How does the last sentence refine the idea that there is a link between birth rate and the possibility of famine in sub-Saharan Africa?

Which the number is main idea? i need . (1) aside from its enormous population—an estimated 280 million—the lowly cockroach has some amazing characteristics. (2) in the first place, the bug is fast. (3) even the smallest roach can run up to three miles an hour. (4) what’s more, cockroaches can change course as often as twenty-five times in a second, making them the most nimble animals known. (5) roaches are tough to catch. (6) with sense organs located front and back, they can start to flee from predators within 0.054 seconds of detecting potential danger. (7) second, they are ideally adapted to living with people. (8) they eat almost anything and are comfortable anywhere as long as the temperature is above freezing. (9) most species can fit through a crevice no wider than the thickness of a quarter, so any building gives them endless places to rest and breed. (10) and they’re perfect travelers, hitching a ride in pant cuffs or grocery bags. (11) finally, the cockroach has an astonishing capacity for survival. (12) it can hold its breath for nearly forty minutes under water. (13) because its brain is spread throughout its body, the roach can survive for several weeks if its head is removed carefully enough to prevent it from bleeding to death. (14) and because roaches sample food before it enters their mouth, they have learned to avoid foul-tasting poisons. (15) this makes them hard to kill with insecticides.