AP Environmental Science, AP exams, 6–8%

Unit 1: The Living World: Ecosystems

Unit 1 introduces ecosystem structure and the biogeochemical cycles that move matter through it. Terrestrial and aquatic biomes are defined by temperature and precipitation patterns, and energy flow is governed by primary productivity and trophic efficiency. The cycles introduced here reappear in every pollution and global-change topic later in the course.

Position: 1 of 9 units
Published weighting: 6–8% 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.

Terrestrial and aquatic biomes: How temperature and precipitation define major biomes, and what distinguishes freshwater from marine systems.
The carbon cycle: Reservoirs, fluxes, and the human activities that move carbon from long-term storage into the atmosphere.
The nitrogen and phosphorus cycles: Fixation, nitrification, and denitrification, and why phosphorus has no significant atmospheric reservoir.
The hydrologic cycle: Evaporation, transpiration, precipitation, runoff, and infiltration, and how land use changes each.
Primary productivity: Gross and net primary productivity, the units used to express them, and what limits productivity in different systems.
Trophic levels and food webs: Energy transfer between levels, the approximate 10% rule, and reading a food web for indirect effects.

Study tips

How to practice Ecosystems.

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

Draw each biogeochemical cycle once from memory with reservoirs boxed and fluxes as arrows; the exam tests fluxes more than names.
Practice the 10% rule in both directions, including problems that ask for energy available at a higher trophic level.
Learn biomes by their temperature-and-precipitation signature rather than by a list of species.

Where this fits

Unit 1: The Living World: Ecosystems is 1 of 9 units in the AP Environmental Science guide, and it carries a published weighting of 6–8% 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 Ecosystems 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
In the nitrogen cycle, what is the name of the process that converts atmospheric N2 gas into ammonia that producers can use?
1. ANitrification
2. BNitrogen fixation
3. CDenitrification
4. DAmmonification
Show answer
Answer: B. Nitrogen fixation
Nitrogen fixation is carried out mainly by bacteria, including those in the root nodules of legumes, and also by lightning, and it breaks the very strong triple bond in N2. Nitrification is the tempting distractor, but it converts ammonium into nitrite and then nitrate, a later step in the cycle. Denitrification runs the opposite direction and returns nitrogen gas to the atmosphere.
Key insight: Nitrogen gas is abundant but unusable, so fixation is the gateway step of the entire cycle.
Question 2Foundational
Which major nutrient cycle has no significant gaseous or atmospheric reservoir?
1. AThe carbon cycle
2. BThe nitrogen cycle
3. CThe phosphorus cycle
4. DThe hydrologic cycle
Show answer
Answer: C. The phosphorus cycle
Phosphorus moves among rock, soil, water, and organisms, and it enters the cycle mainly through the slow weathering of phosphate-bearing rock. Because there is no stable gaseous form, the cycle has no atmospheric shortcut and moves slowly, which is why phosphorus so often limits growth. Carbon and nitrogen both cycle rapidly through large atmospheric pools.
Key insight: No atmospheric reservoir means a slow cycle, which is why phosphorus is a common limiting nutrient.
Question 3Intermediate
Producers in an ecosystem store 10,000 kilocalories of energy. Applying the 10 percent rule, how much of that energy is available to secondary consumers?
1. A10 kilocalories
2. B100 kilocalories
3. C1,000 kilocalories
4. D9,000 kilocalories
Show answer
Answer: B. 100 kilocalories
Each transfer between trophic levels passes on about 10 percent, so producers give primary consumers 1,000 kilocalories and primary consumers give secondary consumers 100 kilocalories. Answering 1,000 kilocalories stops after a single transfer and treats primary consumers as the answer. Counting the number of transfers, not the number of levels, keeps the arithmetic straight.
Key insight: Multiply by 0.1 once for every arrow you cross in the food chain, not once for the whole chain.
Question 4Intermediate
An ecosystem has a gross primary productivity of 15,000 kilocalories per square meter per year, and its producers use 5,000 kilocalories per square meter per year for respiration. What is the net primary productivity?
1. A3,000 kilocalories per square meter per year
2. B5,000 kilocalories per square meter per year
3. C10,000 kilocalories per square meter per year
4. D20,000 kilocalories per square meter per year
Show answer
Answer: C. 10,000 kilocalories per square meter per year
Net primary productivity is gross primary productivity minus the energy producers burn in respiration, so 15,000 minus 5,000 gives 10,000 kilocalories per square meter per year. Adding the two values instead of subtracting yields the distractor 20,000. Net primary productivity is the portion actually available to consumers, which is why it, rather than gross productivity, sets the energy budget of the food web.
Key insight: Gross productivity is energy captured; net productivity is what is left after the producers pay their own bills.
Question 5Advanced
Runoff raises the nitrogen supply of a freshwater lake, yet the lake's primary productivity barely changes. What is the most likely explanation?
1. ANitrogen is toxic to algae at every concentration
2. BThe lake already contains too much dissolved oxygen for algae to grow
3. CPhotosynthesis in freshwater does not require nitrogen
4. DA different nutrient, most often phosphorus, is the limiting factor
Show answer
Answer: D. A different nutrient, most often phosphorus, is the limiting factor
Productivity is capped by whichever required resource is scarcest, so adding a nutrient that is already sufficient produces no response. In most freshwater systems phosphorus is the limiting nutrient, while nitrogen more often limits marine systems. The claim that photosynthesis does not need nitrogen is false, since nitrogen is required for chlorophyll and for the enzymes that fix carbon.
Key insight: Only adding the limiting nutrient changes productivity, which is why phosphate loading drives freshwater algal blooms.

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Unit 1: The Living World: Ecosystems