AP Environmental Science

## Learning Outcomes

- Explain why modern human population growth has been driven mainly by falling death rates
- Identify how crude birth rate, crude death rate, fertility, and migration affect population change
- Calculate rate of natural increase, total population growth rate, and doubling time
- Distinguish between total fertility rate, crude birth rate, and replacement-level fertility
- Explain how the demographic transition model shows changing growth drivers over time
- Apply population formulas to AP-style quantitative questions

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand human population growth drivers, with a focus on the following syllabus points:

- Know that population change depends on births, deaths, immigration, and emigration
- Identify the main modern driver of rapid human population growth as falling death rates rather than rising birth rates
- Explain how sanitation, clean water, food supply, and medicine reduce mortality, especially infant mortality
- Understand the meanings of crude birth rate (CBR), crude death rate (CDR), total fertility rate (TFR), and replacement-level fertility
- Calculate rate of natural increase and total population growth rate from data given per 1,000 people
- Explain how migration changes total population size but is separate from natural increase
- Understand why rapid growth is greatest when death rates fall before birth rates fall in demographic transition
- Recognise factors that lower fertility, including women’s education, urbanization, and access to family planning

## 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 has the world’s human population grown so quickly in the last two centuries?
2. What is the difference between crude birth rate and total fertility rate?
3. If a country has a CBR of 22 and a CDR of 8, what is its rate of natural increase?
4. How does net migration differ from natural increase?
5. Why is population growth especially rapid in Stage 2 of the demographic transition model?

## Introduction

Human population size does not change by chance. It changes because people are born, people die, and people move into or out of regions. For AP Environmental Science, the most tested idea is simple but easy to miss: the recent rapid growth of the human population has happened mainly because **death rates fell**, not because birth rates suddenly became much higher.

This matters because exam questions often ask you to separate different drivers of growth. A country can grow because birth rates are high, because death rates are low, because immigration is positive, or because several of these happen at the same time. If you can identify which factor is doing the main work, you can explain population change much more accurately.

> **Key Term: human population dynamics**
> The study of how and why human population size changes over time through births, deaths, fertility, and migration.

### The basic drivers: births, deaths, and migration

The core APES idea is the demographic balancing equation. A population increases when additions are greater than losses.

> **Key Term: population change**
> The difference between additions to a population and losses from it: births and immigration add people, while deaths and emigration remove people.

A useful way to write it is:

Population change = births − deaths + net migration

And:

Net migration = immigration − emigration

If immigration is larger than emigration, net migration is positive and population size rises. If emigration is larger, net migration is negative and population size falls.

> **Key Term: net migration**
> The balance between people entering and leaving a population, calculated as immigration minus emigration.

This seems straightforward, but students often confuse **natural increase** with total population growth. Natural increase only compares births and deaths. Total population growth includes migration as well.

That distinction matters because AP questions may give you a country with a low birth rate and low death rate but continued growth because migration is positive. A city might also shrink even with more births than deaths if emigration is very high.

The textbook pattern is clear: migration can matter a lot regionally, but globally the largest additions to human population come from births. That is why fertility and mortality trends are so central.

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

“Crude” means these rates are calculated for the whole population, without adjusting for age or sex structure. That is why two countries with similar life conditions can still have different crude rates if one has a much younger population.

### Why modern growth happened: falling death rates first

The single most important driver of rapid human growth in modern history is the decline in death rates. The textbook gives a direct example: in the United States, the overall death rate fell from **1.7% in 1900** to **0.8% in 2018**. Children in 1900 were also far more likely to die than children in 2000.

That tells you something crucial. Population growth can speed up even if birth rates do not rise, as long as more people survive.

Why did death rates fall?

- Better food production
- Better food distribution
- Improved nutrition
- Cleaner water supplies
- Better sanitation and hygiene
- Public health programs
- Vaccines
- Antibiotics and other medical advances

These factors reduce mortality in two linked ways. First, they lower deaths from infectious disease and malnutrition. Second, they greatly reduce **infant mortality**, which has a strong effect on overall population growth because more children survive to reproductive age.

A common error is to say the Industrial Revolution increased population because people “had more babies.” That is incomplete. The stronger explanation is that industrial and medical changes improved survival. When births remain high but deaths drop, population grows rapidly.

This is exactly what the demographic transition model shows in its early expanding stage.

> **Key Term: infant mortality rate**
> The number of deaths of children under age 1 per 1,000 live births.
>
> **Test Tip:** If an FRQ asks for the main driver of rapid human growth, lead with falling death rates due to sanitation, water, food supply, and medicine. Do not make fertility your first explanation unless the question is specifically about birth rates.

The textbook also highlights four major growth surges in human history:

- use of tools
- discovery of fire
- first agricultural revolution
- industrial and medical revolutions

The first two improved survival and resource use slowly. Agriculture allowed settled communities and much larger food supplies. The industrial and medical revolutions caused the fastest change because they combined better food systems with modern sanitation and health care.

### Measuring growth: worked calculations you must be able to do

APES often tests human growth using simple math. The most common formulas involve CBR, CDR, migration, and doubling time.

> **Key Term: rate of natural increase (RNI)**
> Population growth from births and deaths only, excluding migration.

Formula when rates are per 1,000:

RNI (%) = (CBR − CDR) / 10

Example 1:

CBR = 20  
CDR = 8

Step 1: subtract death rate from birth rate  
20 − 8 = 12

Step 2: convert from per 1,000 to percent by dividing by 10  
12 / 10 = **1.2%**

So the rate of natural increase is **1.2%**.

Now include migration.

> **Key Term: population growth rate**
> The percent change in population size over time, including births, deaths, and migration.

Formula:

Population growth rate (%) = (CBR − CDR + net migration rate) / 10

Example 2:

CBR = 18  
CDR = 7  
Net migration rate = 5

Step 1: births minus deaths  
18 − 7 = 11

Step 2: add net migration  
11 + 5 = 16

Step 3: divide by 10  
16 / 10 = **1.6%**

So the total population growth rate is **1.6%**.

Now try a shrinking population.

Example 3:

CBR = 9  
CDR = 11  
Net migration rate = −2

Step 1: births minus deaths  
9 − 11 = −2

Step 2: add net migration  
−2 + (−2) = −4

Step 3: divide by 10  
−4 / 10 = **−0.4%**

This population is shrinking.

Students also need the Rule of 70.

> **Key Term: doubling time**
> The approximate number of years it takes a population to double at a constant growth rate.

Formula:

Doubling time ≈ 70 / growth rate (%)

Example 4:

Growth rate = 2%

70 / 2 = **35 years**

Example 5:

Growth rate = 1%

70 / 1 = **70 years**

This shows a cause-and-effect link examiners like: higher growth rate means shorter doubling time.

> **Test Tip:** Watch the units. CBR and CDR are usually given per 1,000 people, not as percentages. After subtracting, divide by 10 to convert to percent. Forgetting that step is one of the most common APES errors.

Common student errors in this topic:

- forgetting to divide by 10
- including migration in RNI
- mixing up immigration and emigration
- using doubling time for a zero or negative growth rate
- confusing CBR with TFR

### Fertility as a growth driver: why birth rates stay high or fall

Fertility shapes future population growth because births are the main source of long-term additions to a population.

> **Key Term: total fertility rate (TFR)**
> The average number of children a woman is expected to have over her lifetime under current age-specific fertility rates.

TFR is different from CBR. CBR is an annual birth rate for the whole population. TFR is a lifetime average per woman. A country can have a low CBR because its population is older, even if family size per woman is moderate. That is why AP questions may use both measures for different purposes.

The textbook examples show large variation:

- Niger: **7.63**
- India: **2.43**
- Mexico: **2.24**
- United States: **1.87**
- Japan: **1.41**

This matters because fertility above replacement tends to increase population over time, while fertility below replacement tends toward aging and decline if migration does not offset it.

> **Key Term: replacement-level fertility**
> The average number of children per woman needed to replace parents in the next generation, usually about 2.1 in developed countries.

Why is replacement-level fertility above 2 instead of exactly 2? Because not all children survive to reproductive age, and not all women have children.

In some developing countries, replacement-level fertility can be much higher because mortality is higher. That is an easy place to lose marks if you assume 2.1 is universal.

What keeps fertility high?

- children needed for labor
- cultural or religious support for large families
- limited access to contraception
- low education levels for women
- lack of retirement systems, so children support parents later

What lowers fertility?

- women’s education
- more women in the workforce
- urbanization
- higher cost of raising children
- delayed marriage and childbearing
- lower infant mortality
- wider acceptance of contraception
- better access to family-planning services

Notice the logic here. Lower infant mortality can reduce fertility because parents are more confident that children will survive, so they may choose to have fewer. Urbanization also lowers fertility because children are less economically useful in cities than on farms, while housing, schooling, and food are often more expensive.

### Demographic transition: how development changes growth drivers

The demographic transition model ties together mortality and fertility changes over time.

> **Key Term: demographic transition model**
> A model showing how populations shift from high birth and death rates to low birth and death rates as societies develop.

Stage by stage:

- **Stage 1:** high birth rate, high death rate, little growth
- **Stage 2:** birth rate stays high, death rate drops, rapid growth
- **Stage 3:** birth rate falls, death rate stays low, growth slows
- **Stage 4:** low birth rate, low death rate, stable or slow growth
- Stage 5 is sometimes added: birth rate below replacement, aging or declining population

The fastest growth usually occurs in **Stage 2**. This is one of the most tested ideas in the whole population unit.

Why Stage 2? Because the death rate drops first. Sanitation improves, water is safer, food is more reliable, and medical care gets better. But cultural habits, limited access to family planning, and lower education for women keep birth rates high for a while. So more babies are born, and far more survive.

That lag between falling mortality and falling fertility creates a wide gap between birth rate and death rate. That gap is the engine of rapid population growth.

In Stage 3, the same development process begins to reduce birth rates too. Urbanization, women’s education, and changing economic conditions make smaller families more common. Growth slows because births and deaths move closer together.

A subtle but important point: even when TFR falls to replacement level, a population may keep growing for some time if it has many young people entering reproductive years. That effect is called population momentum.

> **Key Term: population momentum**
> Continued population growth caused by a large proportion of young people reaching reproductive age, even after fertility falls.

This is why the world population can still grow even though many countries now have sub-replacement fertility. Large younger age groups and above-replacement fertility in some countries still push the total upward.

### Linking growth drivers to environmental context

Human growth is not just a math topic. In APES, it connects directly to resource demand and environmental stress.

As population rises, demand usually rises for:

- food
- freshwater
- housing
- energy
- land

That does not mean every environmental problem is caused by population alone. Consumption per person and technology matter too. But more people increase pressure on systems that already have limits.

Growth can eventually be constrained by density-dependent and density-independent factors, just as with other populations. For humans, density-dependent pressures may include disease spread, reduced clean water access, and food competition. Density-independent pressures include droughts, storms, fires, and other natural disasters.

This does not mean the AP exam expects a dramatic collapse story every time. Usually, it expects you to explain how growth is driven first, then how it may later be limited by environmental conditions and resource availability.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Human population change depends on births, deaths, immigration, and emigration
- Net migration equals immigration minus emigration
- The main modern driver of rapid human population growth has been falling death rates
- Lower death rates result from better sanitation, cleaner water, improved nutrition, and medical advances
- Crude birth rate and crude death rate are measured per 1,000 people per year
- Rate of natural increase uses births and deaths only and excludes migration
- Population growth rate includes both natural increase and net migration
- Total fertility rate is lifetime births per woman, not annual births in the whole population
- Replacement-level fertility is about 2.1 in many developed countries, but can be higher where mortality is higher
- Doubling time can be estimated with the Rule of 70
- The fastest growth usually occurs in Stage 2 of the demographic transition model
- Women’s education, urbanization, and access to family planning tend to lower fertility

## Key Terms and Concepts

- human population dynamics
- population change
- net migration
- crude birth rate (CBR)
- crude death rate (CDR)
- infant mortality rate
- rate of natural increase (RNI)
- population growth rate
- doubling time
- total fertility rate (TFR)
- replacement-level fertility
- demographic transition model
- population momentum


Human Population Dynamics - Drivers of human growth