To identify the detail that supports the main idea of the section “Why this matters,” we should focus on the significance of butterflies in the ecosystem and the implications of their decline.
The most relevant choice is:
"Butterflies pollinate flowers, control plant populations by munching on leaves as caterpillars, and provide food for other insects, birds, amphibians, and mammals."
This statement underscores the crucial roles butterflies play in the ecosystem, which supports the main idea that their decline has broader implications for the environment. The other options, while they discuss the situation, do not directly emphasize the ecological importance of butterflies as this statement does.