In stage 1 death is caused by starvation, disease, and the
general danger of living in the world of pre-history.
In stage 2, which occurs as agriculture becomes more
organized and at least general medical knowledge is starting to diffuse as
people begin to live in larger settlements, food supply improves and death rates
drop, while the birth rate remains constant. It's the gap in birth and death
rates that causes the population to balloon here. (My kids sometimes have
trouble with this and talk about how birth rates drop too, which isn't the case.
Death can still be the result of influenzas or other fast moving diseases, but
survival rates for these improve throughout the stage. Infant mortality drops
In stage 3, as societies continue to advance and medical
care improves drastically the cause of death is more likely to be old age or
progressive diseases like cancer. Birth rates are dropping by choice as people
realize they don't need to have 6 kids to ensure that 2 survive to take care of
the parents, and death rates continue to decline.
Stage 4 is marked by an increasing death rate as a result
of the aging population - cancer, heart attacks and plain aging hit the top
tiers of the population pyramid, and you see zero population growth as the birth
rate roughly equals the death rate.