题目解析：
据prairie riparian forest and Black Hills forest回到原文定位到第二句。照常理修辞目的题首先看例证的前一句，但是前句提到prairie riparian forest, Black Hills forest与其它森林一样都可以与其combination分隔开来，以及后一句，也就是这一段的1-3句都是例子的具体内容，没有主旨句。直到第4句：The fact that ecological communities are indeed, recognizable clusters of species led some early ecologists, particularly those living in the beginning of the twentieth century, to claim that communities are highly integrated, precisely balanced assemblages.这句话明显是承接前3句话，引出了一个总结性的结果，即：科学家们根据这个例子而认为community是integrated的。而B选项提到解释了为什么一些生物学家认为ecological communities as highly integrated.和这个内容完全吻合。 A选项说的是ecologists的意见不同，错误；C选项提到了无关的对比，错误；D选项是例子本身的内容，错误。
The Black Hills forest, the prairie riparian forest, and other forests of the western United States can be separated by the distinctly different combinations of species they comprise. It is easy to distinguish between prairie riparian forest and Black Hills forest—one is a broad-leaved forest of ash and cottonwood trees, the other is a coniferous forest of ponderosa pine and white spruce trees. One has kingbirds; the other juncos (birds with white outer tail feathers). The fact that ecological communities are indeed, recognizable clusters of species led some early ecologists, particularly those living in the beginning of the twentieth century, to claim that communities are highly integrated, precisely balanced assemblages. This claim harkens back to even earlier arguments about the existence of a balance of nature, where every species is there for a specific purpose, like a vital part in a complex machine. Such a belief would suggest that to remove any species, whether it be plant, bird, or insect, would somehow disrupt the balance, and the habitat would begin to deteriorate. Likewise, to add a species may be equally disruptive.
