24.1 Human Population Growth
Key Takeaways
A wide base on a population pyramid means many children and potential growth; a narrow base means slower growth or decline.
A bulge is a cohort, and it moves up the pyramid as that group ages.
Birth rate is births per population per time; fertility rate is children per woman.
In a low-mortality population, replacement fertility is about 2.1 children per woman because of deaths before reproductive age and a slight excess of males at birth.
In the demographic transition, growth is rapid when the death rate falls while the birth rate stays high, and a young age structure can keep a population growing after fertility falls.
24.1 Human Population Growth
Human numbers change through births, deaths, and the ages of the people already alive. The topics here are age composition, birth and fertility rates, and the theory of demographic transition.
Age composition and population pyramids
Age composition is the share of people in each age class. A population pyramid draws those shares as horizontal bars, males on one side and females on the other, youngest at the bottom and oldest at the top. Tomorrow's parents are today's children, so the shape suggests later growth.
A wide base means many children relative to older ages. If those children survive, a large group will enter reproductive ages, and the population has strong potential to grow. A narrow base means relatively few children, which points to slower growth or, if deaths among older people outnumber births, to decline. A bulge is a large cohort, people born in the same period. It does not stay at the bottom. It moves up as that cohort ages: a large group of children becomes a large group of workers, then a large group of older adults.
Compare the youngest bars with the reproductive ages, roughly the late teens through the forties. A bottom much wider than the middle means many people are about to reach childbearing age. A bottom narrower than the middle means fewer people are about to replace today's parents.
What one drawing cannot tell you
A pyramid is a snapshot of who is alive. It does not by itself state the birth rate or the death rate. A wide base may come from years of high fertility or from a shorter rise in births. Read it as potential: many young people mean many future parents if mortality stays low, and a narrow base means fewer future parents.
Slightly more males than females are born, and at older ages women often outnumber men. Unequal left and right bars can show that imbalance. The replacement figure already includes the slight excess of males at birth.
Birth rate, fertility rate, and death rate
The birth rate is births per unit of population per unit of time, often births per 1,000 people per year. The denominator is everyone, including men, children, and older adults. A population of mostly older adults can have a low birth rate even when the remaining young women have several children.
The fertility rate counts children per woman. The total fertility rate is the average number of children a woman would have if current age-specific birth rates applied across her reproductive years. Fertility is children per woman. The birth rate is births relative to the whole population.
In a low-mortality population, replacement fertility is about 2.1 children per woman. That standard introductory figure sits a little above two because it accounts for mortality before reproductive age and for a slight excess of males at birth. Not every newborn survives to reproduce, and slightly more than half of newborns are male, so two births per woman do not quite replace both parents among the next generation of parents. Do not extend 2.1 with extra decimal places. Where many children die before adulthood, more births are required for two children to reach reproductive age, and replacement is higher than this low-mortality figure.
The death rate is deaths per unit of population per unit of time, often deaths per 1,000 people per year. With migration set aside, the population grows when births exceed deaths and shrinks when deaths exceed births. Immigration and emigration still change the count when a case includes them.
A falling death rate by itself increases growth. Fewer deaths leave more survivors. It does not automatically shrink a population. Shrinkage requires births to fall short of the deaths they must replace. If births stay high while deaths fall, growth speeds up.
The theory of demographic transition
The theory of demographic transition is a staged description of how birth and death rates have changed together in many populations. It is a historical pattern, not a schedule every population must keep.
| Stage | Birth rate | Death rate | Growth |
|---|---|---|---|
| High birth and high death | High | High | Slow, because the two high rates nearly cancel |
| Falling death rate | Still high | Falling | Rapid, because births stay high while fewer people die |
| Falling birth rate | Falling | Already low | Slowing, as births move down toward deaths |
| Low birth and low death | Low | Low | Slow again, and sometimes a later decline if fertility stays below replacement |
The first stage pairs many births with many deaths, so growth is slow. Disease, unreliable food, and limited medical care keep the death rate high while families have many children. The second stage is easy to misread. The death rate falls, often with cleaner water, more reliable food, and vaccines, while the birth rate stays high. People are healthier, and the population grows faster, not smaller. In the third stage the birth rate falls, associated with urban life, schooling, lower child mortality, and the ability to limit births. Growth slows, though it need not stop in the year births start down. In the fourth stage both rates are low and growth is slow again. Fertility that stays below about 2.1 can later bring stability or decline, after the cohorts already born have moved through the pyramid.
Population momentum
Population momentum is growth that continues after fertility falls because of age structure. Children and teenagers already alive will enter reproductive ages even if each woman has fewer children. A large group of young adults can produce many births at a modest fertility rate. A wide base carries more momentum, and a narrow base carries less. Fertility near 2.1 does not freeze the headcount that same year.
The bulge keeps moving up after it leaves the base. In the reproductive ages it keeps births numerous. In old age the same cohort can raise the number of deaths simply because many people are dying together, even when the risk at each age is unchanged.
Warning
A falling death rate by itself increases population growth. It does not automatically shrink a population. Shrinkage comes when births fail to replace deaths, not from the fact that people live longer.
Read a wide base as potential growth, a narrow base as slower growth or decline, and continued increase after fertility falls as population momentum.
Which conclusion follows from a population pyramid with a wide base?
A large cohort of older adults is about to move down into the reproductive ages.
The population must already be shrinking, because a wide base means deaths exceed births.
The drawing states the numerical birth rate and death rate for that year.
Many children are present relative to older ages, so the population has strong potential to grow if those children survive.
Why is replacement fertility about 2.1 children per woman in the low-mortality case used in introductory biology?
Because a falling death rate shrinks the population unless fertility is forced below two children per woman.
Because that figure is the crude birth rate per 1,000 people, which is the same quantity as children per woman.
Because some children die before reproductive age and slightly more males than females are born, so two births per woman do not quite replace both parents.
Because every population is required to grow, so replacement is defined as more than two children in every case.
In the demographic transition, what happens in the stage when the death rate falls while the birth rate stays high?
Growth slows, because fewer deaths mean fewer people remain.
Growth stops in that same stage, because births fall as soon as deaths fall.
Growth becomes rapid, because many births continue while fewer people die.
The pyramid must develop a narrow base during that stage, before the birth rate changes.
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