GRE Reading - Identification of Main Idea

Hi, so I have a general question on identifying the main idea in passages. Does the main idea of the passage not depend on the number of sentences that idea is talked about?


For example, here, I figured the main idea is the long distance transportation of plants to Hawaiian islands, not the actual main idea which is, bird were responsible for transport of seeds.
I had thought that because birds transportation is a small chunk of the entire passage, hence, cannot be the main idea.

The main idea is “More probable is…of the seeds”. The sentence before it (“Yet the results…”) is the hammer.

If you break this up by paragraphs, you’ll see that the first sentence is the author’s hypothesis - see that he/she says “the great variety of plants in Hawaii must be a result of…”. The the first sentence of the second paragraph is a concession and the second sentence is the support for the concession. Then the author lays a hammer starting with “Yet”

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Thanks!

Hi, I have a question regarding this passage from prepswift:

Here, why is the main idea the last part of the sentence, and not the sentence “One particular area that has not yet been adequately explored…”?