population of just a few hundred could have driven a small isolated herd to extinction within centuries. Therefore, human activity, not climate, was the decisive factor.