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题目材料：
Pikas are small, rabbitlike mammals that live high in the mountains on rocky slopes in western North America. During the 1990s, ecologist Eric Beever revisited 24 locations-all in the Great Basin of the western United States-where pikas had been observed between 1898 and 1947, and he found that seven of the original sites no longer had pikas. The pika populations that had vanished were those at lower, warmer elevations, which suggests climate warming is involved. Generally, when climate warming changes habitat, animals move either to higher, colder elevations or farther north. However, pikas in the Great Basin are not easily able to migrate in this way. They live on mountain ranges that are separated from other mountains by inhospitable valleys. Further, even the most widely roving pikas tend to move less than one kilometer from their birthplaces during their lifetimes.
以上解析由 考满分老师提供。
