Topic 2.4 Notes – Population Dynamics

What Population Dynamics Are

Population dynamics means change in population size over time. At any scale, from a city to the whole world, the same three pieces drive change:

• Fertility adds people through births.
• Mortality removes people through deaths.
• Migration changes population through movement.
  • Immigration adds people to a place.
  • Emigration removes people from a place.

The core equation ties the whole topic together:

$$\text{Population change} = \text{births} - \text{deaths} + \text{immigration} - \text{emigration}$$

Two smaller pieces come out of that:

• Natural change = births - deaths
• Net migration = immigration - emigration

This is the exam trap to avoid. Total population change is not the same as natural increase. A place can grow even when deaths are higher than births if enough people move in. A place can shrink even with more births than deaths if even more people move out. Age structure from Topic 2.3 matters here, because crude rates can look misleading if a population is very young or very old.

The Main Population Measures

These measures let geographers compare places of different sizes.

Fertility

• Crude birth rate (CBR) = live births per 1,000 total population in one year
  $$\text{CBR}=\frac{\text{live births}}{\text{total population}}\times1000$$
  It is called crude because the denominator includes everyone, not just women who can have children.
• Total fertility rate (TFR) = average number of children per woman over her lifetime.
  This is better than CBR for comparing childbearing behavior.
• Replacement-level fertility is about 2.1 children per woman in low-mortality countries.
• Population momentum means growth can continue even after fertility drops below replacement, because a large young generation is just entering childbearing years.

Mortality

• Crude death rate (CDR) = deaths per 1,000 total population in one year
  $$\text{CDR}=\frac{\text{deaths}}{\text{total population}}\times1000$$
  A high CDR can happen in a rich, healthy country if it has many elderly people.
• Infant mortality rate (IMR) = deaths under age 1 per 1,000 live births
  $$\text{IMR}=\frac{\text{deaths under age 1}}{\text{live births}}\times1000$$
  IMR is a strong clue about health care, sanitation, nutrition, and development.
• Life expectancy = average expected lifespan under current mortality conditions.

Growth

• Rate of natural increase (RNI)
  $$\text{RNI (\%)}=\frac{\text{CBR}-\text{CDR}}{10}$$
• Net migration rate (NMR) = immigrants - emigrants per 1,000 population
• Total growth rate includes both natural increase and migration.
• Doubling time uses the Rule of 70
  $$\text{Doubling time}\approx\frac{70}{\text{annual growth rate \%}}$$
  Use a percent, not a per-thousand rate.

What Changes Fertility, Mortality, and Migration

Population patterns make more sense once you connect them to conditions in real places.

• Economic factors
  • In agricultural economies, children may provide labor and old-age support, which often raises fertility.
  • In urban-industrial economies, children cost more, so fertility often falls.
  • Jobs pull migrants in. Unemployment pushes people out.
• Social and cultural factors
  • Women’s education, later marriage, workforce participation, and contraception usually lower fertility.
  • Ideas about ideal family size, gender roles, and family duty shape birth rates.
  • Education, sanitation knowledge, and maternal care lower mortality.
  • Social networks create chain migration, making movement easier.
• Political factors
  • Health care, vaccines, clean water, and public health systems lower mortality.
  • War, persecution, and instability raise mortality and emigration.
  • Governments can influence fertility and migration, though detailed population policy is covered later in the course.
• Environmental factors
  • Droughts, floods, earthquakes, disease outbreaks, and food insecurity can raise mortality or trigger migration.
  • The impact depends on how vulnerable a place is and how much the state can respond.

Historical and Regional Population Patterns

For most of history, both birth rates and death rates were high, so population grew slowly. Agriculture supported bigger settled populations, but famine and disease still kept growth in check. Industrialization and public health lowered death rates first, so natural increase sped up. The graph below matches that big-picture trend. World population keeps rising even after the annual growth rate peaks a little above 2 percent in the early 1960s and then declines as fertility falls.

Examples you should know:

How to Read Population Questions and Data

On a graph of CBR and CDR, focus on the distance between the birth-rate line and the death-rate line over time.

Crude birth rate and crude death rate over time

• The vertical gap between the lines shows natural increase.
• Converging lines mean growth is slowing.
• CDR above CBR means natural decrease.

On maps, remember this:

• RNI maps show births vs. deaths only.
• They do not show total growth if migration is changing the population a lot.