Passage Summary: Even though certain animals help us spot pollution, we shouldn't blame pollution every time their numbers drop, because nature changes on its own all the time.

Conclusion: Environmentalists should not assume that every instance of an indicator species' population decline is caused by pollution.

Reasoning: While population drops can signal pollution, they are frequently the result of natural ecosystem evolution and the inherent instability of nature.

Analysis: The conclusion is signaled by the phrase 'yet environmentalists would be misguided if...' This is the author's main point, positioned as a correction to a common practice. The sentences following it provide the evidence regarding natural evolution to support this cautionary stance. Focus strictly on identifying this prescriptive claim about what environmentalists should or should not do.