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题目材料：
Sunflower sea stars help maintain certain kelp forest ecosystems by eating quickly reproducing prey species such as urchins, thus keeping populations low.Without the sea stars, the urchin population explodes, which is bad news for kelp forests and everything in them. Giant kelp can grow to 150 feet underwater at a speed of two feet a day, but their weaknesses are their holdfasts, which are akin to tree roots. The holdfasts are home to brittle stars, prawns,and snails, among other creatures. Urchins like to eat the kelp holdfasts.Once the holdfasts are gone,the rest of the kelp drifts off in the tides. In this way, urchins can destroy the forests, which, higher up, are also home to fish, including several types of commercially important rockfish.
以上解析由 考满分老师提供。
