2019年全国统一考试（天津卷）英语独家解析(25)

【48题详解】
推理判断题。根据第二段中In the 1960s, scientists proposed that predators at the top of a food web had a surprising amount of control over the size of populations of other species—including species they did not directly attack.可知，处于食物网顶端的食肉动物对它们没有直接攻击的其他物种的种群数量有着惊人的控制，由此可推断出，如果食物链顶级食肉动物的数量大大下降，其他物种的种群将经历意想不到的变化。故选D。
【49题详解】
推理判断题。根据第四段And unplanned human activities have proved the idea of top-down control by top predators to be true. In the ocean, we fished for top predators such as cod on an industrial scale, while on land, we killed off large predators such as wolves. These actions have greatly affected the ecological balance.可知，无计划的人类活动证明了由顶级掠食者自上而下控制的想法是正确的。在海洋中，我们大规模捕捞鳕鱼等顶级捕食者，而在陆地上，我们杀死狼等大型捕食者。
这些行为极大地影响了生态平衡。由此可推断出，不受控制的人类活动会极大地破坏了生态系统。故选A。
【50题详解】
细节理解题。根据最后一段中Scientists have built an early-warming system based on mathematical models. Ideally the system would tell us when to adapt human activities that are pushing an ecosystem toward a breakdown or would even allow us to pull an ecosystem back from the borderline.可知，早期预警系统通过发出紧急需要采取预防行动的信号帮助我们维持生态平衡。故选D。