As a rule of thumb, how much of the energy at one trophic level is available to the next level up?
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This is a free APES study guide plus a 15-question practice test for AP Environmental Science. Read the unit-by-unit guide below, then test yourself on ecosystems, biodiversity, populations, Earth systems, land and water use, energy, pollution and global change. The questions mix multiple choice, select all that apply and fill in the blank. Answer all 15, then check your score and read the explanation for every question.
All 15 questions at a glance
Prefer to read before you play? Every question in the sampler is listed below. Answers and explanations stay collapsed until you open them.
1.As a rule of thumb, how much of the energy at one trophic level is available to the next level up?
- A. About 1 percent
- B. About 10 percent
- C. About 50 percent
- D. About 90 percent
Answer: B. About 10 percent
Organisms use most of their energy for movement, growth and heat, so only about 10 percent is stored in tissue that the next level can eat. That is why food chains are short and why there are far fewer top predators than producers. If producers store 10,000 units, primary consumers get about 1,000 and secondary consumers about 100.
2.Which step of the nitrogen cycle turns nitrogen gas (N2) from the air into ammonia?
- A. Nitrogen fixation
- B. Nitrification
- C. Denitrification
- D. Ammonification
Answer: A. Nitrogen fixation
Nitrogen fixation, done mostly by bacteria in soil, water and root nodules (and in small amounts by lightning), converts N2 into ammonia. Nitrification then turns ammonium into nitrate. Denitrification returns nitrogen to the air as gas, and ammonification releases ammonium from decaying matter.
3.Which of these are major threats to biodiversity? Select all that apply.
Select all that apply.
- A. Habitat fragmentation
- B. Introduced invasive species
- C. Overharvesting of wild species
- D. Wildlife corridors between reserves
- E. Protected nature reserves
Answer: A. Habitat fragmentation, B. Introduced invasive species, C. Overharvesting of wild species
Habitat loss and fragmentation, invasive species, overharvesting, pollution and climate change drive most biodiversity loss. Wildlife corridors and reserves are conservation tools that work against those threats. Corridors reconnect fragmented habitat so animals can move and breed across a larger area.
4.A species has many small offspring, gives little parental care and has a short life span. Which term fits it best?
- A. K-selected species
- B. Keystone species
- C. r-selected species
- D. Indicator species
Answer: C. r-selected species
r-selected species put their energy into many offspring and tend to grow fast, colonize disturbed areas and show a Type III survivorship curve, as insects and many fish do. K-selected species have few offspring, long lives and lots of parental care, and their populations stay near carrying capacity.
5.Complete the sentence.
Using the rule of 70, a population growing at 2 percent per year doubles in about _____ years, and growth that levels off at the _____ is called logistic growth.
Options for blank 1: 14, 35, 70, 140
Options for blank 2: replacement level, carrying capacity, biotic potential, doubling time
Answer: 35, carrying capacity
The rule of 70 divides 70 by the percent growth rate: 70 / 2 = 35 years. Logistic growth starts out exponential, then slows as resources run short and levels off near the carrying capacity, the largest population the environment can support over time.
6.Why is the leeward side of a tall mountain range often much drier than the windward side?
- A. Cold air on the leeward side sinks and blocks clouds from forming
- B. Winds blow only from the desert toward the leeward side
- C. Mountains absorb water vapor from the air like a sponge
- D. Rising air cools and drops its moisture on the windward side, then descends dry
Answer: D. Rising air cools and drops its moisture on the windward side, then descends dry
Moist air forced up a mountain cools, condenses and falls as rain or snow on the windward side. The air that crosses the top has lost most of its moisture. It warms as it descends, so it can hold water vapor instead of releasing it. This creates a rain shadow, which is why deserts often sit behind mountain ranges.
7.Which soil horizon is the mineral topsoil where humus mixes with sand, silt and clay?
- A. O horizon
- B. A horizon
- C. B horizon
- D. C horizon
Answer: B. A horizon
The A horizon is the topsoil: dark, rich in organic matter mixed with minerals, and where most plant roots and soil organisms live. The O horizon above it is mostly organic litter. The B horizon is subsoil where leached minerals collect, and the C horizon is weathered parent material.
8.Which practices help reduce soil erosion on farmland? Select all that apply.
Select all that apply.
- A. Contour plowing
- B. Overgrazing pastures
- C. No-till farming
- D. Removing windbreaks
- E. Terracing steep slopes
Answer: A. Contour plowing, C. No-till farming, E. Terracing steep slopes
Contour plowing and terracing slow runoff on slopes, and no-till farming leaves roots and residue in place to hold soil. Overgrazing strips the plant cover that anchors soil, and removing windbreaks lets wind carry topsoil away.
9.Which energy source releases essentially no carbon dioxide while the power plant operates but leaves long-lived radioactive waste?
- A. Natural gas
- B. Coal
- C. Nuclear fission
- D. Biomass
Answer: C. Nuclear fission
Nuclear fission produces heat without burning fuel, so operation releases essentially no CO2. The tradeoffs are mining and processing uranium, the risk of accidents and spent fuel that stays hazardous for thousands of years. Coal, natural gas and biomass all release CO2 when burned.
10.Complete the sentence.
Per unit of energy, _____ releases the most carbon dioxide of the common fossil fuels, while _____ is mostly methane.
Options for blank 1: natural gas, crude oil, coal, uranium
Options for blank 2: coal, natural gas, uranium, wood
Answer: coal, natural gas
Coal emits the most CO2 per unit of energy, oil sits in the middle and natural gas emits the least, which is why it is often called a bridge fuel. Natural gas is mostly methane, so leaks matter: methane traps far more heat per molecule than CO2 over a short span.
11.Which of these is a secondary air pollutant?
- A. Carbon monoxide
- B. Sulfur dioxide from a smokestack
- C. Lead particles from old paint
- D. Ground-level ozone
Answer: D. Ground-level ozone
A secondary pollutant forms in the air from reactions between other pollutants. Ground-level ozone forms when nitrogen oxides and volatile organic compounds react in sunlight and is a main part of photochemical smog. Carbon monoxide, sulfur dioxide and lead are primary pollutants, emitted directly from a source.
12.Fertilizer runoff adds extra nitrogen and phosphorus to a lake. Which sequence describes what happens next?
- A. Algae bloom, algae die and decompose, dissolved oxygen falls
- B. Dissolved oxygen rises, fish populations grow, water clears
- C. Water turns acidic, then plants take up the excess nutrients
- D. Bacteria stop decomposing, so dead matter piles up with no oxygen change
Answer: A. Algae bloom, algae die and decompose, dissolved oxygen falls
Extra nutrients cause eutrophication. Algae grow into a bloom that blocks light, then die, and decomposers use up dissolved oxygen as they break the algae down. Low oxygen can cause fish kills and dead zones. Fewer nutrient inputs and buffer strips along shorelines reduce the risk.
13.What does biomagnification describe?
- A. A toxin building up in one organism over its lifetime
- B. A toxin breaking down faster at higher trophic levels
- C. A toxin increasing in concentration at each higher trophic level
- D. A toxin spreading evenly through all the water in a lake
Answer: C. A toxin increasing in concentration at each higher trophic level
Biomagnification happens when a persistent, fat-soluble substance such as DDT or mercury is passed up the food chain and concentrates in the tissues of each higher level, so top predators carry the most. Bioaccumulation is the related build-up inside a single organism over time.
14.Which of these are greenhouse gases? Select all that apply.
Select all that apply.
- A. Carbon dioxide
- B. Nitrogen gas (N2)
- C. Methane
- D. Oxygen gas (O2)
- E. Nitrous oxide
Answer: A. Carbon dioxide, C. Methane, E. Nitrous oxide
Carbon dioxide, methane and nitrous oxide absorb outgoing infrared radiation and warm the lower atmosphere. Nitrogen and oxygen make up about 99 percent of dry air, but they are not greenhouse gases because they do not absorb infrared energy in that way.
15.Complete the sentence.
As the ocean absorbs more carbon dioxide, its pH _____, which makes it harder for corals and shellfish to build _____.
Options for blank 1: rises, falls, stays the same, doubles
Options for blank 2: calcium carbonate shells and skeletons, chlorophyll, nitrate, methane
Answer: falls, calcium carbonate shells and skeletons
Dissolved CO2 forms carbonic acid, which lowers ocean pH. This ocean acidification reduces the carbonate ions that corals, oysters and some plankton need to build calcium carbonate shells and skeletons. Warming water adds to the stress through coral bleaching.
APES study guide: the nine units and their weights
AP Environmental Science has nine units, and the College Board gives each one a range for its share of the multiple-choice section. Unit 1 (ecosystems) and Unit 2 (biodiversity) are 6 to 8 percent each. Units 3 to 6 are 10 to 15 percent each: populations, Earth systems and resources, land and water use, and energy resources and consumption. Units 7 and 8, atmospheric pollution and aquatic and terrestrial pollution, are 7 to 10 percent each. Unit 9, global change, is the heaviest at 15 to 20 percent. Check the current Course and Exam Description for the exact ranges.
Here is what each unit asks you to know. Unit 1: energy flow and the 10 percent rule, plus the carbon, nitrogen, phosphorus and water cycles. Unit 2: genetic, species and ecosystem diversity, succession, keystone and invasive species. Unit 3: r-selected and K-selected species, survivorship curves, exponential and logistic growth, the rule of 70, age structure and the demographic transition. Unit 4: plate tectonics, soil horizons and texture, atmosphere layers, global wind patterns, El Niño and rain shadows. Unit 5: agriculture, irrigation, pest control, fishing, mining, forestry and urban land use. Unit 6: fossil fuels, nuclear power, renewables and efficiency. Unit 7: primary and secondary air pollutants, smog, thermal inversion, acid deposition and indoor air. Unit 8: water pollution, eutrophication, biomagnification, waste and toxicology. Unit 9: ozone depletion, the greenhouse effect, climate change, ocean acidification and biodiversity loss.
Unit 9 is the largest and borrows from every other unit, so study it after the cycles and the energy units make sense. For each unit, learn the process, one real example and one human action that changes it. The free-response section rewards exactly that habit.
What the exam asks and how to use this practice test
The exam is fully digital and taken in the Bluebook app. Section I is 80 multiple-choice questions in 1 hour 30 minutes and counts for 60 percent of your score, which works out to about 67 seconds per question. Many of the questions come in sets built around a data table, graph, model or map, or a short text. Section II is 3 free-response questions in 1 hour 10 minutes and counts for 40 percent. They ask you to design an investigation, analyze and interpret quantitative data, and analyze an environmental problem using calculations. Calculators are allowed. Check the College Board's AP Central page for the current format and exam date, since details can change.
This practice test has 15 questions covering all nine units, with extra weight on the big ones. It adds select all that apply and fill in the blank, which the real multiple choice does not use, because recalling a term is harder than spotting it in a list. Take it in one sitting, read every explanation, and let a wrong answer point you at a unit. For the free response, drill the calculations that come up again and again: unit conversions, percent change, the rule of 70 and energy transfer between trophic levels. Write the formula, plug in numbers with units and label the answer.
How StudyPDF builds a study guide and practice tests from your notes
Fifteen questions can show you which units are shaky. They cannot get you ready on their own, and a generic guide rarely matches what your teacher spent three weeks on.
StudyPDF works from your material instead. You upload your textbook chapter, lab write-ups or the review packet your teacher posted, press New material, and StudyPDF builds a study guide and full practice tests from those exact pages. Every question is grounded in your material, and every explanation cites where the answer comes from. You do not have to upload anything to start: you can also just name a topic, like Unit 9 or eutrophication, and Bo sets it up with its key concepts, so you can build materials on it. Starting is free.
Written by the StudyPDF team. Last updated August 19, 2026.
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What is the best way to study for APES?
Learn the nine units as cause and effect chains, such as nutrients to algae to low oxygen to fish kills. Practice reading graphs and doing short calculations, because the free response leans on both. Save Unit 9 for last since it pulls from the others. Revisit missed topics a few days later, or upload your own notes to StudyPDF to get questions on exactly what your class covered.
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