What do you mean by population decline?

Population decline (enough to become a social problem)

But in fact, this usage is not exact; Because "population" itself is not the main body, "national strength" is.

Du Niang: "Decline" comes from The Book of Songs. "Shi" is a modal particle in classical Chinese, and "Wei" means decline. "Decline" originally refers to the decline of a country or clan, and now it also refers to the decline of things. "Decline" has the color of written language. For example, "the old-fashioned weaving industry here is declining."