AP Environmental Science

## Learning Outcomes

- Understand what the demographic transition model shows and why it is used
- Identify the main characteristics of Stages 1, 2, 3, and 4
- Explain why death rates usually fall before birth rates during development
- Distinguish between crude birth rate, crude death rate, total fertility rate, and rate of natural increase
- Interpret how age-structure diagrams connect to demographic transition
- Apply simple demographic calculations, including rate of natural increase and doubling time
- Evaluate the strengths and limits of the demographic transition model

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand demographic transition, with a focus on the following syllabus points:

- Know that the demographic transition model describes how birth rates and death rates change as a society develops from preindustrial to industrial and postindustrial conditions
- Identify the characteristics of Stage 1, Stage 2, Stage 3, and Stage 4, especially the pattern of birth rate, death rate, and population growth in each
- Explain why Stage 2 has rapid population growth, including improved sanitation, clean water, food supply, and health care
- Explain why birth rates tend to decline in Stage 3, including urbanization, women’s education, access to contraception, and reduced need for child labor
- Understand how crude birth rate, crude death rate, total fertility rate, replacement fertility rate, and rate of natural increase relate to the model
- Interpret how age-structure diagrams and population momentum help predict future growth
- Recognise that some sources add a Stage 5, but that this extension is not universal
- Understand that the model has limitations, especially because it does not fully account for migration, policy changes, war, or regional variation

## Test Your Knowledge

_Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, look more closely at that area during your revision._

1. Why does population growth usually rise fastest in Stage 2 rather than Stage 1?
2. What changes in society usually cause birth rates to fall in Stage 3?
3. How do you calculate rate of natural increase from crude birth rate and crude death rate?
4. Why can a country still keep growing even if its fertility rate has fallen close to replacement level?
5. How does an age-structure diagram help you predict a country's future population trend?

## Introduction

The demographic transition model, often shortened to **DTM**, is one of the most tested ideas in AP Environmental Science Unit 3 because it connects population change to development. It helps explain why some countries have rapid population growth, why others have slow or near-zero growth, and how changes in health, education, and urban life affect birth and death rates over time.

For the exam, you need more than the stage names. You need to understand the mechanism behind each stage: what causes death rates to fall, why birth rates stay high for a time, why fertility later drops, and why population can keep rising even after families begin having fewer children.

> **Key Term: demographic transition model**
> A model that predicts population trends by showing how birth rates and death rates change as a society becomes more industrialized and developed.

## Introduction to the model and the key demographic measures

### What the model is actually tracking

The demographic transition model is not mainly about total population size. It is about the relationship between **birth rate** and **death rate** over time. That matters because population growth depends on the gap between the two.

A country can have a large population and still grow slowly. Another can have a smaller population but grow quickly. The model focuses on rates, not just raw numbers.

> **Key Term: crude birth rate (CBR)**
> The number of live births per 1,000 people per year.
>
> **Key Term: crude death rate (CDR)**
> The number of deaths per 1,000 people per year.

The simplest way to think about growth in the model is this:

- if birth rate is much higher than death rate, population grows quickly
- if birth rate is only a little higher than death rate, population grows slowly
- if birth rate equals death rate, population is stable
- if birth rate falls below death rate, population declines unless migration changes the picture
  > **Key Term: rate of natural increase (RNI)**
  > The difference between crude birth rate and crude death rate, usually excluding migration.

If CBR and CDR are given per 1,000 people, then:

- **RNI per 1,000** = CBR − CDR
- **RNI as a percent** = (CBR − CDR) / 10

Worked example:

Suppose a country has:

- CBR = 28 births per 1,000
- CDR = 8 deaths per 1,000

Step 1: Subtract death rate from birth rate

- 28 − 8 = 20 per 1,000

Step 2: Convert to a percent

- 20 / 10 = **2.0%**

So the rate of natural increase is **2.0%**.

A 2.0% growth rate is high enough to produce fast population growth.

> **Test Tip:** If the exam gives CBR and CDR "per 1,000," students often forget to divide by 10 when asked for a percentage. Always check the units.

### Why the model matters in AP Environmental Science

APES is not only interested in population as a human geography topic. It matters because population size and growth affect:

- resource use
- freshwater demand
- land conversion
- food production
- waste generation
- pollution
- biodiversity loss

A rapidly growing population can put heavy pressure on land, forests, fisheries, and water supplies. But APES also expects balance: the model does not say that development is bad. In many cases, development lowers death rates first and later lowers birth rates too.

That is why the demographic transition model is useful. It helps explain both the social causes and the environmental consequences of population change.

## The four main stages of the demographic transition model

### Stage 1 and Stage 2: from high stationary to rapid growth

Stage 1 is the **preindustrial** stage. It has:

- high birth rate
- high death rate
- little or unstable population growth

In this stage, families tend to have many children, but many people also die young. Harsh living conditions keep death rates high. Common reasons include:

- poor sanitation
- limited medical care
- unsafe drinking water
- low food security
- infectious disease

Because birth and death rates are both high, they largely offset each other. This is why Stage 1 does **not** have the fastest growth. Students often get this wrong because they see the high birth rate and forget the equally high death rate.

> **Key Term: preindustrial state**
> The first stage of the demographic transition model, in which birth rates and death rates are both high and population growth is low.

Stage 2 is the **transitional** stage. It has:

- birth rate still high
- death rate falling quickly
- rapid population growth

This is the stage of fastest growth in the model. The key idea is timing: **death rates fall before birth rates do**.

Why do death rates fall first?

Because improvements in survival can happen relatively quickly:

- cleaner water reduces waterborne disease
- better sanitation lowers infection rates
- improved food supply lowers starvation and malnutrition
- vaccinations and basic medical care reduce mortality

But birth rates often stay high at first because social and economic attitudes change more slowly. Families may still want many children because:

- children may be needed for labor
- child mortality may still be feared
- cultural or religious expectations may favor large families
- women may have limited access to education or contraception

That delay creates a wide gap between births and deaths, producing rapid growth.

> **Key Term: transitional state**
> The second stage of the demographic transition model, in which birth rates remain high but death rates decline, causing rapid population growth.

A cause-and-effect chain for Stage 2 looks like this:

- sanitation and medicine improve
- fewer infants and adults die
- birth rates remain high
- more children survive to adulthood
- population grows very quickly

This is why Stage 2 often produces short doubling times.

Using the Rule of 70:

- doubling time = 70 / growth rate (%)

If a country has a 4% growth rate:

- 70 / 4 = **17.5 years**

That is very fast doubling.

> **Test Tip:** If a question asks why doubling time is short in the middle stages, do not give a vague answer like "more people are born." The stronger answer is that death rates have fallen while birth rates remain high, so the growth rate is large.

### Stage 3 and Stage 4: slowing growth and near stability

Stage 3 is the **industrial** stage. It has:

- death rate low
- birth rate falling
- population still growing, but more slowly

The common student mistake here is to say Stage 3 means zero growth. It does not. Growth usually continues, but the gap between births and deaths becomes smaller.

> **Key Term: industrial state**
> The third stage of the demographic transition model, in which birth rates decline and begin to approach death rates, slowing population growth.

Why do birth rates fall in Stage 3?

This is one of the most important "why" questions in the topic. Several forces act together:

- **urbanization** makes children more expensive and less useful as labor
- **women’s education** tends to delay marriage and first childbirth
- **workforce participation** gives women more choices beyond early, repeated childbearing
- **contraception and family planning** make smaller families more practical
- **lower infant mortality** means parents do not feel they need as many births to ensure some children survive
- **retirement systems and higher incomes** reduce the need to depend on many children for old-age support

Notice the economic shift here. In rural subsistence settings, more children can increase family labor supply. In urban-industrial settings, more children often increase costs instead.

This is why the model is linked to development, not just biology.

Stage 4 is the **postindustrial** stage. It has:

- low birth rate
- low death rate
- low, stable, or near-zero growth
  > **Key Term: postindustrial state**
  > The fourth stage of the demographic transition model, in which birth rates and death rates are both low and population growth is near zero.

Stage 4 is often associated with **zero population growth**, but be precise. Birth and death rates may be very close without being exactly equal. Also, total population can still grow for a while because of age structure.

That means a Stage 4 country can have:

- low natural increase
- slightly positive growth
- or even decline, depending on fertility and age structure

A useful distinction:

- **Stage 3** = birth rate falling, growth slowing
- **Stage 4** = birth and death both low, growth near zero

Students often confuse these because both stages have low death rates. The real difference is what the birth rate is doing.

## Age structure, fertility, and the limits of the model

### Why age-structure diagrams matter

An age-structure diagram helps predict future growth because it shows whether a population has many young people, many middle-aged people, or many older people.

> **Key Term: age-structure diagram**
> A graph showing the numbers or percentages of people in different age groups within a population.

In APES, the main age groups are often:

- **pre-reproductive**: 0–14
- **reproductive**: 15–44
- **post-reproductive**: 45+

A wide-base pyramid means many young people. Even if fertility later drops, that country may keep growing because so many people are moving toward reproductive age.

> **Key Term: population momentum**
> Continued population growth caused by a large number of young people entering reproductive years, even if fertility falls to replacement level.

This is why population does not stop growing the instant a country reaches replacement-level fertility.

Compare three common age-structure patterns:

- **pyramid-shaped**: rapid future growth
- **more column-shaped or bell-shaped**: stable or slow growth
- **narrow base or urn-shaped**: slow growth or decline

The model and the age structure work together. A Stage 2 or early Stage 3 population often has a broad base, while a Stage 4 population is more even. A declining population may have a narrow base and larger older cohorts.

### TFR, replacement fertility, Stage 5, and model limitations

Do not confuse stage labels with fertility measures.

> **Key Term: total fertility rate (TFR)**
> The average number of children a woman is expected to have over her lifetime in a population.
>
> **Key Term: replacement fertility rate**
> The average number of children per woman needed to replace the parents in a population, about 2.1 in many developed countries.

These are related to the DTM but are not the same thing.

- **DTM** = a stage model of changing birth and death rates
- **TFR** = a measured fertility value
- **replacement fertility** = the level needed for long-term population replacement

A TFR below replacement often suggests long-term decline, but not always immediately, because age structure and migration matter.

Some sources add **Stage 5**, with:

- very low birth rates
- aging populations
- death rates equal to or higher than birth rates
- possible population decline

You may mention Stage 5 if your class has covered it, but use caution. The original model has four stages, and the fifth stage is not universal.

> **Key Term: zero population growth**
> A condition in which births equal deaths, so natural increase is zero.

The model also has limits. It is useful, but it is not a perfect law of history.

Important limitations:

- it was based largely on European development patterns
- it does not fully account for migration
- it can be disrupted by war, famine, pandemics, or natural disasters
- government policy can change fertility faster than the model predicts
- different regions within one country may not fit the same stage

For example, a country with low birth rates may still grow because of immigration. Or a country in a low-mortality stage may suffer a sudden rise in deaths because of conflict or disease.

That means the DTM is best used as a **general framework**, not an exact predictor.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- The demographic transition model tracks changes in birth rates and death rates as societies develop
- Stage 1 has high birth rates and high death rates, so population growth is low
- Stage 2 has high birth rates and falling death rates, so population growth is rapid
- Stage 3 has falling birth rates and low death rates, so growth continues but slows
- Stage 4 has low birth rates and low death rates, so growth is low, stable, or near zero
- Death rates usually fall before birth rates because sanitation, food supply, and health care improve faster than social attitudes about family size change
- Birth rates tend to fall in Stage 3 because of urbanization, women’s education, contraception, lower infant mortality, and changing economic incentives
- Rate of natural increase can be calculated as CBR minus CDR, and as a percent by dividing by 10 when rates are per 1,000
- Age-structure diagrams help predict future growth by showing the proportions of pre-reproductive, reproductive, and post-reproductive people
- Population momentum explains why total population can continue growing even after fertility drops
- Total fertility rate and replacement fertility rate are related to the model but are not the same as the stages themselves
- The model has limitations because it does not fully account for migration, policy changes, or sudden disruptions such as war or pandemics

## Key Terms and Concepts

- demographic transition model
- crude birth rate (CBR)
- crude death rate (CDR)
- rate of natural increase (RNI)
- preindustrial state
- transitional state
- industrial state
- postindustrial state
- age-structure diagram
- population momentum
- total fertility rate (TFR)
- replacement fertility rate
- zero population growth


Demographic Transition - Demographic transition model