AP Biology Unit 8 review: Ecology
Unit 8 connects organism behavior, energy transfer, population growth and community change. It is 10–15% of multiple choice. Expect to read food webs, calculate growth or productivity and use experimental evidence to predict the effects of a disruption.
Unit 8 on the May 2027 exam
| 2027 exam | |
|---|---|
| Weight | 10–15% of the multiple-choice section |
| Exam date | Monday, May 3, 2027, afternoon session |
| Multiple choice | 60 questions in 90 minutes, worth 50% of the exam score |
| Free response | 6 questions in 90 minutes, worth 50%: two long questions (9 points each) and four short questions (4 points each). |
| Calculator | Both sections allow a four-function calculator with square root or a nongraphing scientific calculator. Bluebook includes Desmos scientific. Handheld graphing calculators are not allowed in 2027. |
| Class time | About 19–21 class periods in College Board's pacing guide |
| 2026 results | 15% of students scored a 5 on the whole exam, and about 71% scored 3 or higher |
Sources: College Board's course and exam description, exam page, 2027 exam dates and score distributions.
What Unit 8 covers
College Board's course description splits the unit into 7 topics. The second column is what you should be able to do for each one.
| Topic | What you should be able to do |
|---|---|
| 8.1 Responses to the Environment | Explain how behavioral and physiological responses affect survival and reproduction. |
| 8.2 Energy Flow Through Ecosystems | Track energy and matter through trophic levels and calculate productivity or transfer efficiency. |
| 8.3 Population Ecology | Use birth, death and population-size data to model population change. |
| 8.4 Effect of Density on Populations | Compare exponential and logistic growth and explain density-dependent regulation. |
| 8.5 Community Ecology | Use competition, predation and other interactions to predict changes in community structure. |
| 8.6 Biodiversity | Explain how genetic and species diversity affect responses to environmental change. |
| 8.7 Disruptions in Ecosystems | Use evidence to predict effects of invasive species, habitat change and other disturbances. |
Choose the ecological quantity that answers the question
Keep energy, biomass and population counts separate. State the time interval and area when the data require them.
| Quantity | Relationship | Interpretation |
|---|---|---|
| Net primary productivity | NPP = GPP − producer respiration | Energy stored in new producer biomass after respiration. |
| Transfer efficiency | Next-level production / previous-level production × 100% | Use measured production; do not automatically substitute 10%. |
| Exponential growth | dN/dt = rmaxN | Growth rises with population size when resources do not limit it. |
| Logistic growth | dN/dt = rmaxN(1 − N/K) | Growth slows as population size approaches carrying capacity. |
| Maximum logistic growth | N = K/2 | The absolute growth rate is largest halfway to K. |
| Food-web arrow | Resource → consumer | Shows the direction in which energy and matter are transferred. |
Common mistakes
Each of these loses points on questions you otherwise understand.
- Reversing food-web arrows. Draw an arrow from the organism eaten to its consumer. The CED identifies this error on printed p. 149.
- Placing an organism in a food web from its name alone. Use the feeding data to establish each relationship; some species feed at several trophic levels. CED p. 149.
- Treating energy as recycled like atoms. Matter can cycle; usable energy enters and is eventually dispersed as heat. CED topic 8.2.
- Using exponential growth near carrying capacity. The logistic model includes the factor (1 − N/K), which approaches zero near K. CED topics 8.3–8.4.
6 practice questions with answers
Written by Ashore for this page, in the style of the multiple-choice section. Try each one before you open the answer.
Question 1 · 8.2
Producers fix 1,400 kJ m⁻² yr⁻¹ and use 620 kJ m⁻² yr⁻¹ in respiration. What is NPP?
- A620 kJ m⁻² yr⁻¹
- B780 kJ m⁻² yr⁻¹
- C1,400 kJ m⁻² yr⁻¹
- D2,020 kJ m⁻² yr⁻¹
Show the answer
B. NPP = GPP − respiration = 1,400 − 620 = 780 kJ m⁻² yr⁻¹. Adding respiration would count energy already used by producers as new biomass.
Question 2 · 8.2
Primary consumers produce 180 kJ of new biomass from a producer level that produces 1,500 kJ over the same area and time. What is the transfer efficiency?
- A8.3%
- B10.0%
- C18.0%
- D12.0%
Show the answer
D. Efficiency = 180/1,500 × 100 = 12%. The measured values determine the answer; 10% is only an approximation in some ecological examples.
Question 3 · 8.4
For a logistic population, rmax = 0.30 yr⁻¹, N = 200 and K = 500. What is the instantaneous growth rate?
- A36 organisms per year
- B60 organisms per year
- C90 organisms per year
- D150 organisms per year
Show the answer
A. rmaxN(1 − N/K) = 0.30(200)(1 − 200/500) = 60(0.60) = 36 organisms per year. This is a rate at the stated population size, not a full-year discrete forecast.
Question 4 · 8.5
A food chain is grass → rabbit → fox. Fox numbers fall sharply. With other factors initially unchanged, which response is most likely?
- ARabbit abundance falls and grass increases
- BBoth rabbit abundance and grass increase
- CRabbit abundance rises and grass decreases
- DBoth rabbit abundance and grass decrease
Show the answer
C. Reduced predation can let rabbits increase. Increased grazing can then reduce grass. This is an initial prediction under the stated conditions; later resource limits can alter the response.
Question 5 · 8.4
Which factor is most directly density dependent in a crowded deer population?
- AA volcanic eruption covering the habitat
- BCompetition for a limited supply of forage
- CA sudden regional drop in temperature
- DA windstorm uprooting trees across the region
Show the answer
B. Competition intensifies as more individuals share limited forage. The occurrence of an eruption, cold snap or windstorm is not caused by deer density, though its consequences may interact with population conditions.
Question 6 · 8.1
A researcher tests whether moisture affects pill-bug location. Which design best isolates moisture?
- ACompare a wet dark chamber with a dry bright chamber
- BUse different species on the wet and dry sides
- CPlace food only on the dry side of a chamber
- DKeep light and temperature equal while varying moisture
Show the answer
D. Varying moisture while holding other conditions equal reduces confounding. Randomizing which side is wet across replicates also helps separate moisture preference from a side preference.
How to study Unit 8
Start with a feeding table and draw the food web, labeling every arrow as a transfer from food to consumer. Remove one species and explain an immediate effect and one indirect effect. State what other information could change your prediction.
Work a productivity problem and both growth models with units. For a behavioral experiment, identify the independent variable, measured response, control conditions and replication. Connect your conclusion to the observed difference instead of merely restating the hypothesis.
Ashore's Biology course has a short lesson for every topic in this unit, each ending with a few questions on what it taught.
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- AP Biology Unit 5: Heredity
- AP Biology Unit 6: Gene Expression and Regulation
- AP Biology Unit 7: Natural Selection
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