AP Environmental Science Review Guide: Every Unit Made Visual

APES is the AP science with the widest breadth and the lightest math. The exam tests specific facts (soil horizons, energy sources, pollutants, treaties) AND your ability to reason about environmental trade-offs. Unlike Chem or Physics, APES rewards you for knowing a lot of discrete things rather than mastering a handful of equations.

This guide walks through all 9 units with the facts, math, and diagrams the exam reuses. If you can identify pollutants, interpret data, and reason about environmental choices, you will score well.

What the exam looks like

Exam structure and scoring

• 2 hours 40 minutes total.
• Section I: 80 multiple choice in 90 minutes. Worth 60 percent.
• Section II: 3 free response in 70 minutes. Worth 40 percent.
• FRQ #1: design an investigation (hypothesis, variables, controls).
• FRQ #2: analyze an environmental problem with authentic data (often includes graphs, math).
• FRQ #3: propose and evaluate an environmental solution.
• Calculator allowed throughout. Math is straightforward (percentages, unit conversions, simple formulas).

Unit 1: The Living World, Ecosystems

Biogeochemical cycles and energy flow

• Biomes: defined by climate and vegetation. Major biomes: tropical rainforest (high biodiversity), temperate forest, grassland, desert, tundra, taiga (boreal), chaparral, wetlands, freshwater, marine.
• Carbon cycle: CO2 in atmosphere. Photosynthesis (plants take in CO2, release O2). Respiration (organisms release CO2). Combustion (burning fuels, major human contribution). Decomposition.
• Nitrogen cycle: N2 is inert. Requires fixation (by bacteria, lightning, or Haber-Bosch process). Nitrification (NH4+ to NO2- to NO3-). Assimilation (plants take up nitrates). Denitrification (NO3- back to N2).
• Phosphorus cycle: no atmospheric phase. Weathering of rocks releases phosphate. Taken up by plants and animals. Returned to soil/water by decomposition. Slow cycle.
• Water cycle: evaporation, transpiration, condensation, precipitation, runoff, infiltration, groundwater.
• Food chains and food webs: producers (plants) -> primary consumers (herbivores) -> secondary consumers -> tertiary consumers. Decomposers break down dead matter.
• 10 percent rule: only ~10 percent of energy transfers between trophic levels (the rest is lost as heat). Explains why top predators are rare.
• Primary productivity (GPP and NPP): GPP is total energy fixed by producers. NPP is GPP minus respiration (energy available to consumers).

Unit 2: The Living World, Biodiversity

Ecosystem disturbance and resilience

• Species diversity: number of species (richness) and their relative abundances (evenness). Measured by Simpson's or Shannon indices.
• Ecological tolerance: range of conditions a species can survive. Wider tolerance equals generalist. Narrow equals specialist.
• Natural disruptions: fires, hurricanes, volcanic eruptions, disease. Part of natural cycles; some ecosystems depend on them (fire-adapted forests).
• Ecological succession: primary (bare rock/soil-less surface, starts with pioneer species like lichens) vs secondary (after disturbance, soil remains).
• Stages: pioneer -> grasses and shrubs -> small trees -> mature forest (climax community).
• Adaptations: structural, behavioral, physiological. Natural selection favors traits suited to environment.
• Keystone species: disproportionate impact on ecosystem relative to numbers (sea otters, wolves). Remove them, the ecosystem collapses.
• Indicator species: sensitive to environmental changes, reveal ecosystem health (lichens for air quality, amphibians for water quality).

Unit 3: Populations

Population dynamics

• Generalist (broad diet, wide tolerance, common in disturbed areas) vs specialist (narrow niche, vulnerable to change).
• r-strategists: many offspring, short life, little parental care (insects, weeds). Good when environments are unstable.
• K-strategists: few offspring, long life, much parental care (elephants, humans). Good when environments are stable; population at carrying capacity.
• Carrying capacity (K): max population the environment can support long-term.
• Population growth: exponential when resources abundant (dN/dt = rN). Logistic as resources become limited (dN/dt = rN(K-N)/K).
• Demographic transition: Stage 1 (high birth, high death, low growth: pre-industrial). Stage 2 (death drops, birth high: population surges, developing nations). Stage 3 (birth drops: growth slows). Stage 4 (both low: stable or shrinking).
• Age structure pyramids: broad base = growing population. Even = stable. Narrow base = shrinking (Japan, Germany).
• Human population: ~8 billion. Growth rate 1.1 percent per year. Global variations: highest in sub-Saharan Africa, shrinking in Japan, parts of Europe.

Unit 4: Earth Systems and Resources

Physical and geological systems

• Plate tectonics: convergent (plates collide, subduction, mountains, earthquakes, volcanoes), divergent (plates separate, mid-ocean ridges, rift valleys), transform (plates slide past, San Andreas Fault).
• Soil horizons (top to bottom): O (organic, leaf litter), A (topsoil, mix of organic and mineral), E (eluviated, leached), B (subsoil, accumulation), C (parent material), R (bedrock).
• Atmospheric layers: troposphere (weather), stratosphere (ozone layer, jets), mesosphere, thermosphere, exosphere.
• Watershed: area that drains to a common waterway.
• El Nino and La Nina: El Nino (warm Pacific waters near South America, disrupts normal trade winds, affects global weather). La Nina (opposite, cooler waters, intensifies normal patterns).
• Solar energy and Earth's tilt cause seasons, not distance from sun.

Unit 5: Land and Water Use

Human impacts on land

• Agriculture: slash and burn (temporary fertility, destroys forests), monoculture (efficient but fragile, pests thrive), irrigation (can cause salinization), Green Revolution (high yields but heavy fertilizer/pesticide use).
• Livestock: CAFOs (concentrated animal feeding operations, efficient but waste problem, antibiotic overuse, disease risk). Overgrazing (damages grasslands, desertification).
• Deforestation: for agriculture, logging, development. Leads to erosion, biodiversity loss, climate change (reduces carbon sequestration).
• Mining: open-pit (huge surface scars), strip mining (coal), mountaintop removal (destroys ecosystems), acid mine drainage (contaminates water).
• Overfishing: bycatch, bottom trawling destroys habitats, species collapse (cod, bluefin tuna).
• Urbanization: impervious surfaces increase runoff, urban heat island, pollution.
• Sustainability: integrated pest management, contour plowing, no-till farming, crop rotation, aquaculture (with caveats), sustainable forestry.

Unit 6: Energy Resources and Consumption

Energy sources and trade-offs

• Nonrenewable: fossil fuels (coal, oil, natural gas) and nuclear. Finite supplies.
• Coal: cheap, abundant. BUT highest CO2, particulates, mercury emissions. Mining destroys landscapes.
• Oil: energy-dense, easy to transport. BUT CO2, spills devastate ecosystems, geopolitics (Middle East, OPEC).
• Natural gas: cleaner than coal/oil (less CO2 per unit energy). BUT methane leaks are potent greenhouse gas. Fracking causes water contamination concerns.
• Nuclear (fission): low operational CO2. BUT produces long-lived radioactive waste. Meltdown risks (Three Mile Island, Chernobyl, Fukushima). Uranium mining and processing.
• Renewable: solar, wind, hydro, geothermal, biomass.
• Solar (PV and thermal): zero operational emissions. BUT intermittent, requires battery storage, manufacturing has environmental costs.
• Wind: clean, increasingly cheap. BUT intermittent, bird/bat deaths, visual/noise impact.
• Hydroelectric: renewable, reliable. BUT dams destroy ecosystems, displace people, emit methane from flooded vegetation.
• Geothermal: reliable, clean. BUT location-specific (volcanic areas).
• Biomass: renewable. BUT burning produces CO2 (theoretically carbon-neutral if regrown), competes with food crops.
• Energy efficiency: using less energy for same service. Cheapest way to cut emissions.

Unit 7: Atmospheric Pollution

Air pollutants and their effects

• Primary pollutants: emitted directly (CO, SO2, NOx, particulate matter, VOCs).
• Secondary pollutants: form from reactions in atmosphere (O3 ground-level ozone, acid rain).
• Photochemical smog: NOx + VOCs + sunlight -> ozone. Worst in cities (LA, Beijing).
• Industrial smog (London smog): coal burning -> SO2 + particulates. Thermal inversions trap it.
• Acid rain: SO2 + NOx react with water to form H2SO4 and HNO3. Harms lakes, forests, buildings. Worse in areas downwind of industry.
• Thermal inversions: warm air traps cold air near ground, concentrating pollutants. Common in valleys.
• Indoor air pollution: radon (naturally occurring), asbestos (old buildings), VOCs (cleaners, paint), combustion byproducts (biomass burning in developing countries).
• Stratospheric ozone depletion: CFCs (chlorofluorocarbons) destroy O3. Ozone hole over Antarctica. Montreal Protocol (1987) banned CFCs, ozone recovering.

Unit 8: Aquatic and Terrestrial Pollution

Water and land pollution

• Point source: identifiable, single source (factory pipe, sewage plant).
• Nonpoint source: diffuse (agricultural runoff, urban runoff). Much harder to regulate.
• Eutrophication: nutrient pollution (nitrogen, phosphorus from fertilizer) -> algal bloom -> algae die, decomposers consume O2 -> dead zone (low O2, kills fish). Gulf of Mexico dead zone from Mississippi runoff.
• Thermal pollution: warm water (from power plants) reduces dissolved O2, stresses fish.
• Oil spills: Exxon Valdez, Deepwater Horizon. Devastate marine life, hard to clean up.
• Solid waste: landfills (leachate can contaminate groundwater), incineration (reduces volume but releases pollutants), ocean garbage patches (plastics).
• Hazardous waste: radioactive, toxic chemicals. Superfund (US program for cleaning contaminated sites).
• Dose-response curves: LD50 (dose lethal to 50 percent of test population). Lower LD50 = more toxic.
• Bioaccumulation: contaminant builds up in organism over time.
• Biomagnification: contaminant concentrates UP the food chain. Top predators have highest concentrations. DDT (eggshell thinning in raptors), mercury (in predatory fish like tuna), PCBs.

Unit 9: Global Change

Climate and planetary-scale change

• Greenhouse effect: natural process. Greenhouse gases (GHGs) trap heat. Without it, Earth would be frozen.
• Enhanced greenhouse effect: human-caused increase in GHGs (CO2, CH4, N2O, CFCs) raises global temperature.
• Main GHGs by warming potential per molecule: CFCs (highest) > N2O > CH4 > CO2. But CO2 is dominant contributor by total quantity.
• Sources: CO2 (burning fossil fuels, deforestation), CH4 (cattle, rice paddies, landfills, natural gas leaks), N2O (fertilizer, combustion), CFCs (refrigerants, banned).
• Climate change consequences: rising sea levels (melting ice, thermal expansion), changing precipitation patterns, more extreme weather, species migration, ocean acidification, coral bleaching.
• Ocean acidification: CO2 dissolves in water, forms H2CO3 (carbonic acid), lowers pH. Harms shellfish, corals (calcium carbonate shells dissolve).
• Invasive species: non-native species that outcompete natives. Examples: kudzu (US South), zebra mussels, Asian carp, rabbits in Australia.
• Human health impacts: expanded disease vectors (mosquitoes spreading malaria, dengue, Zika), heat waves, air pollution, food/water insecurity.

Math that shows up

• Population growth rate: r = (births - deaths + immigration - emigration) / population.
• Doubling time: 70 / percent growth rate (Rule of 70).
• Percent change: (new - old) / old * 100.
• Dimensional analysis (unit conversions): always show work. Convert m to km, kg to metric tons, etc.
• Half-life calculations: how much contaminant remains after n half-lives? (1/2) to the n times original amount.
• Energy calculations: 1 kWh = 3.6 million joules. Power (watts) times time (hours) = energy (kWh).
• Reading graphs: trends over time, correlations, identifying variables.

Key laws and treaties

• Clean Air Act (1970, 1990): regulates air pollutants. Cap-and-trade for SO2 reduced acid rain.
• Clean Water Act (1972): regulates water pollution.
• Endangered Species Act (1973): protects threatened and endangered species.
• Montreal Protocol (1987): banned CFCs. Ozone recovering.
• Kyoto Protocol (1997): first international climate treaty. US did not ratify.
• Paris Agreement (2015): nations set own climate targets. Goal to limit warming to 1.5-2 degrees C above pre-industrial.
• CITES: international treaty to protect endangered species from trade.
• Superfund (CERCLA 1980): cleans hazardous waste sites.
• EPA: federal agency enforcing environmental laws.

How to score a 5 on APES

1. Master the 9 units as a whole, not as silos. An FRQ on nitrogen pollution pulls from Unit 1 (cycle), Unit 5 (agriculture), Unit 8 (eutrophication), and Unit 9 (climate).
2. Memorize pollutants and their sources. What causes smog? Acid rain? Dead zones? Ozone depletion? These appear constantly.
3. Practice the math. Doubling time, percent change, unit conversions. Always show work and units.
4. Know the major laws and treaties. Montreal, Paris, Clean Air, Endangered Species. What did each do?
5. Practice the FRQ types. Investigation design (hypothesis, variables, controls). Problem analysis (read graphs, interpret data). Solution evaluation (pros, cons, trade-offs).
6. On solution FRQs, propose environmentally AND economically viable solutions. The grader rewards practical thinking.

Common mistakes

• Confusing weather (day-to-day) with climate (decades-to-centuries). Weather is a specific day; climate is a long-term average.
• Assuming all nuclear power is the same as nuclear weapons. Fission power plants do not explode like bombs. But they do produce waste.
• Saying the ozone hole causes global warming. They are SEPARATE issues. Ozone depletion is UV radiation; climate change is GHGs.
• Skipping units on APES calculations. That is a guaranteed point loss. Show work AND units.
• Confusing the carbon cycle steps. Photosynthesis (CO2 to plants). Respiration (plants/animals to CO2). Combustion (fossil fuels to CO2).
• Forgetting renewable does not equal sustainable. Biomass is renewable but unsustainable if deforestation is involved. Hydroelectric is renewable but dam construction has huge impacts.
• Misidentifying primary vs secondary pollutants. Primary emitted directly (CO, SO2). Secondary forms in atmosphere (O3, acid rain).
• Treating humans as separate from ecosystems. We are part of them. Human impact is always on the exam.

APES is wide, not deep. Make flashcards of the specific facts (pollutants, energy sources, treaties), practice the calculations, and understand the trade-offs. That is the exam.