Remove such a cog and the whole system is affected. This point is well illustrated in the following case from mainland China.
被認為愛吃稻穀的麻雀，好像沒啥用處，如果它絕種了，會有什麼影響？大陸上曾經有段期間，把麻雀趕盡殺絕，城市、鄉村都難見到蹤影，結果那幾年蟲害增加，稻米、蔬菜、雜糧全部欠收。