AP Biology, AP exams, 10–15%

Unit 8: Ecology

Unit 8 scales up from individuals to populations, communities, and ecosystems. It covers behavioral and physiological responses to the environment, energy flow and trophic efficiency, population growth models, species interactions, and how disruptions propagate through a system. Many questions ask you to predict a cascading effect from a single change.

Position: 8 of 8 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 Biology. Treat them as a study checklist rather than an exhaustive list of what a single exam form can sample.

Responses to the environment: Behavioral and physiological responses, communication between organisms, and their effects on survival.
Energy flow through ecosystems: Trophic levels, the roughly tenfold energy loss between them, and productivity as a limit on biomass.
Population ecology: Exponential and logistic growth, carrying capacity, and how density-dependent factors change growth rate.
Community ecology: Competition, predation, symbiosis, and the effect of species interactions on community composition.
Biodiversity: Species richness and evenness, keystone species, and why diversity is linked to ecosystem resilience.
Disruptions to ecosystems: Invasive species, human activity, and natural disturbance, and how each propagates through a food web.

Study tips

How to practice Ecology.

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

Practice one cascade question per session: remove a species and write three ordered consequences with reasons.
Keep exponential and logistic growth on one page with their equations and the shape each produces.
Connect this unit back to Unit 7; selection pressure is the mechanism behind most ecological explanations the exam rewards.

Where this fits

Unit 8: Ecology is 8 of 8 units in the AP Biology 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 Biology guide Official exam information

Sample Ecology 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
Approximately what percentage of the energy stored at one trophic level is transferred to the next trophic level in a typical ecosystem?
1. AAbout 1 percent
2. BAbout 50 percent
3. CAbout 90 percent
4. DAbout 10 percent
Show answer
Answer: D. About 10 percent
Most energy at each level is lost as heat through cellular respiration or is never consumed, so only about a tenth is incorporated into the next level. Answering 90 percent inverts the relationship: about 90 percent is the fraction lost, not the fraction transferred. This inefficiency is why food chains rarely support more than four or five levels.
Key insight: Energy transfer is roughly tenfold lossy at every step, which limits the length of food chains.
Question 2Foundational
What term describes the maximum population size that a particular environment can support indefinitely given its available resources?
1. ACarrying capacity
2. BBiotic potential
3. CPopulation density
4. DExponential growth
Show answer
Answer: A. Carrying capacity
Carrying capacity is the population size at which resource supply and population demand balance, and it is the upper plateau of a logistic growth curve. Biotic potential is the tempting distractor, but it is the maximum reproductive rate under ideal, unlimited conditions. Population density is simply individuals per unit area and sets no limit by itself.
Key insight: Growth slows as a population nears carrying capacity because density-dependent factors intensify.
Question 3Intermediate
What term describes a symbiotic relationship in which one species benefits while the other is neither helped nor harmed?
1. AMutualism
2. BCommensalism
3. CParasitism
4. DCompetition
Show answer
Answer: B. Commensalism
Commensalism is a plus-zero relationship, such as barnacles riding on a whale without measurably affecting it. Mutualism is the tempting distractor, but it is plus-plus, with both partners gaining. Parasitism is plus-minus, and competition is minus-minus.
Key insight: Classify species interactions by writing the effect on each partner as plus, minus, or zero.
Question 4Intermediate
True or false: a keystone species is defined as the species that makes up the greatest biomass in its community.
1. ATrue
2. BFalse
Show answer
Answer: B. False
A keystone species has an effect on community structure that is disproportionately large compared with its abundance, so it is often rare rather than dominant. The species with the greatest biomass is called a dominant species, which is a separate idea. Sea otters and sea stars are classic keystone examples precisely because their small biomass belies their influence.
Key insight: Keystone is about disproportionate influence; dominant is about sheer abundance.
Question 5Advanced
Sea otters eat sea urchins, and sea urchins graze on kelp. If sea otters are removed from a coastal ecosystem, what is the most likely short-term effect on the kelp forest?
1. AKelp increases because urchins decline without otters to concentrate them
2. BBoth urchins and kelp decline because the food web loses a producer
3. CKelp declines because the urchin population grows and overgrazes it
4. DNeither population changes because otters are not producers
Show answer
Answer: C. Kelp declines because the urchin population grows and overgrazes it
Removing the predator releases urchins from top-down control, and the expanding urchin population grazes the kelp down, a trophic cascade that can leave barren rock. The tempting answer is that kelp increases, which would require urchins to decline, but nothing in the food web now limits them. Otters are consumers, so their removal changes the community even though they are not producers.
Key insight: Removing a top predator changes producers two links away, which is the signature of a trophic cascade.

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