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题目材料：
In the late 1970s, bird populations were found to be declining in India's Bharatpur Bird Sanctuary in Rajasthan. Grazing cattle and buffalo were the suspected culprits: they were entering the preserve in sufficient numbers to disrupt what was believed to be an otherwise balanced ecosystem. Accordingly, grazing was banned in Bharatpur. Yet by the mid.-1980s, studies found that bird diversity within the preserve had declined further since the ban, and it continued to plummet. The absence of grazing animals, it turns out, had disturbed the park's ecology. Weed species were taking over wetlands and choking canals, thereby reducing the fish populations that had once attracted so many birds, and avian species then went elsewhere in search of more suitable nesting places.
以上解析由 考满分老师提供。
