AP Environmental Science, AP exams, 10–15%

Unit 3: Populations

Unit 3 covers population dynamics for organisms generally and humans specifically. Life-history strategies, survivorship curves, carrying capacity, and growth models set up the quantitative work, and the human section adds age structure diagrams, total fertility rate, and the demographic transition. Growth-rate calculations from this unit appear regularly on the exam.

Position: 3 of 9 units
Published weighting: 10–15% in the official framework
Topics covered: 6 topics in this guide

What is covered

Topics inside this unit.

These topics describe the scope students most often review for this part of AP Environmental Science. Treat them as a study checklist rather than an exhaustive list of what a single exam form can sample.

Generalist and specialist species: Niche breadth and why specialists are more vulnerable to environmental change.
K-selected and r-selected species: Reproductive strategies, parental investment, and the survivorship curve each pattern produces.
Carrying capacity and population growth: Exponential versus logistic growth, overshoot, dieback, and the factors that set carrying capacity.
Age structure diagrams: Reading a population pyramid for growth, stability, or decline, and connecting shape to future demand.
Total fertility rate and human population dynamics: Replacement-level fertility, the drivers that lower fertility rate, and their effect on growth.
Demographic transition: The stages of the model and the birth-rate and death-rate pattern that defines each one.

Study tips

How to practice Populations.

Section-specific guidance that follows from how this material is usually tested.

Practice population growth-rate and doubling-time calculations with units written out on every line.
Sketch the three survivorship curves and attach an organism to each so recall is concrete.
Rehearse describing an age structure diagram in one sentence that names the shape and the implication.

Where this fits

Unit 3: Populations is 3 of 9 units in the AP Environmental Science guide, and it carries a published weighting of 10–15% in the official framework. Use the full guide to balance it against everything else you have to review.

Back to the AP Environmental Science guide Official exam information

Sample Populations questions

Practice questions, not yet expert-reviewed

Original practice questions written to the published framework for this section. No expert has reviewed them, and they are not taken from any official exam. Try each one before you open the answer.

Question 1Foundational
Which reproductive strategy is characterized by many small offspring, little or no parental care, early maturity, and a Type III survivorship curve?
1. AK-selected
2. Br-selected
3. CSpecialist
4. DDensity-dependent
Show answer
Answer: B. r-selected
An r-selected species maximizes reproductive rate rather than offspring survival, which suits unstable or newly opened habitats where most young die young. K-selected species take the opposite approach with few offspring, heavy parental investment, and a Type I curve. Many invasive species are r-selected, which is part of why they spread so quickly after introduction.
Key insight: r-selected species bet on quantity; K-selected species bet on the survival of each individual.
Question 2Foundational
What demographic measure reports the average number of children a woman is expected to have in her lifetime?
1. ACrude birth rate
2. BDoubling time
3. CTotal fertility rate
4. DInfant mortality rate
Show answer
Answer: C. Total fertility rate
Total fertility rate is expressed per woman, so it can be compared directly with the replacement level of about 2.1 children per woman. Crude birth rate is the tempting distractor, but it counts births per 1,000 people in the whole population and is therefore affected by age structure. A total fertility rate below replacement eventually shrinks a population once population momentum fades.
Key insight: Replacement level is slightly above 2 because not every child survives to reproduce.
Question 3Intermediate
A country has a crude birth rate of 22 per 1,000 people and a crude death rate of 8 per 1,000 people, with no net migration. What is its annual population growth rate as a percentage?
1. A0.14 percent
2. B1.4 percent
3. C3.0 percent
4. D14 percent
Show answer
Answer: B. 1.4 percent
Subtracting the death rate from the birth rate gives 14 per 1,000, which is 14 divided by 1,000, or 0.014, and multiplying by 100 gives 1.4 percent. Answering 14 percent reports the rate per 1,000 as if it were already a percentage. Writing the units on every line is what prevents that slip.
Key insight: A rate per 1,000 becomes a percentage by dividing by 10, since a percentage is a rate per 100.
Question 4Intermediate
Using the rule of 70, approximately how many years will it take a population growing at 2.0 percent per year to double?
1. A14 years
2. B20 years
3. C35 years
4. D70 years
Show answer
Answer: C. 35 years
The rule of 70 estimates doubling time by dividing 70 by the growth rate expressed as a percentage, so 70 divided by 2.0 gives 35 years. Answering 70 years uses the constant itself and ignores the growth rate. Because the relationship is inverse, doubling the growth rate cuts the doubling time in half.
Key insight: Doubling time is 70 divided by the percent growth rate, so small rate changes shift it dramatically.
Question 5Advanced
In which stage of the demographic transition model does the death rate fall sharply while the birth rate stays high, producing the fastest population growth?
1. AStage 1
2. BStage 2
3. CStage 3
4. DStage 4
Show answer
Answer: B. Stage 2
Improved sanitation, medicine, and food supply push death rates down quickly in stage 2, while cultural and economic reasons keep birth rates high, so the gap between the two curves is widest and growth peaks. Stage 1 is the tempting distractor, but there both rates are high and roughly equal, so the population is nearly stable. Growth slows in stage 3 as birth rates finally fall.
Key insight: Population growth is the gap between the birth and death curves, not the height of either one.

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Unit 3: Populations