Estimates indicate that the aquifer
contains enough water to fill Lake Huron, but
unfortunately, under the semiarid climatic conditions that presently exist in
the region, rates of addition to the aquifer are minimal, amounting to about
half a centimeter a year.(TPO 3)
Analysis-
主语：
谓语：
宾语：
其他成分：
还有这个
Many ecologists now think that the relative
long-term stability of climax communities comes not from diversity but from the
“patchiness” of the environment, an environment that varies from place to place
supports more kinds of organisms than an environment that is uniform. (TPO 3)
Analysis-
主语：
谓语：
宾语：
其他成分：
两句是转折，不是后句是主句么
追答你要搞清楚简单句、复杂句和主句、从句的区别。复杂句是由一个以上完整的简单句，中间用and、but等连词连接（当然还有not only...but also...等固定表达），两个句子在语法地位上是平等的。
而你说的主句、从句，则是另一个概念。主句占据主导，从句只是整体上作为主句的某一个成分
这里很清楚，but连接了前后两个句子，两个句子都有完整的句式结构，表达一种转折的意思
明白了，谢谢
追答说错了，是复合句，不是复杂句，不好意思