Keeps soil and moisture in place, stores carbon

Restores soil nitrogen, breaks pest cycles

Integrated pest management (IPM)

Cuts pesticide use by combining controls

Overgrazing / monoculture

Degrades soil, lowers biodiversity

How is the AP Environmental Science exam scored?

The exam is scored on the standard AP 1–5 scale. Section I (multiple choice) is worth 60% and Section II (free response) is worth 40%. A score of 3, 4, or 5 generally earns college credit, though each college sets its own policy.

What units are on the AP Environmental Science exam?

There are nine College Board units: (1) The Living World: Ecosystems, (2) Biodiversity, (3) Populations, (4) Earth Systems and Resources, (5) Land and Water Use, (6) Energy Resources and Consumption, (7) Atmospheric Pollution, (8) Aquatic and Terrestrial Pollution, and (9) Global Change. Unit 9 is the most heavily weighted.

Which AP Environmental Science units are weighted the most?

Unit 9 (Global Change) is the largest at 15–20% of the multiple-choice section. Units 3 through 6 (Populations, Earth Systems, Land and Water Use, and Energy) are each 10–15%. Units 1 and 2 are the smallest at 6–8% each. Focus your study time accordingly.

Is AP Environmental Science hard?

APES is considered one of the more accessible AP science courses because it has less heavy math than chemistry or physics. The challenge is its breadth — nine units spanning ecology, geology, energy, pollution, and policy — and the free-response calculations, which reward showing clean, labeled work.

Do I need a calculator on the AP Environmental Science exam?

Yes. A four-function, scientific, or graphing calculator is permitted on both sections, and the free-response section includes a question requiring calculations. Practicing dimensional analysis and percent-change problems pays off, since the math itself is straightforward but easy to set up wrong.

How should I use this AP Environmental Science study guide?

Work through the nine units in order, leaning into the highest-weighted ones (Units 3–6 and 9). After each unit, take the checkpoint quiz to find gaps, drill that unit with our free practice questions and flashcards, then revisit flagged topics — especially the biogeochemical cycles, energy flow, and climate change — before exam day.

Is this AP Environmental Science study guide free?

Yes — the full guide, the checkpoints, the glossary, the practice questions, and the flashcards are 100% free, with no account required.

What is the 10% rule in energy flow?

The 10% rule says that only about 10% of the energy stored at one trophic level is passed up to the next; roughly 90% is lost as heat through cellular respiration and as undigested waste. That loss is why food chains rarely have more than four or five links. Energy flow is a core Unit 1 (Ecosystems) idea. Energy enters as sunlight, is fixed by producers, and decreases at each higher trophic level — it flows one way and is not recycled. Because so much is lost at each step, biomass and available energy form a pyramid with producers at the wide base.

How does the carbon cycle move carbon between reservoirs?

Photosynthesis pulls CO₂ out of the atmosphere into producers; respiration, decomposition, and combustion return it. Carbon is also stored long-term in oceans, soils, sediments, and fossil fuels. Burning fossil fuels moves geologic carbon into the air faster than natural processes remove it. Biogeochemical cycles are a high-yield Unit 1 topic. The fast carbon cycle (air, life, surface ocean) turns over in years; the slow cycle (rock and fossil fuels) takes millions of years. Human emissions are the main driver of rising atmospheric CO₂ and the enhanced greenhouse effect.

What is the difference between species richness and species evenness?

Species richness is simply the number of different species in a community. Species evenness describes how close in numbers each species is. A community high in both richness and evenness has the greatest biodiversity, which generally makes it more resilient to disturbance. Biodiversity is the heart of Unit 2. Higher genetic, species, and ecosystem diversity buffers a community against disease, climate shifts, and other stresses. An ecosystem dominated by one species (low evenness) is more vulnerable even if many species are present.

What is ecological succession?

Succession is the gradual, predictable change in a community over time after a disturbance. Primary succession starts on bare rock with no soil (pioneer species are lichens and mosses); secondary succession rebuilds on existing soil after a fire or farming and is much faster. Succession connects Unit 2 (Biodiversity) to ecosystem stability. It proceeds toward a relatively stable climax community in equilibrium with the local climate. Pioneer species are r-selected (fast-growing); later climax species are K-selected (shade-tolerant, slow-growing).

What is carrying capacity?

Carrying capacity (K) is the maximum population size an environment can support indefinitely, given its food, water, habitat, and other resources. A population grows quickly when small, then levels off near K as limiting factors increase death rates and reduce birth rates. Carrying capacity is central to Unit 3 (Populations). Exponential (J-shaped) growth happens with unlimited resources; logistic (S-shaped) growth occurs as a population approaches K. Overshooting K causes a population crash (a die-off) until numbers fall back toward what the environment can sustain.

What is the demographic transition model?

The demographic transition model describes how a country's birth and death rates change as it develops, through four stages: pre-industrial (high birth and death rates), transitional (death rate falls, rapid growth), industrial (birth rate falls), and post-industrial (low, stable rates). Human population dynamics are a key Unit 3 topic. Rapid growth in the transitional stage occurs because death rates drop (better medicine and food) before birth rates do. Education, access to family planning, and a higher status for women all lower total fertility rate over time.

How do El Niño and La Niña affect global weather?

El Niño is a periodic warming of the eastern Pacific that weakens the trade winds, shifting rainfall and warming the ocean off South America. La Niña is the opposite — cooler eastern Pacific water and stronger trade winds. Both alter temperature, precipitation, and storm patterns worldwide. El Niño-Southern Oscillation (ENSO) is a high-yield Unit 4 topic. El Niño suppresses upwelling off Peru, cutting nutrients and harming fisheries. These are natural climate cycles, distinct from long-term human-caused climate change.

What is the difference between weather and climate?

Weather is the short-term state of the atmosphere — temperature, precipitation, and wind on a given day. Climate is the long-term average of weather in a region, typically measured over 30 years or more. “Climate is what you expect; weather is what you get.” This distinction underlies Unit 4 and Unit 9. Soil and rock weathering, plate tectonics, and atmospheric layers also fall in Unit 4 (Earth Systems and Resources). A single cold day does not disprove a warming climate, because climate is a long-term statistical average.

What is the tragedy of the commons?

The tragedy of the commons occurs when a shared, unregulated resource is overused because each individual gains the full benefit of using it while the cost of depletion is spread across everyone. Overfishing, overgrazing, and air pollution are classic examples. This concept anchors Unit 5 (Land and Water Use). The solution is shared management — quotas, regulation, or privatization — that aligns individual incentives with sustaining the resource. It explains why open-access fisheries collapse without catch limits.

What are the environmental effects of agriculture?

Agriculture can cause soil erosion, salinization and waterlogging from irrigation, nutrient runoff that triggers eutrophication, pesticide resistance, and loss of habitat. Sustainable practices — contour plowing, no-till, crop rotation, and integrated pest management — reduce these impacts. Land and water use is the focus of Unit 5. The Green Revolution raised yields with fertilizer, irrigation, and high-yield crops, but at a high environmental and water cost. Concentrated animal feeding operations (CAFOs) generate large amounts of waste that can pollute nearby water.

What is the difference between renewable and nonrenewable energy?

Renewable energy comes from sources that replenish on a human timescale — solar, wind, hydro, geothermal, and biomass. Nonrenewable energy comes from finite stocks that took millions of years to form — coal, oil, natural gas, and nuclear fuel — and cannot be replaced once used. Energy resources are the subject of Unit 6. Fossil fuels still supply most global energy, but burning them releases CO₂ and air pollutants. Each source has trade-offs: nuclear is low-carbon but produces radioactive waste; hydropower is renewable but disrupts rivers and habitat.

How does a nuclear power plant generate electricity?

A nuclear plant splits uranium-235 atoms in a controlled fission chain reaction. The heat released boils water into steam, which spins a turbine connected to a generator. It produces no CO₂ during operation, but generates long-lived radioactive waste and carries a small risk of meltdown. Nuclear energy is a key Unit 6 topic. Control rods absorb neutrons to keep the chain reaction steady; a coolant carries away heat. Major trade-offs include high construction cost, waste storage, and the consequences of accidents such as Chernobyl and Fukushima.

How does photochemical smog form?

Photochemical smog forms when sunlight reacts with nitrogen oxides (NOₓ) and volatile organic compounds (VOCs) from vehicle exhaust, producing ground-level ozone and other irritants. It is worst on hot, sunny, windless days in cities with heavy traffic. Air pollution is the focus of Unit 7 (Atmospheric Pollution). A thermal inversion — warm air trapping cool air below — traps pollutants near the ground and makes smog worse. Ground-level ozone is a pollutant that harms lungs and crops; stratospheric ozone, by contrast, protects life from UV.

What causes acid rain and what are its effects?

Acid rain forms when sulfur dioxide (SO₂) and nitrogen oxides (NOₓ), mostly from burning fossil fuels, react with water in the atmosphere to make sulfuric and nitric acids. It acidifies lakes and soils, harms fish and trees, and corrodes buildings and statues. Acid deposition is a high-yield Unit 7 topic. Lakes with limestone (calcium carbonate) bedrock can buffer the acid; granite-bottomed lakes cannot and acidify quickly. Scrubbers on smokestacks and cap-and-trade rules under the Clean Air Act have cut U.S. SO₂ emissions sharply.

What is eutrophication?

Eutrophication is the nutrient enrichment of water — usually by nitrogen and phosphorus from fertilizer or sewage — that triggers an algal bloom. When the algae die, decomposing bacteria consume the dissolved oxygen, creating hypoxic “dead zones” where fish and shellfish suffocate. Water pollution is the focus of Unit 8. Cultural eutrophication is the human-accelerated version caused by runoff. Solutions include buffer strips, reduced fertilizer use, and treating sewage to remove nutrients before discharge.

What is bioaccumulation and biomagnification?

Bioaccumulation is the buildup of a persistent toxin (like DDT or mercury) in an individual organism's tissues over time. Biomagnification is the increase in that toxin's concentration at each higher trophic level, so top predators carry the highest, most dangerous doses. These linked concepts are a high-yield Unit 8 topic. Fat-soluble, slow-to-break-down pollutants are the ones that magnify. DDT thinned the eggshells of birds of prey (like eagles) because they sit at the top of the food chain — the classic example.

What is the greenhouse effect?

The greenhouse effect is the natural warming that occurs when greenhouse gases — CO₂, methane, water vapor, and nitrous oxide — absorb infrared heat the Earth radiates and re-emit it back toward the surface. Without it Earth would be frozen; too much of it drives global warming. This is the core mechanism of Unit 9 (Global Change). Human emissions of CO₂ and methane have intensified the natural effect — the enhanced greenhouse effect. Methane traps far more heat per molecule than CO₂ but is less abundant and shorter-lived in the atmosphere.

How does stratospheric ozone depletion happen?

Chlorofluorocarbons (CFCs), once used in refrigerants and aerosols, drift to the stratosphere where UV light frees chlorine atoms. Each chlorine atom catalytically destroys many ozone molecules, thinning the ozone layer and letting more harmful UV reach the surface. Ozone depletion is a high-yield Unit 9 topic. The Montreal Protocol phased out CFCs and is allowing the ozone layer to slowly recover. Do not confuse this with the greenhouse effect: stratospheric ozone protects life from UV, while ground-level ozone is a pollutant.

A feeding position in a food chain: producers (1st), primary consumers (2nd), secondary consumers (3rd), and so on. Energy decreases at each higher level.

Only about 10% of the energy at one trophic level is passed to the next; roughly 90% is lost as heat through respiration. This limits food-chain length.

Gross primary productivity

The total rate at which producers capture and store energy through photosynthesis (GPP). Net primary productivity (NPP) is GPP minus the energy producers use for their own respiration.

The movement of an element (carbon, nitrogen, phosphorus, water) through the living and nonliving parts of an ecosystem.

Conversion of inert N₂ gas into ammonia (NH₃/NH₄⁺) that organisms can use, done by bacteria, lightning, and the industrial Haber-Bosch process.

Nutrient enrichment of water (usually nitrogen and phosphorus) that causes algal blooms; when the algae die, decomposers use up the oxygen, creating dead zones.

The variety of life at three levels — genetic, species, and ecosystem diversity. Higher biodiversity generally makes an ecosystem more resilient.

A species whose presence has a disproportionately large effect on its community; removing it causes the ecosystem to change dramatically (e.g., sea otters).

The maximum population size (K) an environment can support indefinitely given its resources. Populations level off near K.

J-shaped growth that occurs when resources are unlimited; the population grows by a constant percent each interval.

S-shaped growth that slows as a population nears its carrying capacity and limiting factors increase.

Species with many small offspring, little parental care, and short lifespans (e.g., insects, weeds). They reproduce fast and dominate early succession.

Species with few large offspring, high parental care, and long lifespans (e.g., elephants). They stabilize near carrying capacity.

Demographic transition

A model of how birth and death rates change as a country develops, moving from high birth and death rates to low ones in four stages.

The average number of children a woman has in her lifetime. A TFR of about 2.1 is replacement level in developed nations.

A periodic warming of the eastern Pacific that weakens trade winds and shifts global weather; La Niña is the cooler opposite phase.

The warming caused when greenhouse gases (CO₂, methane, water vapor, nitrous oxide) absorb and re-emit Earth's infrared heat toward the surface.

The fraction of sunlight a surface reflects. Ice and snow have high albedo (reflective); dark land and ocean have low albedo (absorbent).

FREE AP Environmental Science Study Guide 2026 + Interactive

This free AP Environmental Science study guide teaches to the College Board APES exam — every one of the nine official units, organized the way the course is built.^[1] AP Environmental Science (APES) is an interdisciplinary course that blends ecology, geology, chemistry, and policy to study how natural systems work and how humans affect them.

The guide is interactive, not a wall of text: every unit has a built-in checkpoint quiz, hover-able glossary terms, labeled diagrams of the carbon and nitrogen cycles, worked calculations, and concept questions, so you learn by doing. Read it unit by unit, test yourself at each checkpoint, then round out your free APES prep with our practice questions and flashcards.

Taking more than one AP science? The science-process and data-analysis skills here carry straight over to our AP Biology study guide.

AP Environmental Science is one of the 17 AP exams — explore our AP study guides to compare and prep across the whole family.

AP Environmental Science Exam Snapshot

AP Environmental Science exam at a glance (2026)
Detail	AP Environmental Science
Section I	80 multiple-choice questions · 90 minutes · 60% of score
Section II	3 free-response questions · 70 minutes · 40% of score
Total time	About 2 hours 40 minutes
Score scale	1–5 (a 3 or higher generally earns college credit)
Units	9 College Board units, ecology through global change
Calculator	Four-function, scientific, or graphing — allowed on both sections
Math	Dimensional analysis, percent change, and rates (no calculus)
Publisher	College Board

How the AP Environmental Science exam is built

Two sections, scored together on the 1–5 AP scale. This guide and our practice questions focus on the Section I multiple-choice content, which is also the foundation for the FRQs.

Section I — Multiple Choice80 questions · 90 minutes · 60% of the score. Includes individual questions plus sets tied to a text, model, visual, or quantitative data.
Section II — Free Response3 questions · 70 minutes · 40% of the score. One design-an-investigation, one analyze-an-environmental-problem-and-propose-a-solution, and one with calculations.

About 3 hours total. A score of 3 or higher generally earns college credit. Bring an approved calculator — Section II rewards clean, labeled calculations.

Because Section I is 60% of your score and the foundation for the free-response questions, this guide focuses on mastering the multiple-choice content unit by unit.^[2] Spend your study time across all nine units, but know that Unit 9 (Global Change) is the single biggest unit and that Units 3–6 together make up roughly half the exam:

AP Environmental Science units (2026 exam weights)

9 · Global Change18% · 15–20%

3 · Populations13% · 10–15%

4 · Earth Systems & Resources13% · 10–15%

5 · Land & Water Use13% · 10–15%

6 · Energy Resources13% · 10–15%

7 · Atmospheric Pollution9% · 7–10%

8 · Aquatic & Terrestrial Pollution9% · 7–10%

1 · Ecosystems7% · 6–8%

2 · Biodiversity7% · 6–8%

College Board reports unit weights as approximate ranges, so the exact mix shifts slightly each year.^[1] This guide teaches all nine units in order, from how ecosystems work to how humans are changing the planet.

1 · The Living World: Ecosystems

6–8% of the exam. How ecosystems are structured, how energy flows through them, and how matter cycles. This is the foundation the rest of the course builds on.^[1]

Ecosystem Structure & Food Webs

A is a feeding position in a food chain: producers (plants and algae that make their own food), primary consumers (herbivores), secondary consumers (carnivores that eat herbivores), and so on, with decomposers recycling everything. A food web is many interlocking food chains. A has an outsized effect on its community — remove sea otters and kelp forests collapse.

Energy Flow & the 10% Rule

Energy enters as sunlight, is captured by producers, and flows one way up the food chain — it is never recycled. The says only about 10% of the energy at one level reaches the next; ~90% is lost as heat through respiration. That loss is why food chains are short and why matters.

Energy flow & the 10% rule

Tertiary consumers — 10 units

Secondary consumers — 100 units

Primary consumers — 1,000 units

Producers — 10,000 units

Only about 10% of the energy at one trophic level is passed to the next; the other ~90% is lost as heat through respiration. That is why food chains rarely exceed four or five links and why eating lower on the chain feeds more people per acre.

Biogeochemical Cycles

A moves an element through the living and nonliving environment. The two most tested are carbon and nitrogen:

The carbon cycle — reservoirs and fluxes

Atmosphere — CO₂Photosynthesis removes it ↓ · respiration, combustion & decomposition return it ↑

ProducersFix CO₂ into sugars via photosynthesis

Consumers & soilRespiration and decomposition release CO₂

Oceans & sedimentsDissolved CO₂, shells, and the slow sink: fossil fuels & limestone

The fast cycle moves carbon between air, life, and surface ocean in years to decades. Burning fossil fuels (the slow reservoir) moves geologic carbon into the atmosphere far faster than it is removed — the root of the enhanced greenhouse effect.

The nitrogen cycle — five key transformations

1 · Nitrogen fixationN₂ gas → ammonia (NH₃ / NH₄⁺). Done by bacteria (often in legume root nodules), lightning, and industrially by the Haber-Bosch process for fertilizer.
2 · NitrificationNH₄⁺ → nitrite (NO₂⁻) → nitrate (NO₃⁻), by soil bacteria. Nitrate is the form plants take up most easily.
3 · AssimilationPlants absorb nitrate/ammonium and build it into proteins and DNA; animals get nitrogen by eating plants.
4 · AmmonificationDecomposers break down dead tissue and waste, returning nitrogen to the soil as ammonium (NH₄⁺).
5 · DenitrificationAnaerobic bacteria convert nitrate (NO₃⁻) back to N₂ gas, returning nitrogen to the atmosphere and closing the cycle.

Nitrogen, not carbon, usually limits plant growth on land — which is why fertilizer (extra nitrate) boosts crops, and why fertilizer runoff drives eutrophication downstream.

turns inert N₂ gas into usable ammonia; denitrification returns it to the air. The phosphorus cycle has no atmospheric (gas) phase — it moves through rock, soil, and water, which is why phosphorus often limits aquatic plant growth.

Checkpoint · Unit 1 · Ecosystems

Question 1 of 10

In the carbon cycle, which process transfers carbon from the atmosphere into the tissues of living producers?

2 · The Living World: Biodiversity

6–8% of the exam. Why biodiversity matters, the services ecosystems provide, and how communities recover from disturbance.^[1]

Biodiversity & Ecosystem Services

exists at three levels — genetic, species, and ecosystem diversity — and higher diversity generally makes an ecosystem more resilient to disease, climate shifts, and other stresses. Ecosystems provide free services: pollination, water filtration, flood control, climate regulation, and food. Greater richness (number of species) and evenness (how balanced their numbers are) both raise biodiversity.

Disturbance & Succession

is the predictable change in a community after a disturbance. Primary succession starts on bare rock with no soil (pioneer species are lichens and mosses) and takes the longest; secondary succession rebuilds on existing soil after a fire or abandoned farm and is much faster, ending in a relatively stable climax community.

Ecological succession — from disturbance to climax community

DisturbancePrimary: bare rock with no soil (after a glacier or lava flow). Secondary: soil remains (after a fire or abandoned farm).
Pioneer speciesPrimary starts with lichens and mosses that break rock into soil; secondary starts with fast-growing grasses and weeds.
Intermediate communityShrubs and small, sun-loving (r-selected) trees colonize as soil and shade develop.
Climax communityA relatively stable community of shade-tolerant (K-selected) species, in equilibrium with the local climate.

Primary succession begins where there is no soil and takes the longest; secondary succession rebuilds on existing soil and is much faster.

Checkpoint · Unit 2 · Biodiversity

Question 1 of 10

A wetland filters pollutants from water, stores floodwater, and provides habitat for migratory birds. These benefits that humans receive from natural ecosystems are collectively known as

3 · Populations

10–15% of the exam. How populations grow and what limits them — for wildlife and for humans. A high-yield, math-friendly unit.^[1]

Population Growth & Carrying Capacity

happens when resources are unlimited; slows as a population nears its (K). Limiting factors — density-dependent ones like food, disease, and predation, and density-independent ones like weather and natural disasters — keep populations in check.

r- and K-Selected Species

have many small offspring, little parental care, and short lives (insects, weeds) — they boom and bust and dominate early succession. have few large offspring, high parental care, and long lives (elephants, oak trees) — they stabilize near carrying capacity. A survivorship curve shows the pattern: Type I (humans) survive long then die off; Type III (fish, insects) die young in huge numbers.

Human Populations & Demographics

The tracks how birth and death rates fall as a country develops, through four stages. (TFR) of about 2.1 is replacement level. An age-structure diagram with a wide base predicts rapid future growth; a narrow base predicts a shrinking population.

The four stages of the demographic transition
Stage	Birth & death rates	Growth
1 · Pre-industrial	Both high	Slow / stable
2 · Transitional	Death rate falls, birth rate high	Rapid growth
3 · Industrial	Birth rate falls	Slowing growth
4 · Post-industrial	Both low	Stable or declining

Checkpoint · Unit 3 · Populations

Question 1 of 10

A field biologist observes that a lake trout population stops increasing and fluctuates around 8,000 fish year after year. The value around which the population levels off represents the lake's

4 · Earth Systems & Resources

10–15% of the exam. The physical Earth — plate tectonics, soil, the atmosphere, and the global movement of air and water that drives climate.^[1]

Plate Tectonics, Soil & Rocks

Plate boundaries cause earthquakes, volcanoes, and mountain-building. Soil forms slowly from weathered rock plus organic matter; its texture (the mix of sand, silt, and clay) controls how well it holds water and nutrients — loam (a balanced mix) is best for farming. A soil horizon profile runs from the O (organic litter) and A (topsoil) layers down to bedrock.

Atmosphere & Global Wind Patterns

The atmosphere’s layers — troposphere (weather), stratosphere (the protective ozone layer) — and global circulation cells (Hadley, Ferrel, polar) set up the world’s wind belts and rain patterns. Air rises at the equator (wet) and sinks around 30° latitude (where most deserts sit).

The Water Cycle & ENSO

The water cycle moves water through evaporation, condensation, precipitation, runoff, and infiltration into aquifers (underground water stored in permeable rock). and La Niña are periodic Pacific Ocean cycles that shift global temperature and rainfall — El Niño suppresses the cold, nutrient-rich upwelling off South America and harms fisheries.

Checkpoint · Unit 4 · Earth Systems & Resources

Question 1 of 10

A chain of volcanic islands in the middle of the Pacific Ocean has formed in a line, with the oldest island farthest from an active volcano and progressively younger islands toward it. This pattern is best explained by a tectonic plate moving over

5 · Land & Water Use

10–15% of the exam. How humans use land and water — agriculture, mining, fishing, and urban development — and the trade-offs each brings.^[1]

The Tragedy of the Commons

The explains why shared, unregulated resources get overused: each person reaps the full benefit of using more while the cost of depletion is spread across everyone. Overfishing and overgrazing are classic cases. The fix is shared management — quotas, regulation, or assigning ownership.

Agriculture & Its Impacts

Farming feeds the world but can cause soil erosion, salinization from irrigation, fertilizer runoff that triggers , and pesticide resistance. The Green Revolution boosted yields with fertilizer, irrigation, and high-yield seed. Sustainable practices — contour plowing, no-till, crop rotation, and integrated pest management — cut the damage.

Sustainable vs. damaging land-use practices
Practice	Effect
Contour plowing / terracing	Reduces soil erosion on slopes
No-till farming	Keeps soil and moisture in place, stores carbon
Crop rotation	Restores soil nitrogen, breaks pest cycles
Integrated pest management (IPM)	Cuts pesticide use by combining controls
Overgrazing / monoculture	Degrades soil, lowers biodiversity

Mining, Fishing & Sustainability

Surface mining (strip mining, mountaintop removal) is cheaper but scars the land; subsurface mining is costlier and more dangerous. Overfishing collapses fisheries — the answer is catch limits and aquaculture, though fish farming has its own pollution. The goal across the unit is sustainable yield: using a resource no faster than it can renew.

Checkpoint · Unit 5 · Land & Water Use

Question 1 of 10

A shared ocean fishery is open to all boats, and each crew benefits directly from catching more fish while the cost of the declining stock is spread across everyone. The depletion of this shared resource because individuals act in their own self-interest is best described as

6 · Energy Resources & Consumption

10–15% of the exam. Where our energy comes from, how each source works, and the environmental trade-offs of fossil fuels, nuclear power, and renewables.^[1]

Fossil Fuels & Nonrenewables

Coal, oil, and natural gas are nonrenewable — finite stocks formed over millions of years from buried organisms. They still supply most global energy, but burning them releases CO₂ (driving climate change) plus air pollutants.

Among them, natural gas burns cleanest and coal the dirtiest. Combustion converts chemical energy to heat to electricity, losing energy at each step.

Renewable & Nuclear Energy

Renewables — solar, wind, hydro, geothermal, biomass — replenish on a human timescale but each has trade-offs (intermittency, habitat disruption). Nuclear fission of uranium-235 produces no CO₂ during operation but creates long-lived radioactive waste and a small meltdown risk. Energy conservationand efficiency are the cheapest “source” of all.

Energy sources and their main trade-offs
Source	Renewable?	Key trade-off
Coal	No	Cheap but dirtiest — CO₂, SO₂, mercury, particulates
Natural gas	No	Cleanest fossil fuel, but methane leaks and still CO₂
Nuclear	No	No operating CO₂, but radioactive waste and meltdown risk
Solar / wind	Yes	Clean but intermittent; needs storage and land
Hydropower	Yes	Reliable but dams disrupt rivers and fish
Geothermal	Yes	Steady and clean, but location-limited

Checkpoint · Unit 6 · Energy Resources

Question 1 of 10

Which of the following best explains why coal, oil, and natural gas are classified as nonrenewable resources?

7 · Atmospheric Pollution

7–10% of the exam. The major air pollutants, how smog and acid rain form, and the laws and devices that control them.^[1]

Air Pollutants & Smog

Primary pollutants are emitted directly (CO, SO₂, NOₓ, particulates); secondary pollutants form in the air (ground-level ozone). forms when sunlight reacts with NOₓ and VOCs from car exhaust, worst on hot, sunny, windless days. A traps pollutants near the ground and makes it worse.

Acid Rain & Indoor Air

forms when SO₂ and NOₓ react with water to make sulfuric and nitric acids, acidifying lakes and soils and corroding stone. Lakes with limestone bedrock can buffer it; granite-bottomed lakes cannot. Indoor air pollutants — radon, carbon monoxide, asbestos, and VOCs — are a major, often-overlooked health risk.

Checkpoint · Unit 7 · Atmospheric Pollution

Question 1 of 10

Photochemical smog forms most readily under which combination of conditions?

8 · Aquatic & Terrestrial Pollution

7–10% of the exam. Water and solid-waste pollution, toxins that move through food chains, and how pollution affects human health.^[1]

Water Pollution & Eutrophication

Point-source pollution comes from a single identifiable pipe; nonpoint source pollution (farm and street runoff) is diffuse and harder to control. from nitrogen and phosphorus runoff fuels algal blooms; when the algae die, decomposers use up the oxygen, creating hypoxic dead zones. Sewage adds a high biochemical oxygen demand (BOD) that starves water of oxygen too.

Toxins, Waste & Human Health

Persistent, fat-soluble toxins like mercury and DDT undergo in an organism and up the food chain, so top predators carry the highest doses — the reason DDT thinned the eggshells of eagles. A dose-response curve shows how toxicity rises with exposure; the LD50 is the dose lethal to half a test population. Solid waste goes to sanitary landfills (lined to protect groundwater) or incinerators, but reducing, reusing, and recycling come first.

Checkpoint · Unit 8 · Aquatic & Terrestrial Pollution

Question 1 of 10

A coastal estuary repeatedly experiences large summer dead zones where bottom-dwelling shellfish die off. These hypoxic dead zones are most directly the end result of which process?

9 · Global Change

15–20% of the exam — the single biggest unit. The large-scale, human-driven changes to the planet: ozone depletion, climate change, and biodiversity loss. Master this unit.^[6]

Ozone Depletion

happens when chlorine freed from CFCs (old refrigerants and aerosols) catalytically destroys stratospheric ozone, letting more harmful UV reach the surface and raising skin-cancer risk. The phased out CFCs and is the great success story — the ozone layer is slowly recovering.

Greenhouse Effect & Climate Change

The is natural and necessary — greenhouse gases (CO₂, methane, water vapor, nitrous oxide) trap heat and keep Earth livable. Human emissions have enhanced it, raising global temperatures. Consequences include melting ice (which lowers and amplifies warming), sea-level rise, ocean acidification (CO₂ + seawater → carbonic acid, harming coral and shellfish), and shifting climate zones.

Invasive Species & Biodiversity Loss

The main drivers of biodiversity loss are summarized as HIPPCO: Habitat destruction, Invasive species, Pollution, Population (human), Climate change, and Overexploitation. spread fast because they have no natural predators (zebra mussels, kudzu). A can flip an ecosystem — like a coral reef — abruptly and irreversibly to a degraded state.

Checkpoint · Unit 9 · Global Change

Question 1 of 10

The greenhouse effect refers to the process by which certain atmospheric gases

How to Use This Study Guide

A study guide is a map, not the whole territory — use it alongside official College Board practice and our free tools. Because APES is broad but not deeply mathematical, the winning strategy is wide coverage plus mastery of the high-yield mechanisms: the biogeochemical cycles, energy flow, population math, pollution types, and climate change. Lean into the heaviest units (3–6 and 9), and practice the calculations until the setup is automatic.

AP Environmental Science by unit (2026 exam weights)

9 · Global Change

15–20%

3 · Populations

10–15%

4 · Earth Systems

10–15%

5 · Land & Water Use

10–15%

6 · Energy Resources

10–15%

7 · Atmospheric Pollution

7–10%

8 · Aquatic & Terrestrial Pollution

7–10%

1 · Ecosystems

6–8%

2 · Biodiversity

6–8%

Unit 9 (Global Change) is the single biggest unit, and Units 3–6 together make up roughly half the exam — weight your study accordingly.

A study loop that actually works

1
Read a unit here
Work through one unit at a time, in order, watching for the diagrams and worked examples.
2
Take the checkpoint
The quick check at the end of each unit exposes what didn't stick.
3
Drill the gaps
Send your weak unit straight into the free practice questions and flashcards.
4
Practice the math + FRQs
Work percent-change, dimensional-analysis, and energy problems, and outline free-response answers.

AP Environmental Science Concept Questions

Common APES concepts the exam actually measures — at least one per College Board unit. Tap any card for a short, exam-ready answer backed by an official source (College Board, EPA, USGS, NOAA, NASA), then test yourself on them as flashcards.

AP Environmental Science Glossary

Quick definitions for the terms you’ll see most across the AP Environmental Science exam:

10% rule: Only about 10% of the energy at one trophic level is passed to the next; roughly 90% is lost as heat through respiration. This limits food-chain length.
Acid rain: Precipitation acidified by sulfuric and nitric acids that form when SO₂ and NOₓ from burning fossil fuels react with water in the air.
Albedo: The fraction of sunlight a surface reflects. Ice and snow have high albedo (reflective); dark land and ocean have low albedo (absorbent).
Bioaccumulation: The buildup of a persistent toxin in one organism's tissues over its lifetime.
Biodiversity: The variety of life at three levels — genetic, species, and ecosystem diversity. Higher biodiversity generally makes an ecosystem more resilient.
Biogeochemical cycle: The movement of an element (carbon, nitrogen, phosphorus, water) through the living and nonliving parts of an ecosystem.
Biomagnification: The increase in a toxin's concentration at each higher trophic level, so top predators carry the highest doses.
Carrying capacity: The maximum population size (K) an environment can support indefinitely given its resources. Populations level off near K.
Demographic transition: A model of how birth and death rates change as a country develops, moving from high birth and death rates to low ones in four stages.
Ecological succession: The gradual, predictable change in a community over time after a disturbance, proceeding toward a stable climax community.
El Niño: A periodic warming of the eastern Pacific that weakens trade winds and shifts global weather; La Niña is the cooler opposite phase.
Eutrophication: Nutrient enrichment of water (usually nitrogen and phosphorus) that causes algal blooms; when the algae die, decomposers use up the oxygen, creating dead zones.
Exponential growth: J-shaped growth that occurs when resources are unlimited; the population grows by a constant percent each interval.
Externality: A cost (or benefit) of an activity that falls on people not involved in it — such as the health costs of pollution borne by society.
Greenhouse effect: The warming caused when greenhouse gases (CO₂, methane, water vapor, nitrous oxide) absorb and re-emit Earth's infrared heat toward the surface.
Gross primary productivity: The total rate at which producers capture and store energy through photosynthesis (GPP). Net primary productivity (NPP) is GPP minus the energy producers use for their own respiration.
Invasive species: A non-native species that spreads rapidly and harms the local ecosystem, often because it has no natural predators.
K-selected species: Species with few large offspring, high parental care, and long lifespans (e.g., elephants). They stabilize near carrying capacity.
Keystone species: A species whose presence has a disproportionately large effect on its community; removing it causes the ecosystem to change dramatically (e.g., sea otters).
Logistic growth: S-shaped growth that slows as a population nears its carrying capacity and limiting factors increase.
Montreal Protocol: The 1987 international treaty that phased out ozone-depleting CFCs, allowing the ozone layer to slowly recover.
Nitrogen fixation: Conversion of inert N₂ gas into ammonia (NH₃/NH₄⁺) that organisms can use, done by bacteria, lightning, and the industrial Haber-Bosch process.
Ozone depletion: Thinning of the stratospheric ozone layer by chlorine from CFCs, which lets more harmful UV radiation reach Earth's surface.
Photochemical smog: A brownish haze of ground-level ozone formed when sunlight reacts with NOₓ and VOCs from vehicle exhaust on hot, sunny days.
r-selected species: Species with many small offspring, little parental care, and short lifespans (e.g., insects, weeds). They reproduce fast and dominate early succession.
Thermal inversion: A layer of warm air sitting over cooler air near the ground, trapping pollutants and worsening smog.
Tipping point: A threshold beyond which a system shifts abruptly and often irreversibly to a new state, such as the loss of a coral reef.
Total fertility rate: The average number of children a woman has in her lifetime. A TFR of about 2.1 is replacement level in developed nations.
Tragedy of the commons: The overuse and degradation of a shared, unregulated resource because individuals benefit from using it while the cost is shared by all.
Trophic level: A feeding position in a food chain: producers (1st), primary consumers (2nd), secondary consumers (3rd), and so on. Energy decreases at each higher level.

Free AP Environmental Science Study Materials & Resources

Everything you need to prepare for the APES exam is free here — no paywall, no sign-up. This guide is the foundation; pair it with the rest of our free AP Environmental Science study materials for active recall, timed practice, and last-minute review:

AP Environmental Science Practice Test — exam-style questions across all nine units, with explanations.
AP Environmental Science Flashcards — active-recall decks for the cycles, laws, pollutants, and key concepts.
AP Biology Study Guide — the related life-science course, for students taking both.

AP Environmental Science Study Guide FAQ

The AP Environmental Science exam has 80 multiple-choice questions in Section I (90 minutes, 60% of the score) and 3 free-response questions in Section II (70 minutes, 40% of the score). Total testing time is about 2 hours and 40 minutes.

References

1.College Board. “AP Environmental Science Course and Exam Description.” College Board. ↑
2.College Board. “AP Environmental Science Exam — AP Students.” College Board. ↑
3.U.S. Environmental Protection Agency. “Environmental Topics.” U.S. EPA. ↑
4.U.S. Geological Survey. “The Carbon Cycle.” USGS. ↑
5.National Oceanic and Atmospheric Administration. “What are El Niño and La Niña?.” NOAA. ↑
6.U.S. Global Change Research Program. “Fourth National Climate Assessment.” USGCRP. ↑
7.U.S. Environmental Protection Agency. “What is Acid Rain?.” U.S. EPA. ↑

Sources for the concept answers

Every answer in the AP Environmental Science concept questions above is drawn from an official primary source:

NASA. “What's the Difference Between Weather and Climate?.” NASA.
U.S. Environmental Protection Agency. “Agriculture and the Environment.” U.S. Environmental Protection Agency.
U.S. Environmental Protection Agency. “Sources of Energy.” U.S. Environmental Protection Agency.
U.S. Environmental Protection Agency. “Nuclear Explained.” U.S. Environmental Protection Agency.
U.S. Environmental Protection Agency. “Ground-level Ozone Basics.” U.S. Environmental Protection Agency.
U.S. Environmental Protection Agency. “The Effects: Dead Zones and Harmful Algal Blooms.” U.S. Environmental Protection Agency.
U.S. Environmental Protection Agency. “Persistent Organic Pollutants: A Global Issue.” U.S. Environmental Protection Agency.
U.S. Environmental Protection Agency. “Basic Ozone Layer Science.” U.S. Environmental Protection Agency.

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