AP Environmental Science Populations Guide
Return to the AP Environmental Science exam guide.
Populations in the AP Environmental Science blueprint
College Board assigns Unit 3, Populations, 10–15% of AP Environmental Science multiple-choice questions. This range describes the multiple-choice section, not a percentage of the total exam score.
Populations
- May 2027
- Bluebook
- Clean-room teaching
- Read the evidence surface.
- Trace the environmental mechanism.
- Check units, scale, and claim scope.
What Populations covers
Within Populations, use these routes to connect official topics to evidence, calculations, and environmental decisions.
Unit 3: Populations
3.1 Generalist and Specialist Species
When studying Populations, generalists use broad resources and tolerate varied conditions, whereas specialists occupy a narrower niche.
3.2 K-Selected and r-Selected Species
3.3 Survivorship Curves
3.4 Carrying Capacity
3.5 Population Growth and Resource Availability
3.6 Age Structure Diagrams
3.7 Total Fertility Rate
3.8 Human Population Dynamics
3.9 Demographic Transition
Vocabulary anchors
Generalist and Specialist Species; K-Selected and r-Selected Species; Survivorship Curves; Carrying Capacity; Population Growth and Resource Availability; Age Structure Diagrams; Total Fertility Rate; Human Population Dynamics; Demographic Transition are the official topic anchors used throughout this unit.
Evidence lab: Cohort survival in two coastal species
Cohort survival in two coastal species Each cohort begins with 1,000 offspring. Age interval | Species R alive | Species K alive birth | 1000 | 1000 juvenile | 120 | 910 reproductive | 62 | 730 late life | 8 | 140
Which species shows the stronger Type III survivorship pattern?
Species R — Most mortality occurs before the juvenile stage, followed by higher survival among the remaining individuals.
K retains most individuals through early life, closer to Type I. Their early-life survival differs greatly. Survivorship curves describe the timing of mortality, not immortality.
Reads when mortality is concentrated from a cohort table.
Which reproductive strategy is most consistent with Species R?
Many small offspring with little parental care — High early mortality is often offset by producing many offspring.
That strategy is more consistent with high juvenile survival. Population replacement does not require synchronized adult death. That would not replace the large early losses shown.
Connects survivorship shape to life-history allocation.
What fraction of Species K reaches the reproductive interval?
0.73 — 730 of 1,000 initial individuals remain, so the fraction is 730/1000=0.73.
This is the fraction lost by that stage. The decimal place is shifted by a factor of ten. This omits conversion from percent to fraction.
One denominator choice and fraction conversion.
A contaminant increases juvenile mortality in Species K. Which comparison best detects that effect?
Juvenile survival in exposed and unexposed cohorts matched for starting size and habitat — Matched cohorts isolate exposure while preserving the relevant life stage and denominator.
It measures the wrong species and life stage. One adult cannot estimate cohort juvenile mortality. Pre-exposure mass does not measure the new survival effect.
Requires exposure control, matched cohorts, and stage-specific response.
Why might the same proportional juvenile loss threaten K more than R?
K invests in fewer offspring, so an added early loss can remove a larger share of expected recruits that the strategy does not rapidly replace. — Risk depends on baseline life history, not only the proportional toxic effect.
The table shows many K individuals reaching reproductive age. Life-history type does not eliminate density-dependent limits. High fecundity can buffer recruitment but does not remove toxicity.
Integrates baseline survivorship, reproductive allocation, and population-level consequence.
Evidence lab: Deer population under two harvest policies
Deer population under two harvest policies The habitat's estimated carrying capacity is 1,200 deer. Year | No harvest | Managed harvest 0 | 300 | 300 2 | 570 | 520 4 | 910 | 720 6 | 1120 | 790 8 | 1190 | 805
Which feature of the no-harvest series is most consistent with logistic growth?
Growth slows as the population approaches 1,200. — Successive gains shrink near the stated carrying capacity.
The increments are not constant. It rises well above 300. The series levels rather than accelerating.
Identifies logistic deceleration from one time series.
Why can managed harvest stabilize below carrying capacity?
Harvest removes individuals while density-dependent reproduction adds them, allowing the flows to balance at a lower stock. — A stable population occurs when additions and removals balance.
Removing deer does not eliminate habitat limits. The managed series grows above 300. No such mechanism is given or required.
Explains a stable stock through opposing population flows.
What was the average annual increase in the no-harvest population from year 0 to year 4?
152.5 deer per year — The increase is 910-300=610 deer over four years; 610/4=152.5.
This is the total increase, not the annual average. This divides the final population by four rather than the change. This divides the four-year change by eight years.
Two-step change-over-time calculation with units.
Which evidence would best test whether 1,200 is still the carrying capacity after a severe drought?
Measure forage, water availability, body condition, and population trend during and after the drought. — Carrying capacity depends on limiting resources and should be evaluated with both habitat and population responses.
A drought can change the resource base. Antlers do not measure total stock or habitat limits. Several drought-related mechanisms can reduce population size.
Requires updating a model parameter with resource and demographic evidence.
A proposal sets harvest at the largest historical annual population increase. Why is that risky?
The maximum observed increase occurred far below carrying capacity; growth is slower near the current stock and may fall further after drought. — A fixed harvest based on peak growth ignores density dependence and changing resources.
Managed harvest can be sustainable when updated with evidence. Logistic growth predicts the opposite. The table provides five values for each policy.
Combines logistic growth, state-dependent yield, and environmental uncertainty.
Evidence lab: Age structure in two regions
Age structure in two regions Percentages refer to the total population in each region. Age group | Region X (%) | Region Y (%) 0-14 | 34 | 16 15-44 | 43 | 39 45-64 | 15 | 27 65+ | 8 | 18
Continue in the A+ teaching layer for the remaining evidence labs, figure, and final audit.
How the AP Environmental Science assesses Populations
| Item | Weight / count | What it means |
|---|---|---|
| Official topic denominator | 9 | |
| Multiple choice | 80 questions in 90 minutes | |
| Free response | 3 questions in 70 minutes | |
| Calculation credit | ||
| Administration | May 2027 |
Net population change
Read each question as it appears on the exam paper, choose your answer, then tap it to see the feedback.
Practice 1: Calculate net population growth
A population of 12,000 has 840 births, 510 deaths, 190 immigrants, and 280 emigrants in one year.
What is the population's percent growth during the year?
Select the best answer.
A. (840 − 510 − 190 − 280) ÷ 12,000 × 100 = −1.2%
❌ Not quite 😭 Immigration is incorrectly treated as a loss. Individuals entering the population must be added to net change.
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B. (840 + 190 − 510 − 280) ÷ 12,000 × 100 = 2.0%
✅ Correct! 😊 Births and immigration are additions; deaths and emigration are removals. Dividing the net gain by the starting population gives the requested rate.
C. (840 + 510 + 190 + 280) ÷ 12,000 × 100 ≈ 15.2%
❌ Not quite 😭 This adds deaths and emigration instead of subtracting them. It measures total recorded movement, not net population growth.
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D. (840 − 510) ÷ 12,000 × 100 ≈ 2.8%
❌ Not quite 😭 This natural-increase calculation omits both immigration and emigration. The question supplies migration flows because they affect total change.
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Full explanation
- Step 1Add gains and subtract losses: 840 + 190 − 510 − 280 = 240 individuals.
- Step 2Divide net change by the starting population: 240 ÷ 12,000 = 0.020.
- Step 3Multiply by 100 to report 2.0% growth.
Practice 2: Evaluate carrying capacity after drought
A deer habitat has an estimated carrying capacity of 1,200. After a severe drought, managers consider setting harvest equal to the largest historical annual population increase.
Explain why carrying capacity may change, identify evidence needed to reassess it, and evaluate the proposed fixed-harvest rule using logistic growth.
Show your work.
A. Keep 1,200 permanently because carrying capacity is a fixed species trait.
❌ Not quite 😭 Carrying capacity depends on changing habitat resources and conditions, not just species identity. Drought can lower water and forage availability.
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B. Measure resources, body condition, and population trend, then adjust harvest because growth slows near carrying capacity and may decline after drought.
✅ Correct! 😊 This updates both habitat and demographic evidence and recognizes that peak historical growth need not match current replacement.
C. Use the largest historical increase as a fixed harvest because logistic growth is fastest at carrying capacity.
❌ Not quite 😭 Logistic growth slows near carrying capacity. A maximum observed increase from a lower stock can overstate replacement at current density.
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D. Count antlers on one day and use that count as the new carrying capacity.
❌ Not quite 😭 Antler counts do not measure the full population or the resource base. A one-day observation also misses survival and recovery trends.
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Full explanation
- Step 1Carrying capacity depends on limiting resources, habitat, disease, climate, and management, so drought can change it.
- Step 2Measure forage, water, body condition, survival, recruitment, and population trend during and after drought.
- Step 3Logistic growth decelerates near carrying capacity; maximum historical increase may have occurred at a much lower density.
- Step 4Set a conservative, adjustable harvest tied to updated population and habitat evidence rather than one fixed historical value.
Key terms for Populations
- Generalist and Specialist Species
- K-Selected and r-Selected Species
- Survivorship Curves
- Carrying Capacity
- Population Growth and Resource Availability
- Age Structure Diagrams
- Total Fertility Rate
- Human Population Dynamics
- Demographic Transition
Populations FAQ
How do exponential and logistic population models differ?
Exponential growth assumes a roughly constant per-capita rate without a limiting ceiling. Logistic growth slows as density-dependent limits intensify near carrying capacity, producing an S-shaped trajectory.
Why can carrying capacity change through time?
Carrying capacity depends on resources, habitat, competition, disease, climate, and human management. Because those conditions change, the value is a model-dependent estimate rather than a permanent population number.
What does a survivorship curve actually describe?
It displays the fraction of a cohort surviving across age. Type I, II, and III patterns summarize mortality timing; they do not by themselves state population size or future growth.
How should age-structure diagrams be interpreted?
Compare pre-reproductive, reproductive, and post-reproductive cohorts while considering migration and changing fertility. A broad base suggests momentum, but it does not guarantee a fixed growth rate.
Where does the rule of seventy fit in human demography?
Divide seventy by a positive annual percentage growth rate to estimate doubling time. State that it is an approximation and avoid using it when growth is zero, negative, or changing rapidly.
Continue through the APES system
How to study Populations
Sketch exponential and logistic curves on the same axes, then label where density-independent shocks, density-dependent limits, and carrying capacity would alter each prediction. Explain what the curve cannot reveal about age structure or migration.
Calculate net population change with births, deaths, immigration, and emigration before using a shortcut. Next, inspect whether the rate is a decimal or percentage and whether the question asks for a change, final population, or doubling time.
Read age-structure diagrams from cohorts to momentum rather than from shape to slogan. Compare the reproductive pipeline, then qualify the prediction with fertility, mortality, migration, policy, and the time required for cohorts to age.
