首先,第一句是个复合句,but连接前后两个完整的句子,表达一种转折的语意.前面一个分句的主语是estimates,谓语是indicate,宾语是一个that引导的宾语从句；后面一个分局,rates （of addition to the aquifer）是主语,are（be动词）作系动词,后面的minimal是表语,under the semiarid climatic conditions that presently exist in the region和amounting to about half a centimeter a year都是状语,unfortunately也是状语
其次,第二句根据语意,an environment that varies from place to place supports more kinds of organisms than an environment that is uniform是补充解释前面the environment的状语,主句在前面.many eclolgists是主语,think是谓语,后面又是一个that引导的宾语从句
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