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AP Environmental Science · Unit 3 of 9
Populations
10–15% of the exam4 lessons · 51 min17 terms
What this unit covers
The topics below follow the published Env. Science course framework for Unit 3. This unit is worth 10–15% of the exam, so budget your time against that rather than against how long the unit takes to teach.
Population dynamicsSurvivorshipCarrying capacityDemographics
Lessons in this unit
- Population Dynamics & Growth13 min · 3 objectivesIdentify the four factors that change population size and combine them into a growth rate · Distinguish exponential growth from logistic growth by their limiting conditions · Calculate a population’s growth rate as a percentage from birth, death, and migration data
- Survivorship & Reproductive Strategies12 min · 3 objectivesInterpret Type I, II, and III survivorship curves · Contrast r-selected and K-selected species by their life-history traits · Connect a species’ reproductive strategy to its survivorship pattern
- Carrying Capacity & Limiting Factors13 min · 3 objectivesDefine carrying capacity and describe overshoot and dieback · Distinguish density-dependent from density-independent limiting factors · Explain how resource availability drives populations toward carrying capacity
- Human Demographics13 min · 3 objectivesRead an age-structure diagram to predict a country’s future growth · Trace a population through the four stages of the demographic transition · Use the rule of 70 to calculate a population’s doubling time
Formulas in Unit 3
Population growth rate
growth rate (%) = [(births + immigration) − (deaths + emigration)] / N × 100
N is the starting population. A positive result means growth; a negative result means decline. If migration is negligible, growth rate ≈ (CBR − CDR) / 10 as a percent.
Matching strategy to curve
r-selected → many offspring, little care → Type III · K-selected → few offspring, much care → Type I
r-strategists bet on quantity and rebound fast; K-strategists bet on quality and recover slowly. Type II (constant mortality) sits between the extremes.
Two kinds of limiting factors
density-dependent: competition, predation, disease · density-independent: weather, natural disasters
If the factor gets worse as the population gets more crowded, it is density-dependent. If it strikes regardless of crowding (usually abiotic events), it is density-independent.
Rule of 70 (doubling time)
doubling time (years) ≈ 70 / annual growth rate (%)
Enter the growth rate as a plain percent number (a 2% rate → 70/2 = 35 years). Only works for steady exponential growth, and the growth rate must be positive.
Every term in Unit 3
All 17 terms we publish for Populations, with definitions. Reading them through is the fastest way to find the ones you cannot define — then drill those in cram mode until you can produce them without the prompt.
- Rule of 70
- Doubling time in years ≈ 70 divided by the percentage growth rate. A 2% growth rate doubles a population in about 35 years.
- Population momentum
- A population keeps growing after fertility falls to replacement, because a large cohort of young people has yet to reach reproductive age.
- Generalist vs specialist species
- Generalists tolerate a wide range of conditions and diets and adapt to change; specialists are efficient in a narrow niche and vulnerable to disruption.
- r-selected species
- Many small offspring, little parental care, early maturity, short life. Recover quickly from disturbance and often become invasive.
- K-selected species
- Few offspring with heavy parental investment, late maturity, long life. Slow to recover from population crashes, so more extinction-prone.
- Survivorship curves
- Type I high survival until old age, Type II constant mortality, Type III heavy early mortality with few reaching adulthood.
- Carrying capacity
- The population an environment can sustain indefinitely. Exceeding it causes an overshoot followed by a die-off as resources are depleted.
- Population growth rate calculation
- r = [(births + immigration) − (deaths + emigration)]/N. APES expects all four terms.
- Age structure diagram
- A broad base means rapid growth ahead, a column means stability, a narrow base means decline and an aging population.
- Total fertility rate
- Average children per woman. Replacement level is about 2.1 — above 2 because not every child survives to reproduce.
- Demographic transition
- Stage 1 high birth and death rates; stage 2 death rate falls and population booms; stage 3 birth rate falls; stage 4 both low and population stabilizes.
- Factors that lower birth rates
- Education of women, access to family planning, urbanization, lower infant mortality and greater cost of raising children.
- Infant mortality rate
- Deaths under age one per 1,000 live births. Used as a proxy for healthcare quality and correlates inversely with development.
- Calculating percent growth rate
- Growth rate (%) = [(births − deaths)/N] × 100, or (CBR − CDR)/10 when using crude rates per 1,000.
- Crude birth and death rates
- Births or deaths per 1,000 people per year. Subtracting them and dividing by 10 gives the percentage natural increase.
- Replacement level fertility
- About 2.1 in developed countries and higher where child mortality is greater, since more births are needed for two to survive to adulthood.
- IPAT equation
- Impact = Population × Affluence × Technology. Explains why a small wealthy population can outweigh a large poor one.
What examiners penalize here
- On the AP exam, "growth rate" almost always wants a percentage: compute the net change, divide by the starting population N, and multiply by 100. Show every step — even a wrong final number earns partial credit if the setup is right. Watch your units and don’t drop the ×100.
- Link the concepts in one chain for the exam: r-selected → many offspring, little care → Type III → fast recovery / often invasive. K-selected → few offspring, much care → Type I → slow recovery / often threatened. Being able to run this chain both directions answers most population-strategy questions.
- On graphs, carrying capacity K appears as a horizontal line the population fluctuates around. If the curve shoots above the line then plunges, label it overshoot and dieback. If it rises and flattens smoothly onto the line, that is logistic growth reaching K.
- Read population pyramids by their base: wide base → growing (young population), straight sides → stable, pinched base → shrinking (aging population). Pair this with the demographic transition — Stage 2 shows the widest base and the fastest growth.
Practice Env. Science
Our practice bank is drawn from across the whole course rather than filtered to one unit, which is closer to how the exam asks anyway — it will not tell you which unit a question is testing.
Questions about this unit
How much of the AP Environmental Science exam is Unit 3?
Unit 3, Populations, is worth 10–15% of the Env. Science multiple-choice section according to the published course framework. Across all 9 units that makes it a substantial share — heavier than an even split would give it.
What topics are covered in Env. Science Unit 3?
Populations covers Population dynamics, Survivorship, Carrying capacity and Demographics. We publish 17 terms with definitions for this unit, all of them on this page.
How should I study Env. Science Unit 3?
Read the 4 lessons below first — about 50 minutes — then drill the 17 terms in cram mode until you can produce each definition from memory rather than just recognize it. Recognition is what makes a unit feel finished when it is not. Finish with practice questions and read the explanation for every one you get right by elimination as well as the ones you miss.
All 9 units of AP Environmental Science
Unit names, topics and exam weights follow the published College Board course framework for AP Environmental Science. AP® is a trademark registered by the College Board, which does not endorse this site.