Replaced themselves.....Why does "Invasion of the Body Snatchers" come to mind.
A common term in demographic analysis is "replacement rate": i.e. given mortality rates, average life expectancy, fertility rates, etc. how many children does each woman in a population have to have in order for the population to remain stable?
In other words - what rate represents replacement, what rate represents decline, what rate represents growth?
It's statistics, not science fiction.